GeolCarp_Vol49_No3_193

195

period in inclination and declination, and also more rapid swings

particularly marked in the inclination. Strong easterly declinations

of 80

to 90

in the top half of the profile, which cause the virtual

geomagnetic pole to migrate clockwise to around 50

away from the

geographical pole, has been named the Nørre Lyngby declination

excursion.

The Allerød section

A palaeomagnetic investigation of a 7 m sequence of freshwater

sediments from a formerly small lake, which is now dried out in an

open cliff profile, composed by 7 m sand, silt and gyttja was re-

cently reported (Abrahamsen & Readman 1997). The sequence

spans the time interval between ca. 12,000 and ca. 10,700 BP, and

is exposed in the classic Late Glacial Allerød site at Nørre Lyngby

in North Jutland (Abrahamsen & Readman 1997). Magnetically

the sequence shows about 5 cycles in declination and 2 cycles in

inclination with amplitudes of ca. ±10

(Fig. 1). Secular variation

features as those observed at Nr. Lyngby are also recognisable at

sites in southern Sweden (e.g. Björk & Sandgren 1986), Finland

(Saarinen 1994) and Soviet Karelia (Bakhmutov et al. 1994). The

secular variation therefore may be a usefull tool for local and re-

gional stratigraphical correlation of young sediment series on a

much more detailed timescale than possible by the global magnet-

ic reversal timescale, which off course is very useful for older se-

quences of longer duration.

Late glacial sediments from southern Baltic Sea

Secular variation results have also been obtained from three 5 to

12 m long sediment cores from the Bornholm Basin (Abrahamsen

1995), coverning Late Glacial varves and Ancylus-Yoldia clays.

Here the magnetic susceptibility, as well as density, NRM intensity

and q-ratio are useful for correlating the cores locally. In this case

shallow values of the inclination may be interpreted as caused by

sediment compaction of ca. 50% by younger sediments, and may

indicate later removel of some 58 m of the young sediments by

erosion. In two cores a fairly regular inclination secular variation

pattern was also seen in varvic clays, while a detailed record of

declination variations was only resolved in one core.

References

Abrahamsen N., 1982: Pleistocene-Holocene magnetostratigraphy at Sol-

berga, Brastad and Moltemyr, SW Sweden. SGU Ser. C, 93119.

Abrahamsen N., 1995: Paleomagnetism. Paleomagnetic Investigation. In: E.

Emelyanov, C. Christiansen & O. Michelsen (Eds.): Geology of the

Bornholm Basin. Aarhus Geoscience, 5, 5563.

Abrahamsen N. & Readman P.W., 1980: Geomagnetic variations recorded

in Older (>23000 BP) and Younger Yoldia Clay (14 000 BP) at

Nørre Lyngby, Denmark. Geophys. J.R. Astr. Soc., 62, 329344.

Abrahamsen N. & Readman P., 1997: Geomagnetic Secular Variation in

late Weichselian Allerød sediments from Nr. Lyngby (Denmark). Bull.

Geol. Soc. Denmark, 4, 4558.

Bakhmutov V., Yevzerov V. & Kolka V., 1994: Geomagnetic secular varia-

tions of high-latitude glaciomarine sediments: data from the Kola

Peninsula, northwestern Russia. Physics Earth Planet. Interiors, 85,

143153.

Björk S. & Sandgren P., 1986: A 2000 year geomagnetic record from two

Late Weichselian sequences in south-east Sweden. GFF, 108, 2129.

Saarinen T., 1995: Palaeomagnetic study of the Holocene sediments of Lake

Päijänne (Central Finland) and Lake Paanajärvi (North-West Russia).

Bull. Geol. Surv. Finland, 376, 187.

NEOGENE PALAEOMAGNETIC

ROTATIONS OF THE CHENOUA MASSIF

(NORTHERN ALGERIA)

T. AÏFA

, D. BELHAÏ

and O. MERLE

Géosciences-Rennes, CNRS UPR4661, Université de Rennes 1, Bt 15,

Campus de Beaulieu, 35042 Rennes Cedex, France; aifa@univ-rennes1.fr

USTHB, IST, BP32 El-Alia, 16000 Algiers, Algeria

Université Blaise Pascal, Dpt Sciences de la Terre, 5 rue Kessler,

63038 Clermont Cedex, France

The tectonics of the Chenoua Massif suggests rotation of the

Neogene nappes related to the African-European plate conver-

gence through a NNWSSE direction. It is associated with norther-

ly dipping reverse faults. Seismicity and surface ruptures caused

by the seismic event in the Cherchell area on October the 29th

1989 and the focal mechanism of this seismic event also reveal the

same compressional direction (mean deformation velocity around

0.25 cm/yr) (Meghraoui 1991). Sedimentary and eruptive forma-

tions which crop out west of Algiers are related to basins whose

subsidence started in the Lower Eocene. The Chenoua Basin is

part of the Mitidja and Cherchell basins where sediments lie un-

conformably on already deformed Meso-Cenozoic sediments and

older Paleozoic basement (Belhaï 1996). These so-called post-

nappes Neogene terranes were not affected by the two superim-

posed Alpine phases of Upper Eocene and Lower Miocene age

(Belhaï et al. 1990). From bottom to top, stratigraphic analysis of

the Neogene section shows that it is composed of white sandy

limestones with Amphiope of the Lower Miocene (N7N8 Bio-

zone of Blow), a red continental event, composed of conglomer-

ates, sandstones and pelites, a well developped facies from Chen-

oua to Ténès. Volcanics overlie these continental formations and

are covered by a marly marine formation (1611 Ma) showing lo-

cally yellow limestones. Andesites and rhyolites which outcrop in

the southern and western area of the Chenoua have been dated at

1516 Ma and 1113 Ma respectively by K-Ar method (Bellon et

al. 1977). The structural map shows that the Neogene have been

deformed by post-Alpine phases. It is overthrusted by the Creta-

ceous flyschs in the central part of the massif. This overthrusting

marked by an E-W striking reverse fault visible in the central part

of the massif progressively dies out along the strike towards the

eastern and western sides of the massif. Accordingly, the EW fold

axes direction in the central part rotates to N140

E in the west,

which gives a striking arcuate pattern to the Chenoua Massif. The

Neogene is cut by numerous faults. Structural analysis of the fault

pattern, particularly the syncline of Cap Blanc which is cut by two

major active faults oriented N30

E and N340

E, reveals sinistral

NE and dextral NW strike-slip faults suggesting an overall

NNW shortening direction. The dipping Neogene strata present in-

ter-bedded volcanics of the same vertical dip. It has been shown

within the overlying sediments that magmatic vertical intrusion oc-

curs during distension (Hellinger & Sclater 1983). Southward over-

thrusting of the Cretaceous sediments over the Neogene in the cen-

tral part of the massif is associated with a lateral displacement

gradient leading to an arcuate structure of the massif. This arcuate

structure of the Neogene strongly suggests block rotations associat-

ed with differential transport. A palaeomagnetic study of 14 sam-

pling sites, distributed in the NW, the SE and in the South of the

Chenoua Massif, has been performed to test this hypothesis and to

better understand the Neogene geodynamic history of this massif.

The NRM values are scattered for the sediments (3.10

A/m)

and clustered for the volcanics (3.10

A/m). Apart from most of

the volcanics, AF demagnetization is often insufficient for sedi-

196

ments because of a resistive component of magnetization, proba-

bly carried by goethite. Magnetic susceptibilities measured at each

heating step for the sediments (10

SI) and volcanics (3.10

SI)

allowed us to track the mineralogical changes which may occur at

the temperatures of 250

C and 450500

Demagnetization reveals two components of magnetization: a

viscous component below 200250

C and another one eliminated

up to 250

C. Orthogonal projections show that declination varia-

tion is linked to demagnetization, whereas inclination remains un-

changed. Principal component analysis confirms these observa-

tions and makes it possible to find an accurate direction for the

high-temperature component (

= 5.7

, 6.9

and 8.7

in situ). Af-

ter unfolding, the mean directions become scattered and the fold

test is negative. We interpret this as follows: the Lower Miocene

formations of the Cap Blanc syncline were folded and remagne-

tized during the Pliocene. Then, rotation of the whole structure

took place during the Plio-Quaternary.

Demagnetization diagrams show, first a component of magnetiza-

tion close to the present day field which is eliminated around 75

then a reverse one which is destroyed above 200

C and finally a

characteristic (Chr) normal component. This may suggest that re-

magnetization occurred during at least one polarity change.

Two types of mineralogical behaviour have been observed in the

volcanics. IRMs curves of type (I) show saturation before 0.2 T.

This is coherent with the existence of one mineralogical phase of

weak coercivity which can probably be attributed to magnetite.

IRMs curves of type (II) show the presence of two kinds of magnet-

ic carriers for which saturation is not reached before 1.2 T. They

may be goethite and magnetite or titanomagnetite types. Similar be-

haviour type (II) is also observed in sites within sedimentary rocks.

A thermomagnetic study has been performed under vaccum condi-

tions (V) and at atmospheric pressure (A) in some specimens in or-

der to determine the magnetic carriers. It enabled us to identify a

Curie temperature around 580

C suggesting the presence of magne-

tite. A mineralogical phase around 300

C is observed (A) for speci-

mens of behaviour (II). This mineralogic phase is still well observed

(V) in specimens of behaviour (II). This could explain the presence

of pyrrhotite like the type which transformed into magnetite up to

300

C. The carrier of the Chr component is probably magnetite.

This is coherent with the IRMs curves. Pyrrhotite type probably car-

ry the secondary component.

The paleomagnetic study of the Neogene in the Chenoua Massif

shows post-tectonic remagnetizations. Frequent normal and re-

verse polarities probably indicate that remagnetization took place

during a quite long time span. In most cases, detailed analysis of

orthogonal projections reveals that these remagnetized directions,

if isolated after an efficient cleaning, are well grouped and have

accurately carried the magnetic field.

It can be assumed that the clockwise rotations of 30±14

record-

ed by volcanics and 14±8

recorded by sediments occurred since

Tortonian times. The clockwise rotation evidenced in Cap Blanc

syncline took place since 8 Ma (i.e. since the age of the folding

event which shortly predates the remagnetization). Other sites of

the Cap Blanc structure registered a counterclockwise rotation of

10±5

. This part of the structure is probably related to N140

sin-

istral strike-slip. This could be explained by a model of indenta-

tion used by Laubscher (1972) related to lateral extrusion.

These rotations are related to recent deformations in this region,

particularly by the conjugated NE-SW sinistral strike-slip with

NWSE dextral strike-slip, at the origin of the shifted tectonic

markers. This classical model predicted the clockwise rotations of

blocks that we observed through palaeomagnetic results. The tec-

tonic results are coherent with a model suggesting clockwise rota-

tions at the Western part of the Chenoua (Cap Blanc) and counter-

clockwise rotations at its South-Eastern part, deduced from fold

axis changes. The directions of maximum constraints

deduced

from faults and slikensides and from their directions using the

right dihedra method allowed us to determine a NS subhorizontal

mean direction (due mainly to strike-slip) compatible with EW

folds (observed in the field) and to overthrusts to the South of fly-

schs on the Neogene and to conjugated NE sinistral and NW dex-

tral strike-slips. To the West of the massif, synclinal folds (Cap

Blanc and Koudiet Beida), of NS direction plunge 5

to the North

on the left side of the Hachem River and 5

to the South on its

right side. They also show a strike-slip fault system with a mean

direction of N10

deduced from the same method.

The bloc rotations in the Chenoua mount since the Pliocene are

comparable to rotations determined in the Chelif Basin (80 km south

of this studied area) for the same period (Aïfa et al. 1992).

References

Aïfa T., Feinberg H., Derder M.E.M. &Merabet N., 1992: Rotations paléo-

magnétiques récentes dans le bassin du Chéliff (Algérie). C.R. Acad.

Sci. Paris, t. 314, série II, 915922.

Belhaï D., Merle O. & Saadallah A., 1990: Transpression dextre à lEocéne

supérieur dans la chaîne des Maghrébides (massif du Chenoua, Algé-

rie). C.R. Acad. Sci. Paris, t. 310, série II, 795800.

Belhaï D., 1996: Evolution tectonique de la zone ouest-algéroise (Ténès-

Chenoua): approche stratigraphique et structurale. PhD Thesis, Univer-

sity of Algiers, 1163 (unpubl.).

Bellon H., Lepvrier C., Magné J. & Raymond D., 1977: Lactivité éruptive dans

lAlgérois: nouvelles données géochronologiques. Rev. Géol. Méditer. ann.

Univ. Provence, 4, 291298.

Hellinger S.J. & Sclater J.G., 1983: Some comments on two-layer extensional

models for the evolution of sedimentary basins. J. Geophys. Res., 88,

82518269.

Laubscher H.P., 1972: Some overals aspects of Jura dynamics. Am. Journal.

of Sciences, 293304.

Meghraoui M., 1991: Blind reverse faulting system associated with the Mont

Chenoua-Tipaza earthquake of 29 October 1989 (north-central Alge-

ria). Terra Nova, 3, 8493.

PALEOMAGNETIC CHARACTERISATION

OF THE BULGARIAN PART

OF THE MOESIAN PLATFORM

H. HAUBOLD

, H.J. MAURITSCH

, TZ. TZANKOV

K. KOURTEV

and G. NIKOLOV

Paleomagnetic Laboratory Gams, Montanuniversitaet Leoben, Austria;

herbert.haubold@grz08u.unileoben.ac.at

Department for Geophysics, Gams 45, A-8130 Frohnleiten

Institute of Geology, Bulgarian Academy of Sciences,

Acad. G. Bonchev Str. Bl. 24, Bg-1113 Sofia

The Moesian micro-continent is a peri-Gondwanan continental

fragment which broke its attachment in the Early Paleozoic and

accreted to the sub-Carpathian and Dobrudzha segments of the

southern periphery of the Eurasian continent after the closing of

the Paleotethys in the late Paleozoic.

We obtained paleomagnetic results from five sites of Neogene

silt- and sandstones (Odarska and Karvunska Formations), five

sites of the Neogene basalts along the NNESSW trending Suhin-

dol-Svishtov fault zone (K-Ar age 22 Ma), five sites of Paleogene

siltstones (Dikilitash and Aladan Formations), two sites of Paleo-

gene carbonate rocks (Komarevo Formation), and eleven sites of

Upper Cretaceous carbonate rocks (Nikopol, Mezdra, and Kailaka

Formations) from the broader vicinities of Pleven (middle to west-

197

ern part of northern Bulgaria) and Varna (eastern part of northern

Bulgaria).

The Neogene clastic sediments contain characteristic magnetiza-

tion components that typically are stable up to a temperature of

more than 500

C, indicating magnetite as the predominant carrier

of the remanence. Unfortunately, all of these sites show very large

within-site scatter and, thus, had to be rejected. The behaviour of the

Neogene volcanics during demagnetization clearly indicates magne-

tite as the only carrier of the remanence. After removal of a viscous

random component at very low alternating fields (AF), the remain-

ing magnetization decays in a univectorial fashion towards the ori-

gin and is stable up to 560

C. Owing to the extremely weak natural

magnetization of the samples from the Paleogene sediments, only

three sites of clastics and one site of carbonates yielded interpret-

able results. Again, the behaviour during demagnetization indicates

magnetite as the predominant remanence carrier. The Cretaceous

carbonates show magnetite phases of various hardnesses, as the

samples typically lose approximately half of their intensity when

exposed to an AF of only 5 mT, but their magnetization gradually

decays at higher fields and remains stable up to 30 mT or more than

500

C, respectively. Some of these samples show univectorial mag-

netizations but do not reach a final component, indicating the pres-

ence of a hard magnetic mineral, probably hematite. Of almost half

of the Upper Cretaceous sites no stable magnetization components

could be isolated due to very weak magnetizations, however the re-

maining sites show good within-site groupings. Isothermal rema-

nence acquisition experiments support the above observations as to

the magnetic mineralogy.

The following area means were calculated: Upper Cretaceous:

4.4/53.4/7.7/5 (declination, inclination, confidence limit, amount of

sites), Paleogene: 22.6/53.6/15.8/3, Neogene volcanics: 14.8/54.7/

14.3/5. Importantly, the difference between the observed Upper Cre-

taceous direction and the present day field direction is statistically

significant. Because most strata are in a horizontal position, field

tests could not be conducted. Three sites in the eastern part of the

sampling area show unusual site means and were excluded from the

calculation of the area means. By comparison of these values with

the expected magnetizations for this area with respect to the Eur-

asian apparent polar wander path, rotation and flattening were cal-

culated. In each case, these values are small and statistically not sig-

nificant, because none of them exceed their error limits. This result

implies that the investigated area was contingent upon Europe since

the Late Cretaceous and has not undergone significant movements

relative to the European continental interior since this time.

During the Alpine orogeny, the Moesian microplate formed a part

of the active continental margin of the Eurasian continent, and its

southern parts were subjected to repeated polyphase deformations.

The regional stress field was characterized by sub-horizontal and

generally north oriented compression and sub-vertical extension.

However, our data indicate that the cratonized parts of the Moesian

microplate were not significantly affected by these events. During

the neotectonic (post-Oligocene) a SSWNNE trending extensional

stress field formed in the area, and since the beginning of the Neo-

gene, the Bulgarian part of the microplate was progressively tecton-

ized. The latter process is probably responsible for the anomalous

site mean directions mentioned above.

In another study (presented at EGS meeting, Nice, F, this year)

we could show that Jurassic carbonates from the Fore Balkan and

Stara Planina mountain ranges just to the south of the Moesian

platform were pervasively remagnetized in an Oligocene field,

likely reflecting the thermal events associated with the active con-

tinental margin between the African and Eurasian continents dur-

ing that time. The concordant nature of our data from the Moesian

platform with respect to the European continent and the absence of

remagnetizations implies that the structural motions controlled by

extension within the general Aegean region and lateral movement

along the North Anatolian transform fault still affected the Fore

Balkan and Stara Planina mountain ranges, but ceased at the south-

ern rim of the cratonized Moesian micro-continent.

PALEOMAGNETIC OVERPRINTS AND

ROCK MAGNETISM OF PALEOZOIC

SERPENTINITES FROM THE

GOGO£ÓW-JORDANÓW MASSIF,

SUDETES (SOUTH POLAND)

M. K¥DZIA£KO-HOFMOKL* and M. JELEÑSKA

Institute of Geophysics, Polish Academy of Sciences,

01-452 Warsaw, Ks. Janusza 64, Poland; *magdahof@igf.edu.pl

The serpentinite massif of Gogo³ów-Jordanów (GJ) belongs to the

dismembered Paleozoic ophiolite sourrounding the Sowie Góry Mts.

block (SG) and are was recently intepreted as a surviving frag-

ment of an obducted rock series (Dubiñska & Gunia 1997). Paleo-

magnetic results for its other members: le¿a gabbroic unit (SL)

contacting the GJ from NE, and Nowa Ruda and Z¹bkowice massifs,

are given in Jeleñska et al. (1995). To the NW the seprentinites con-

tact the Strzegom-Sobótka (SS) granitoid massif, to S - with the the

SG block. The GJ rocks are older than the SL gabbros dated to

420 Ma (Oliver et al.,1993) which penetrate them with udisturbed

apophyses. According to Van Breemen et al. (1988) the GJ massif

came into direct contact with the GS which uplifted during the De-

vonian (380 Ma). This event, as well as intrusion of the SS grani-

toids (299 Ma and 323 Ma, Puziewicz 1990) strongly influenced the

GJ rocks. The first stage of serpentinization took place in an oceanic

environement, several later stagesin continental conditions. Dur-

ing each stage the temperature did not exceed hydrothermal temper-

atures (Jêdrysek 1988; Dubiñska & Gunia 1997). Sampling was

done in four quarries labelled G, K, S and P, where 31 hand samples

and 9 drill cores were taken. Optical microscope and SEM investi-

gations revealed the presence of secondary magnetite of various ori-

gins and ages. Chromites present in great abundance altered into

magnetite passing through the phase of iron-chromium spinel. Apart

from post-chromite magnetite there are also post-pyroxene magne-

tite lamellae, post-serpentinite magnetites, blastic magnetites and

small (1

m and less) magnetite grains residing within pyroxenes,

olivines and serpentines. Some of the small magnetite grains are

partly altered into martite. Magnetite is the only magnetic mineral

identified by IRM acquisition curves and by thermomagnetic meth-

ods applied to isothermal remanence Ir in non-magnetic space and

to magnetization I acquired in various fields. In some specimens the

curves obtained show a sudden drop of Ir at temperatures of 150

200

C and an increase of I measured in fields of several mT up to

300

C. Higher fields screen this effect. Study of hysteresis parame-

ters reveals decreases of Hc, Hcr and saturation remanence Mr after

annealing to 100300

C. Saturation magnetization Ms and bulk

susceptibility increase in more or less the same temperature range.

Similar behaviour was observed in Mongolian ophiolite by Didenko

(1992). He interprets it as due to the presence of small (<1

magnetite grains with maghemite coating that complete its oxidation

to maghemite in 150300

C. We surmise, that similar processes

take place in the GJ serpentinites. Hematite, visible as martite under

the microscope, manifests itself only during thermal demagnetiza-

tion experiments as the carrier of a component forming several per-

cent of the initial NRM. 68 specimens were demagnetized, nearly all

198

of them thermally. Analysis of demagnetization results revealed the

7 components of NRM shown in Table 1. Their age was estimated

from comparison with reference data for Baltica and Stable Europe.

The J component is a Lower Jurassic overprint (Westphal et al.

1986) and was isolated in the low temperature range LT<250

The A component dated as Carboniferous and probably related to

the intrusion of SS granitoids was obseved on hematite, in some cas-

es only as a point on the demagnetization curve in the VHT (650

685

C) range. The A1 component dated as Silurian is carried by

magnetite (HT range, 500575

C). All other components were iso-

lated in either the HT or VHT ranges. Component D fits the Devo-

nian reference datait probably originated during uplift of the GS.

Three remaining components are not easily interpreted in terms of

the post Silurian geomagnetic field and are probably artefacts. The

results obtained here support the idea expressed in Jeleñska et al.

(1995) concerning closeness of the studied ophiolitic units to Balti-

ca since the Silurian.

nence. In both the allochthon and the parautochthon the remaining

high temperature stable paleomagnetic signal appears to be older

and forms tails in a counterclockwise direction with respect to the

vertical axis of rotation. This signal is originating from a thermo-

chemical remanence of pure hematite lamellae exsolved in silicates.

The similar character of the tails for both the allochthon and the

parautochthon suggests that the paleomagnetic component was re-

corded before the allochthon was separated from the parautochthon.

The dimensions of the tails indicate that the preexisting block pri-

or to separation rotated about 45

. The counterclockwise direction

of this motion is consistent with the majority of sinistral vertical

faults in this area and with the low temperature recrystallization of

titanhematite grains. Once separated, the allochthon was thrust over

the parautochton. During this motion the allochthon rotated counter-

clockwise around a vertical axis by 162

. At the end of this rotation-

al thrust, mafic dikes were implaced in the allochthon and recorded

the Grenville pole direction. This suggests that the suggested coun-

terclockwise rotation had to occur during latest Grenville time, ca.

1000 Ma. This structural tectonic event is consistent with sinistral

oblique plate convergence.

NEW NAMURIAN PALEOMAGNETIC POLE

FROM THE WESTERN AFRICAN CRATON

N. MERABET, B. HENRY*,

H. BOUABDALLAH and S. MAOUCHE

Lab. Paleomagnetism et Geodynamique, 4 av. de Neptune,

94107 Saint-Maur Cedex, France; *henry@ipgp.jussieu.fr

Thermal analyses of rock samples, collected at 12 sites in the Na-

murian Reouina redbeds (Tindouf Basin; Algeria), showed that the

natural remanent magnetization consists of two juxtaposed compo-

nents, apart from a weak viscous component A which is destroyed at

low temperature (200300

C). Component B was defined at tem-

peratures ranging between 200300

C and 550580

C in 115 spec-

imens from all the 12 sites. After dip correction, its mean magnetic

direction is defined by D = 134.0

, I = 6.6

, k = 235,

= 2.8

Component C was isolated in 193 specimens (covering all the sites)

at temperatures higher than 550580

C. Its mean direction after dip

correction is D = 126.9

, I = 10.8

, k = 276,

= 2.5

. A representa-

tive set of specimens has been submitted to rock-magnetic experi-

ments in order to discover out the origin of these components. He-

matite and a titanomagnetite (probably magnetite) have been found

as probable carriers of the C and B components respectively. In such

a case, the component carried by the hematite is classically consid-

ered as being a magnetic overprint (chemical remanent magnetiza-

tion), while the magnetite should be the carrier of the primary mag-

netization. However, the general trend of the evolution of the

paleomagnetic inclination during the Upper Paleozoic period is a

decrease in algebraic values. Accordingly, the component C is the

oldest one, although it is carried by the hematite. It was thus proba-

bly acquired just after the deposition process. The B component was

then acquired later by the sediments and corresponds to a post-Na-

murian overprint. Its associated paleomagnetic pole (35.4

S, 53.6

E) appears to be close to the Stephano-Autunian poles for the Sahar-

an craton (38.5

S, 57.5

E El Adeb Larache, Henry et al. 1992

33.8

S, 61.1

E Derder et al., 1994 29.1

S, 57.8

E Mera-

bet et al. 1998 32.5

S, 56.7

E Merabet et al. 1997). The pale-

omagnetic pole associated with the C component is situated at 28.4

S and 56.9

E. This pole is in good agreement with those of Hassi

Bachir (Upper NamurianLower Moscovian; 26.8

S and 56.6

Daly & Irving 1983) and El Adeb Larache (Moscovian; 28.7

S and

dir

est.age

N/n D I

PlatN PlongE

GJ A L.Permian 3/24 202 5

14 80 -33 351

GJ D M.Devonian 1/5 243 20 11 48 -8

314

GJ A1 M.Silurian 3/11 195 39 6 70 -16 2

GJ B ?

1/6 284 2 8 74 13 278

GJ C1 ?

4/19 303 43 8 18 40 277

GJ C2 ?

3/12 276 70 5 74 42 324

GJ J

L.Jurassic

3/8 24 54 11 17 66 141

Table 1: Mean directions and pole positions obtained for four exposures of

Gogo³ów-Jordanów serpentinites (lat: 50.9 E, long: 16 N) dir direction,

est. age age in Ma estimated from APWP reference curve for Baltica after

Torsvik & Smethurst (1992), N/n number of exposures/number of speci-

mens,

, k parameters of Fisher statistics, PlatN pole latitude, PlongE

pole longitude, letters denoting directions as in Jeleñska et al. (1995).

References

Didenko,1992: Phys. Earth. Pl. Int.

Dubiñska & Gunia, 1997: Geol. Quart., 41, 1, 120.

Jeleñska et al., 1995: Geol. J. Int., 122, 658674.

Jêdrysek,1988: Miner. Pol., 22, 1, 6176.

Puziewicz, 1990: Arch. Min., XLV, 12, 136153.

Torsvik & Smethurst, 1992: GMAP V.9.0.

Van Breemen et al., 1988: Ann. Soc. Geol. Pol., 58, 319.

Westphal et al., 1986: Tectonophysics, 123, 3782.

PALEOMAGNETIC EVIDENCE FOR OBLIQUE

CONVERGENCE IN THE GRENVILLE

G. KLETETSCHKA* and J.H. STOUT

Department of Geology and Geophysics, University of Minnesota, 310

Oillsbury Drive SE, 55455 Minneapolis, Minnesota, USA;

*klet0001@gold.tc.umn.edu

Paleomagnetic evidence from granulite facies gneisses in central

Labrador indicates that the Wilson Lake allochthon has rotated 162

about a vertical axis relative to underlying parautochthonous rocks.

The Wilson Lake allochthon has a distinct paleomagnetic signature

when compared to the paleomagnetic signature of the surrounding

parautochthon. Our data indicate that the allochthonous paleomag-

netic pole (295

, 54

; trend and plunge, respectively) is displaced

horizontally by 162

from the pole of parautochthon (97

, 55

). The

horizontal direction of displacement is constrained by an offset of

the magnetization that remained stable at high temperature (over

600

C) demagnetization from the direction of the dominant rema-

199

55.9

E; Henry et al. 1992) formations belonging to the stable Sahar-

an craton. The Upper Carboniferous segment of the African appar-

ent polar wander path appears now very well documented. As in the

Stephanian Merkala formation of the Tindouf Basin (Merabet et al.

1997), two juxtaposed components have been found, the older one

being carried by the hematite.

References

Derder M.E.M., Henry B., Merabet N. & Daly L., 1994: Palaeomagnetism

of the Stephano-Autunian Lower Tiguentourine formations from sta-

ble Saharan craton (Algeria). Geophys. J. Int., 116, 1222.

Henry B., Merabet N., Yelles A., Derder M.M. & Daly L., 1992: Geodynami-

cal implications of a Moscovian paleomagnetic pole from the stable Sa-

haran craton (Illizi Basin, Algeria). Tectonophysics, 201, 8396.

Merabet N., Henry B., Bouabdallah H. & Maouche S., 1997: Lower

Stephanian paleomagnetic pole from the West African craton. EUG 9,

Strasbourg.

Daly L. & Irving E., 1983: Paléomagnétisme des roches carbonifères du Sa-

hara central; analyse des aimantations juxtaposées; configuration de la

Pangée. Annales Geophysicae, 1, 207216.

Merabet N., Bouabdallah H. & Henry B., 1998: Paleomagnetism of the Low-

er Permian redbeds of the Abadla basin. Tectonophysics, in press.

PALEOMAGNETISM OF MIOCENE -

PLIOCENE ROCKS OF THE CENTRAL

(SIERRA DE LAS CRUCES, MEXICO BASIN)

AND EASTERN SECTOR

OF THE MEXICAN VOLCANIC BELT

M.L. OSETE

, V.C. RUIZ-MARTÍNEZ

, R. VEGAS

C. CABALLERO

, J. URRUTIA-FUCUGAUCHI

and D. TARLING

Dpto. Geofísica, F. CC. Fisicas, Universidad Complutense,

Madrid 28040, Spain; *mlosete@eucmax.sim.ucm.es

Dpto. Geodinámica, F. CC. Geológicas, Universidad Complutense,

Madrid 28040, Spain

Instituto de Geofísica, Univ. Nacional Autónoma de Mexico,

04150 Mexico D.F., Mexico

Dep. Geological Sciences, Plymouth University, Drake Circus,

Plymouth PL4 8AA, U.K.; d.tarling@plymouth.ac.uk

Despite the amount of studies of the so-called Mexican Volcanic

Belt (MVB) there is no consensus about its time of onset, aerial ex-

tension and origin. Many models have been proposed to explain the

tectonic origin and subsequent development of this magmatic activi-

ty, related directly to, or caused indirectly by, the subducting slab of

the Cocos Plate at the Acapulco trench. But any geodynamic model

proposed should consider the rotational-deformational history of the

magmatic arc that palaeomagnetic investigations provide.

Palaeomagnetic data available from the central sector of the

TMVB show a significant divergence from the expected directions,

with negative R parameters ranging from 10 to 56 degrees. These

results have been interpreted in terms of counterclockwise rotations

of the studied areas. The eastern sector of the TMVB has practically

not been palaeomagnetically investigated till today. In order to de-

termine the spatial distribution of the proposed block rotations, two

systematic palaeomagnetic studies have been carried out.

A local palaeomagnetic study has been carried out on the Sierra

de las Cruces (western margin of the Mexico Basin, central part of

the MVB) in a NNW-SSE profile of 25 sites of Pliocene andesites. A

total amount of 241 samples has been demagnetized and an easy di-

rectional behaviour, related to Titanomagnetites of low Titanium

content, has been observed in most samples. Normal and reversed

directions pass the reversal test. The mean direction obtained in this

study is D = 353.1, I = 30.8 (N = 25; K = 27.7;

= 5.6), in line

with the expected direction. Therefore this area has not been affect-

ed by significant block rotations in contrast to the counterclockwise

rotations observed in the central-western sector of the MVB. A mag-

netic zoning has also been observed, as a result of that, a geochrono-

logical study has been carried out. Results indicate a southern mi-

gration of the volcanism.

To present a wider view of the rotational pattern of the MVB, a

regional palaeomagnetic study has been carried out in 221 samples

of 20 volcanic sites from the eastern sector of the magmatic arc. The

investigated volcanism extends from the central part of the MVB to

the Gulf of Mexico: 17 sites are grouped in the Altiplano area, (dat-

ed from 2.4 to 9 Ma by means of geochronological studies), and 3

sites in the Palma Sola Massif, Eastern Volcanic Province (from 1

2 Ma to 17.0 Ma). Most characteristic directions are carried by Tita-

nomagnetites of low Titanium content, but Titanohematites are also

found. No directional difference has been observed between the in-

vestigated rocks that are older or younger than 5 Ma. Both normal

and reversed polarities have been observed and the characteristic di-

rections pass the reversal test. The obtained mean direction (D =

350.4; I = 38.3; K = 22.7;

= 7.0; N = 20) indicates that little (if

any) counterclockwise rotation has taken place in this sector of the

MVB since the Late Miocene.

References

Cantagrel J.M. & Robin C., 1979: K-Ar Dating on Eastern Mexican Volcanic

Rocks Relations between the andesitic and the alkaline provinces. J.

Volc. Geoth. Res., 5, 99114.

Irving E. & Irving G.A., 1982: Carboniferous Through Cenozoic and the

Assembly of Gondwana. Geophysical Surveys, 5, 141188.

Mooser F., 1970: Condiciones geológicas acerca del Pozo Texcoco PP I, V.

Reunión Nacional Mexicana de Suelos, 2, 143161.

Mooser F., A.E.M. Nairn &Negendank F.W. , 1974: Palaeomagnetic Investi-

gations of the Tertiary and Quaternary Igneous Rocks: VIII. A Palaeo-

magnetic and Petrologic Study of Volcanics of the Valley of Mexico.

Geologische Rundschau, 63, 451483.

Mora-Alvarez G., Caballero C., Urrutia-Fucugauchi J. & Uchiumi Sh.,

1991: Southward migration of volcanic activity in the Sierra de Las

Cruces, basin of Mexico? A preliminary K-Ar dating and palaeomag-

netic study. Geofísica Internacional, 30, 2, 6170.

Pasquaré G., Vezzoli C. & Zonchi A., 1987: Morphological and structural

model of Mexican Volcanic Belt. Geofísica Internacional, 26, 2, 159

176.

Shubert D.H. & Cebull S.E., 1984: Tectonic interpretation of the Trans-Mexi-

can Volcanic Belt. Tectonophysics, 101, 159165.

Urrutia-Fucugauchi J. & Böhnel H., 1988: Tectonics along the Trans-Mexi-

can volcanic belt according to palaeomagnetic data. Physics of the

Earth and Planetary Interiors, 52, 320329.

MAGNETIC AND PALEOMAGNETIC

STABILITY OF NEOVOLCANIC ROCKS

OF DISTINGUISHABLE TYPES

OF MAGNETIC MINERALS

O. ORLICKÝ

Geophysical Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 842 28

Bratislava, Slovak Republic; geoforky@savba.sk

Seven dominant groups (B, C, D, F, G, I, J) of magnetism carri-

ers in volcanic rocks have been distinguished by Orlický (1998).

Group B the dominant phase of T

≈

130220

C corresponds

to quasi homogeneous to partly oxidized titanomagnetites (TMs),

200

second phase of T

≈

570

C (in minor portion) corresponds to

oxidized TMs. Group C the first phase of T

≈

210

C corre-

sponds to quasi homogeneous to partly oxidized TMs, second

phase of T

≈

575

C corresponds to oxidized TMs. The share of

both phases is thought to be equal in the rocks. Group D the

first-dominant phase of T

≈

480

C and the second magnetic

phase of T

≈

590

C, (in a minor portion in the rock). Both

phases correspond to oxidized TMs. Hematite-ilmenites can be

present in these minerals. In group F the first magnetic phase of

≈

420

C or T

≈

530

C, and the second phase of T

≈

600

C or

≈

590600

C. Both magnetic phases correspond to oxidized

TMs with presence of hematite-ilmenites. Group G contains domi-

nantly pure multi-domain magnetite of T

≈

580

C (small portion of

hematite-ilmenites can be present in this group). Group I the first

phase of T

≈

580

C corresponds to non stoichiometric magnetite

and the second phase of T

≈

620630

C contains hematite-ilmeni-

tes. Group J contains only one magnetic phase of T

≈

620640

hematite-ilmenites are present in the rocks of group J.

The thermal demagnetization of rock samples was performed by

the MAVACS system with an automated feed-back compensation of

external field to detect a stable component of RMP. The 50

C step

was applied for demagnetization of each sample within the interval

from room temperature to 650

C. JR-4 instrument was placed in the

centre of the Helmholtzs coils; a container from mumy metal was

used for transportation of rocks in the laboratory in order to protect

the sample against acquiring a parasitic remanence.

The examples of thermal demagnetization of representative sam-

ples of rocks of the groups F, G (the most unstable rocks) and I, J

(the most stable rocks) have been presented in Figs. 1, 2.

Fishers statistical parameters (k-precision parameter, and

semiangle of cone confidence for P = 0.05) were computed from

the results of individual steps of demagnetization for each tested

sample (the interval from room temperature up to the T

of each

individual sample was taken into account; rock samples with high

dispersion of RMP directions have low k and high value

, while

samples of low dispersion of RMP directions point to a high level

of k and low value of

). The mean values of k (k

) and

(

95M

) were computed for rocks of the respective groups of mag-

netism carriers (B, C, D, G, F, I ,J).

The following results have been obtained: Group B: k

= 396.6;

95M

= 6.1; n = 65 (n-number of samples). Group C: k

= 210.0;

95M

= 8.0; n = 38. Group D: k

= 148.0;

95M

= 8.4; n = 46. Group

F: k

= 140.0;

95M

= 15.6; n = 165. Group G: k

= 110.7;

95M

21.2; n = 89. Group I: k

= 1073.5;

95M

= 3.9; n = 192. Group J:

= 1756.0;

95M

= 2.1; n = 72.

We see that while the rocks with the quasi homogeneous, or

partly oxidized TMs of the groups B, C and those of partly oxi-

dized TMs of the group D have pointed out relatively good mag-

netic and paleomagnetic stability, the highly oxidized TMs of the

group F and those of the nearly pure magnetites of the group G are

of low paleomagnetic as well as magnetic stability. The rocks with

the magnetism carriers of both I and J groups-mostly hematite-il-

menites, are the most stable among the neovolcanics under study.

References

Orlický O., 1998: The carriers of magnetic properties in neovolcanic rocks

of central and southern Slovakia (Western Carpathians). Geol. Car-

pathica, 49, 181192.

PALEOMAGNETIC EVIDENCE

FOR THE MODE OF EMPLACEMENT

OF THE TRIASSIC EVAPORITES

IN THE EASTERN MAGHREB

H. ROUVIER, B. HENRY*, M. LE GOFF,

N. HATIRA, E. LAATAR, A. MANSOURI,

V. PERTHUISOT and A. SMATI

*Lab. Paleomagnetism et Geodynamique, 4 av. de Neptune,

94107 Saint-Maur Cedex, France; henry@ipgp.jussieu.fr

The mode of emplacement of the Triassic evaporites in the east-

ern Maghreb is presently the subject of debate. These evaporites are

considered, either as belonging to diapiric bodies taking root under

them (Perthuisot et al. 1997), or as being interbedded in the Albian

strata; in this last assumption, they should come on the surface by

small sized drain and be redeposited on the floor of the Albian sea,

giving salt glaciers (Vila et al. 1996). The aim of this study is to

show that paleomagnetism allows to resolve this controversy.

Northwest of the Jebel ed Debadib, Albian marls and limestones

are covered by the evaporitic body of El Kef. This formation has

been considered to be related to diapir evolution but in an over-

turned position, or as the floor of a salt glacier in an upright position

(Vila et al. 1996, plate 13B). It has been chosen as one of the sam-

Fig. 1. Thermal demagnetization, Zijderveld diagrams and stereographic

projections of RMP of selected rocks. 225/9 Pyroxene andesite; 168/9

Propylitized pyroxene andesite.

Fig. 2. Thermal demagnetization, Zijderveld diagrams and stereographic pro-

jections of RMP of selected rocks. St-217/1 Biotite-hornblende andesite;

TR24/2 Hyperstene-biotite hornblende andesite.

201

since the Albian, and this ChRM cannot be an overprint. Assuming

the Albian strata of the studied sites are in upright position, this mag-

netization cannot be true because the polarity of the Earths magnetic

field was always normal during the Upper Aptian and Albian periods.

The reversed polarity of the primary magnetization implies that the

studied series are in overturned position. The scattering of the ob-

tained directions shows that the tilting occurred around axes with var-

ious orientations. At Koudiat ed Delaa, the relatively high value of the

inclination shows that the magnetization was acquired when a moder-

ate dip existed due to the beginning of the tilting.

The implication of these data about the emplacement mode of the

Triassic evaporitic bodies is unequivocal. Not only all the Albian

strata presented as reference of the floor in normal position of the

salt glacier appear in overturned position, but the various axes of

tilting can only be explained by the emplacement of diapirs. There-

fore, the notion of a salt glacier interbedded within the Albian sedi-

ments has to be discarded.

References

Besse J. & Courtillot V., 1991: Revised and synthetic Apparent Polar Wan-

der Paths of the African, Eurasian, North American and Indian plates,

and True Polar Wander since 200 Ma. J. Geophys. Res., 96 (B3),

40294050.

Perthuisot V., Aoudjehane M., Bouzenoune A., Hatira N., Laatar E., Man-

souri A., Rouvier H. & Smati A., 1998: Les corps triasiques des

Monts du Mellègue sont-ils des diapirs ou des «glaciers de sel» ?

Bull. Soc Géol. France, sous presse.

Smati A., 1986: Les gisements de Pb, Ba et Fe du Jebel Slata (Tunisie cen-

trale-nord): minéralisations épigénétiques dans le Crétacé néritique de

la bordure dun diapir de Trias. Gisements de Sidi Amor ben Salem et

de Slata-fer. Thèse 3

ème

cycle, Paris, 1250.

Vila J.M., Ben Youssef M., Charrière A., Chikhaoui M., Ghammi M., Ka-

moun F., Peybernes B., Saadi J., Souquet P. & Zarbout M., 1994: Dé-

couverte en Tunisie, au SW du Kef, de matériel triasique interstratifié

dans lAlbien: extension du domaine à glaciers de sel sous-marins

des confins algéro-tunisiens. Compt. Rend. Acad. Sc. Paris, 318, II,

16611167.

Vila J.M., Ben Youssef M., Chikhaoui M., Ghammi M. & Kechid-Benkher-

ouf, 1996: Les grands glaciers de sel sous-marins albiens des confins

algéro-tunisiens. Proc. 5th. Tun. Petrol. Conf., Tunis, ETAP Mém., 10,

273322.

PALEOMAGNETIC INVESTIGATION OF

THE SEDIMENTARY AND

VOLCANO-SEDIMENTARY ROCKS

IN THE EAST SLOVAK BASIN

I. TÚNYI

, E. MÁRTON

and D. VASS

Geophysical Institute SAS, Dúbravská cesta 9,

842 28 Bratislava, Slovak Republic; geoftuny@savba.sk

Eötvös Loránd Geophysical Institute,

Colombus út 17-23, 1145 Budapest, Hungary

Slovak Geological Survey, Mlynská dolina 1,

817 04 Bratislava, Slovak Republic

After the paleomagnetic study of the Tertiary units from Western

and Central Slovakia (Márton et al. 1996) as well as paleomagnet-

ic study of the Neogene Volcanics from Eastern Slovakia (Nairn

1967; Orlický 1996) the measurements on sedimentary and volca-

no-sedimentary rock samples from the East Slovak Basin were car-

ried out. The aim of this investigation is to answer the question of

wheter the counterclokwise rotation also occurred in this Easter-

pling sites for the paleomagnetic analysis. At Koudiat ed Delaa, the

Albian strata could also be either overturned or in upright position

as the floor of a salt glacier according to the authors (Perthuisot et

al. 1998; Vila et al. 1994). Samples were taken at several sites in the

limbs and in the pericline of the Koudiat ed Delaa. The Jebel Slata

was considered as a reference example of a diapir (Smati 1986; Per-

thuisot et al. 1988). This mushroom like structure is also now the

subject of controversy because of the salt glacier assumptions. Sam-

ples for paleomagnetism were taken in the two debated limbs of this

structure, though the presence of post-tectonic ore deposits. The Al-

bian formation of the Jebel Slata pericline at Charen, considered in

upright position by all the authors, was also sampled for a reference

paleomagnetic direction.

The analysis of hysteresis loops and Curie curves of some speci-

mens indicates that magnetite is the main carrier of the magnetiza-

tion. A very low anisotropy of magnetic susceptibility indicates a

lack of significant internal strain. After at least one month in zero

field, the NRM was measured using a JR-4 inductometer. Because

of the acquisition of parasitic magnetizations during heating to tem-

peratures higher than 400

C, the demagnetization of the NRM was

mostly carried out using heating until 300350

C, followed by al-

ternating field demagnetization, and eventually again thermal treat-

ment at higher temperatures. The NRM contains several different

components. After demagnetization of a viscous component (com-

ponent A), one or two components can be isolated according to the

sites. In the two limbs of the Jebel Slata we found a component B

and a ChRM (component C). The B component was acquired after

deposition of a conglomerate containing Triassic and Albian frag-

ments. The orientation of the ChRM C can be estimated only in a

few specimens, because of the formation of parasitic magnetizations

during thermal treatment at high temperatures. However, the polari-

ty of the ChRM is clearly reversed, both before and after correction

for the apparent dip. In all the other sites, only the ChRM C is

present. The fold test performed at Koudiat ed Delaa shows that this

ChRM was acquired before the folding. This ChRM, after correction

for the apparent dip, is of reversed polarity in all the sites, except in

the Jebel Slata pericline. It has various orientations according to the

areas (Fig.).

The strata known to be in their upright position in the Jebel Slata

pericline retain a ChRM direction and polarity that corresponds to that

expected for a middle Cretaceous magnetization (Besse & Courtillot

1991). For the remaining sites, the ChRM direction does not corre-

spond to either that of the Albian reference pole or any reference pole

Fig. Stereographic projection of the component C with the confidence cir-

cle in the different studied sites after correction of the apparent dip. The

full (empty) symbols correspond to the lower (upper) hemisphere.

202

most part of the Inner Western Carpathians. We collected Eggen-

burgian sediments at one locality, zeolitized rhyolite tuffs of Bade-

nian age at three localities as well as Badenian rhyolite domes at

two localities. The number of indepedently and magnetically orient-

ed samples was 92. Standard-size cylinders were measured and

stepwise demagnetized by thermal, AF or by combining AF and

thermal methods. IRM and low susceptibility versus temperature ex-

periments were performed to help the identification of the magnetic

minerals. Stepwise thermal demagnetization was carried out in Bra-

tislava, the other experiments in Budapest.

The samples from the two rhyolite domes yielded excellent pa-

leomagnetic directions. The zeolitized rhyolite tuffs were weakly

magnetic; the demagnetization curves of the NRM were less

smooth than those of the rhyolites. Nevertheless, the components

of the NRM were well defined. The sediments are of different ages

and of different lithologies. The Eggenburgian locality yield a

good paleomagnetic direction. The Sarmatian sediments seemed to

be partly unstable. There was only one locality where 6 of the 8

collected samples gave a good cluster away from the present field

Fig. 1. Typical behaviour of the rhyolites during thermal demagnetization.

Modified Zijderveld diagram and normalized intensity/susceptibility (cir-

cles/dots) curves.

Fig. 2. Site and locality mean paleomagnetic direction with confidence cir-

cles. Numbers refer to Table 1. All inclinations are positive on the plot, i.e.

site mean directions with reversed polarity (l and 5) are shown as equiva-

lent normal polarity directions.

direction. The results are given in Table 1. The typical demagne-

tizing curves of rhyolitic sample are on Fig. 1. Fig. 2 shows mean

paleomagnetic directions.

As a conclusion we say that the above mentioned paleomagnetic

measurements gave information about counterclockwise rotation of

the Neogene units from East Slovak Basin. The results are very simi-

lar to those obtained by Márton & Pécskay (1995) in the Tokaj Mts.

(Hungary). The Eggenburgian sediments (1 loc. 20 spec.) show a

CCW rotation, of about 80

, the zeolitized rhyolite tuffs of the Early-

Middle Badenian age (3 loc. 25 spec.) a CCW rotation of about 60

the rhyolites (1 loc. 19 spec.) of the Late Badenian age a CCW rota-

tion of about 45

and the youngest sediments of the Early-Middle Sar-

matian age (1 loc. 6 spec.) gave a CCW rotation of about 20

References

Márton E. & Pécskay Z., 1995: The Tokaj-Vihorlát-Oas-Ignis Triangle:

Complex evaluation of paleomagnetic and isotope age data from Neo-

gene volcanics. IGCP Project 356, Plate Tectonic Aspect of Alpine

Metallogeny in the Carpatho-Balkan Region, 3rd Annual Meeting.

Athens, 1819 September 1995. Volume of Abstracts, 30.

Márton E., Vass D. & Túnyi I., 1996: Rotation of the North Hungarian Pa-

leogene and Lower Miocene rocks indicated by paleomagnetic data

(S. Slovakia, N-NE Hungary). Geol. Carpathica, 47, 1, 3141.

Nairn A.E.M., 1967: Paleomagnetic investigations of the Tertiary and

Quartenary igneous rocks: III A paleomagnetic study of the East Slo-

vak Province. Geol. Rdsch., 56, 408419.

Orlický O., 1996: Paleomagnetism of neovolcanics of the East-Slovak Lowlands

and Zemplinske Vrchy Mts.: A study of the tectonics applying the paleo-

magnetic data (Western Carpathians). Geol. Carpatica, 47, 1, 1320.

PALEOMAGNETISM OF M. DEVONIAN

TO E. CARBONIFEROUS SEDIMENTS

FROM THE DRAHANY UPLAND,

MORAVIAN ZONE, BOHEMIAN MASSIF

J. SLEPIÈKOVÁ

Institute of Geology, Academy of Sciences of the Czech Republic,

Rozvojová 135, Prague 6, Czech Republic

The study is devoted to the principal results of paleomagnetic in-

vestigations carried out in the Middle Devonian to Lower Carbonif-

erous sediments from the Drahany Upland, Moravian Zone, Bohe-

mian Massif. Pilot samples were collected from 12 localities (in the

vicinity of Jesenec, Slavoòov, Jevíèko, Mohelnice and Vitoov), de-

Locality

n/no

k a°

Age

Hrádok

9/9

183 -53 363 3 183 -53 10.5-13.2 Ma

Lesné

3/4

312 63 249 8 312 63

upper

Badenian

Lesné

16/2 311 66 1080 3 311 66

Oreské

11/11 2 44 28 9 319 60

lower

Badenian

Kuèín

8/9

8 24 61 7 304 75

lower

Badenian

Niný

6/12 95 -54 34 12

63 -20

mid

Hrabovec

Badenian

Lada

20/21 289 57 14 9 281 30 Eggenburgian

Slanèík

6/8

338 +56 24 14

338 46 lower-mid

Sarmatian

Table 1: n/nonumber used/collected samples; D, I (Dc, Ic)declination

before (after) tilt correction; k and

the Fishers statistical parameters;

statistics is based on number of specimen (n); loc. 12brhyolites, loc. 3

5zeolithized rhyolite tuffs, loc. 67sediments.

203

tailed sampling was done in two localities (Jesenec, Slavoòov). Pa-

leomagnetic investigation of further localities continues at the

present time. The principal object of investigation was to find locali-

ties of rocks with properties suitable for paleomagnetic analyses,

and hence for derivation of paleotectonic and paleogeographical pa-

rameters. Selected samples were subjected to progressive A.F. (al-

ternating field) demagnetization (15 samples) and the majority of

samples to progressive thermal demagnetization by means of the

MAVACS apparatus (78 samples). The A.F. demagnetization was

found ineffective for the types of rocks investigated. From the deter-

mination of the unblocking temperature (within the limits of 310

330

C) it could be concluded that the pyrrhotite is the principal car-

rier of magnetization for the localities of Vitoov (limestones),

Jevíèko (limestones), Mohelnice (limestones, greywackes, silt-

stones) and Jesenec. One site only from those of the locality of

Jevíèko yielded Middle Devonian greywacke with a wider spectrum

of unblocking temperatures, within 300580

C. Hematite is con-

tained in some rock samples (shales) from the locality of Slavoòov,

the unblocking temperature is around 660

C. The magnetization of

the Devonian limestones from the locality of Újezd near Boskovice

is carried by minerals with a wider spectrum of unblocking tempera-

tures within 200500

C (obviously due to different Fe-oxides) and

up to 675

C (due to hematite). Generally, the concentration of ferri-

magnetics is low in rocks sampled at all the localities mentioned

above. The rock samples collected from the localities near Jevíèko

and Mohelnice show properties not suitable for paleomagnetic in-

vestigations. Only greywacke samples from the locality of Jevíèko

and from one site from those near Mohelnice showed magnetic

properties suitable for paleomagnetic analyses. A complicated situa-

tion was found at the locality of Slavoòov, where samples show dif-

ferent properties due to different degree of alteration of rocks at this

locality. However, the samples from all the above localities as well

as samples from the localities of Vitoov, Jesenec and Újezd near

Boskovice are under laboratory investigations with the aim of veri-

fying their applicability to paleomagnetic studies. The primary (pa-

leomagnetic) magnetization components of the Devonian age were

found in some samples from the locality of Slavoòov and from one

locality near Mohelnice. The Variscan overprint magnetization com-

ponents were found in rock samples from the localities of Vitoov,

Jesenec and Jevíèko. The derived paleolatitudes are similar to those

derived earlier for rocks of similar age in the Bohemian Massif (Krs

& Pruner 1995). The rocks from the Drahany Upland prove paleo-

tectonic rotation similar to that derived from rocks from the Moravi-

an Karst, approx. 120

clockwise.

Reference

Krs M. & Pruner P., 1995: Paleomagnetism and paleogeography of the

variscan Formations of the Bohemian Massif, comparison with other

regions in Europe. Journal of the Czech Geological Society, 40, 12.

ARCHAEOMAGNETIC STUDY

OF BURNT CLAY STRUCTURES

FROM EMPORION PISTIROS

(CENTRAL SOUTH BULGARIA)

N. JORDANOVA* and M. KOVACHEVA

Geophysical Institute BAN, Acad. Bonchev str., bl. 3, 1113 Sofia,

Bulgaria; *vanedi@geophys.acad.bg

The first archaeomagnetic studies on materials from Emporion

Pistiros were made quite recently (Kovacheva & Gigov 1996). As a

result of this first study, three archaeomagnetic data points were

defined: 1 archaeologically dated about 290 BC; 2 archaeo-

magnetically dated in the time interval 270160 BC; 3 also ar-

chaeomagnetically dated, corresponding to the time interval 430

380 BC.

In 1995 the second sampling campaign was performed. Sam-

ples were obtained from six localities altars in squares B2,

B7, B17, destructions in squares A13, A13/A14 and the slope

near the site.

The questions stated by the archaeologist, leading the excava-

tions, are concerned with the origin of burnt clay found in squares

A13, A13/A14 and the slope whether they are destructions or

remains of ancient ovens. The other problem to be solved by the

archaeomagnetic investigations relates to the time of last burning

of the sampled structures. The large Bulgarian archaeomagnetic

data base available (Kovacheva 1997) is used to construct master

curves (Kovacheva et al. in press), suitable for dating purposes.

A set of rock-magnetic experiments have been carried out in or-

der to obtain information about the suitability of the material for

direction- and intensity-evaluations. The obtained results suggest

that the main ferrimagnetic minerals are magnetite and hematite.

However, significant mineralogical changes occurred during the

laboratory heating experiments, which make the paleointensity de-

terminations problematic.

The archaeomagnetic characteristics for altars in squares B2

and B7 are quite similar, which is in agreement with the archaeol-

ogists opinion. Thus, we regard these two structures as contempo-

rary. The dating procedure carried out (Kovacheva 1995) gives the

most probable dating interval as 282176 BC. Comparison of this

dating with the results obtained for the first collection (Kovacheva

& Gigov 1996), suggests that the present data coincide well with

the most recent structures (dated between 270160 BC). The obvi-

ous advantage here is that we have got not only the intensity, but

also the directional results.

Using measurements of anisotropy of magnetic susceptibility, it

is further proved that the remains of burnt clay, sampled in squares

A13, A13/14 and on the slope, are in fact destructions, rather than

destroyed ovens. However, this means that the direction (D and I)

cannot be obtained for these structures. Thus, only the paleointen-

sity value was used for dating. The obtained results are very close

to those for the altars in B2 and B7.

For destructions by the ancient fire, sampled on the slope near

the site, we obtained neither direction, nor paleointensity.

For samples taken from the altar in square B17, the direction

and intensity results do not fit the archaeomagnetic reference

curves for Bulgaria during the Thracian epoch. Therefore the final

dating of this structure is still questionable.

The results of the present study are in a good agreement with

the previous ones. It is found that most probably the structures of

burnt clay, sampled in squares B2, B7, A13, and A13/A14 all

can be related to the most recent development phase of Emporion

Pistiros.

References

Kovacheva M., 1995: Bulgarian Archaeomagnetic studies. In: D. Bailey &

I. Panayotov (Eds.): Prehistoric Bulgaria. Monographs in World Ar-

chaeology., Prehistory Press, Madison Wisconsin, No. 22, 209224.

Kovacheva M. & V. Gigov, 1996: Emporion Pistiros and archaeomagnetic

studies. In: J. Bousek, M. Domaradzki & Z. Archibald (Eds.): Pistiros

I. Excavations and studies. Charles Univ. Press, Prague, 187196.

Kovacheva M., 1997: Archaeomagnetic database from Bulgaria: the last

8000 years. Phys. Earth Plan. Inter., 102, 145151.

Kovacheva M., Jordanova N. & Karloukovski V., in press: Geomagnetic

field variations as determined from Bulgarian archaeomagnetic data.

Part II: The last 8000 years. Surveys in Geophysics.

204

ARCHAEOMAGNETICAL INVESTIGATIONS

OF ROMAN EXCAVATIONS

D. GREGOROVÁ

Geophysical Institute of SAS, Dúbravská cesta 9,

842 28 Bratislava, Slovak Republic

From the first to the fourth century AD the Romans built a sys-

tem of military bases, forts and watch-towers (Limes Romanus) on

both banks of the Danube to protect the northern borders of the

Roman Empire from barbarian attacks. There are some important

archaeological localities from this period in South Slovakia. From

two of them: Bratislava-Dúbravka and Ia near the city of Ko-

márno archaeomagnetic samples were taken. The samples from

Dúbravka, investigated in 1995, were from roof tiles (Orlický et

al. 1995). The results of measurements by the Thellier method

show, that the rate of the ancient geomagnetic field intensity from

about the third century AD to today´s intensity of the geomagnetic

field is k = 1.24 (Orlický et al. 1995).

The samples from Ia were measured this year. There were four

samples taken from wall-brick or from brick-tile (samples A1A4)

and six samples from roof-tile (B1B6). The time of their origin is

about the same as in the other case about 300 years AD. The ap-

plied investigation method was the Method of sequential paired

heating Thellier method.

The samples were twice in two exactly defined positions, heat-

ed and cooled step by step to temperatures 50, 100, 150, 200, 300,

350(A), 400, 450, 500, 550, 600, 650(B), 700 Celsius degrees and

after each cooling measured on a spinner. After each step magnetic

susceptibility was also measured. The representative examples of

curves of magnetic susceptibility, demagnetization and magnetiza-

tion curves are represented in the enclosed figures.

The shape of the analogous curves for the other A and B samples

are very similar to these examples.

The magnetic susceptibility of samples A before the first heating

lay

in the interval: K

= 6279

-6

7780

-6

u. SI and of samples

B in interval: 10,504

-6

13,227

-6

u. SI. The volumes of natu-

ral remanent magnetic polarization were also different. For the B

samples they were twice as big as for the A samples (A: from 127 to

178 nT, B: from 303 to 373 nT). The two investigated magnetic

characteristics indicate a partially different mineralogical composi-

tion of fragments A and B.

By calculation of coefficient k (=intensity of ancient field/intensity

of recent field) we take in acount only that part of curves, where the

susceptibility values were stable. The average value of koeficient k

for samples A is k

= 1.34 and for samles B is k

= 1.37. It is a little

higher than for samples from the lokality Dúbravka. But these results

correspond to the other data, found in other works for third century

AD and our part of Europe (Krs 1977). These works as well as our re-

sults document, that the intensity of the geomagnetic field in around

the third century AD was about 1.31.4 times higher than intensity of

recent magnetic field.

References

Orlický O., Túnyi I. & Elschek K., 1995: Archeomagnetism of the fragments of

the Roman roof tiles from the Dúbravka locality. Proceedings of the 1st

Slovak geophysical conference. Geophysical institute of SAS.

Carmichael Ch.M. & Thellier E., 1977: Paleomagnetic field intensity, its

measurement in theory and practice. Physics of the Earth and planetary

interiors. 13, 4.

Krs M., 1969: Paleomagnetizmus. Academia, Praha.

Sample A1 - demagnetization and magnetization curves

100

150

200

100

200

300

400

500

600

700

temperature [ Celsius degrees ]

Magnetic Field Intensity: Paleomag. field-Jp [ nT ] Laboratory field-JL [ nT ]

Jp
JL

Sample A1 - magnetic susceptibility heating curve

= 6 841x10

-6

u.SI

0.5

0.6

0.7

0.8

0.9

1.1

1.2

100

200

300

400

500

600

700

temperature [Celsius degrees]

Kn/Kn-max Ks/Ks-max

Kn
Ks

Sample B3 - demagnetization and magnetization curves

100

150

200

250

300

350

400

100

200

300

400

500

600

700

temperature [Celsius degrees]

Magnetic Field Intensity Paleomag. field - Jp [nT] Laboratory field JL [nT]

Jp
JL

Sample B3 - magnetic susceptibility heating curve

= 11 759x10

-6

u. SI

0.5

0.6

0.7

0.8

0.9

1.1

1.2

100

200

300

400

500

600

700

temperature [Celsius degrees]

Kn/Kn-max Ks/Ks-max

Kn
Ks

205

(3)

which enables modelling interpretations in any position of the ob-

served point P in the dipole field (Q). This position is determined by

the angle

The characteristic equation of the intensity gradient of the dipole

is written in the case of spherical coordinates of the form:

(4)

with eigen values of the tensor:

(5)

The own vector a

of the intensity gradient (belonging to eigen-

value

) can be deducted as:

(6)

which it is perpendicular to a plane drawn across the observed

point P and across the dipole axis. The additional two own vectors

and a

will be expressed in the form:

(7)

On the basis of theoretical conclusions mentioned in Eqs. (6) and

(7) it is possible to consider a complete model for the developed in-

terpretative procedures of geophysical measurements. This given

model of own vectors of the intensity gradient in potential (magnet-

ic) field can be aimed at acquiring new magnetometric maps for

some mining, and geological-prospecting purposes.

The simplicity of the dipole structure is sufficient for modelling

basic tasks in magnetometry, especially if it is for the purpose of op-

erative approximation in variability of some real geological struc-

tures. The theoretical principles of modelling confirm the necessity

of introducing concrete hypothetical interpreted parameters into the

interpretative procedures for the investigated geophysical fields of

some mineral deposit inhomogeneities.

References

Andrejev V.J. & Sokolovskij K.J., 1971: Interpretacija materialov podzemnych

gravitacionnych i magnetnych nabljudenij. Nedra, Moskva, 1198.

Okál M., 1972: Invarianty a vlastné vektory gradientu intenzity v interpre-

taènej geofyzikálnej praxi. In: Zborník vedeckých prác VT Koice,

Fig. 1. Scheme of considered dipole.

MODELLING INTERPRETATIVE TASKS

AT THE MINING MAGNETIC SURVEY

V. SEDLÁK

Technical University of Koice, Department of Geodesy & Geophysics,

Park Komenského 19, 043 84 Koice, Slovak Republic;

sedlak@ccsun.tuke.sk

Some theoretical knowledge of the variability of modelling inter-

pretative procedures in mining geophysics are presented. The direct

and inverted magnetometric task in a geological survey of deep min-

eral deposits is solved by means of the mathematical invariant so-

lution. The object of interest is to give precise and prospective new

mineral deposit positions from magnetometric data observed from

underground mine profiles (Andrejev & Sokolovskij 1971; Sedlák &

Gainec 1997). A different view of the interpretative procedures of

the geophysical fields at the inhomogeneities separation and back-

ground from the observed underground data is derived from a math-

ematical modelling of these interpretative procedures. The magnetic

field source is approximated by a dipole (Fig. 1).

II. Paleointensity

Considering also the effect of the gravitational field, in case of a

negative value of the charge (abundance) of the dipole, i.e. m(Q)

< 0 it is the Eq. (1) to be accepted for the intensity K of this dipole

field:

(1)

where p = m(Q)dl is the moment of the dipole, / is the gravita-

tional constant and U is the potential of the dipole magnetic field.

The intensity gradient of the magnetic potential of the dipole

field in terms of spherical coordinates is written as follows (con-

sidering the scalar value p) (Okál 1972):

(2)

The square of the scalar quantity of this intensity in the case of a

potential field dipole is given by solving of the Hamilton (Laplace)

operator, i.e. by partial derivatives of the intensity according to sep-

arate spherical coordinates in the Eq. (2):

K(Q,P) = -ÑU(Q,P) = Ñ

Q,P

G Q











(

)

K(Q,P)=-

e e

r r

−

cos

sin

cos

υ υ

cos

sin

υ λ





































(

)

K (Q,P)=

G p

cos

(

)

and

1 2

4 5

cos

a = f ( )e , where f ( )=

+ +

a f ( )e , where f ( )=

- +

i i

I I

4 5

sin

cos

sin

cos

= ae

206

Bratislava, Alfa, 1723.

Sedlák V. & Gainec J., 1997: Mathematical Modelling at the Mining Geo-

physical Survey. In: Proceedings results from recent Study in seismol-

ogy and Engineering Geophysics, Regional Conf. with Inter.

Participation, (Kaláb, Z.), Ostrava, ÚG ÈAV, April 89, 1997, 168174.

AVERAGED PALEOFIELD AND

GEODYNAMO CURRENT

S.V. STARCHENKO

Longitudinally-averaged magnetic and velocity fields show far

less time dependence than do 3D fields from geomagnetic observa-

tions and MHD dynamo simulations. This allows an additional time-

averaging of the axisymmetric magnetic field on a scale of a few de-

cades. The source of the resulting averaged paleofield is

longitudinal electric currents that have clear extrema near the

boundaries of the liquid core in the Earth. Thus, it is justified to rep-

resent averaged geomagnetic, archaeomagnetic and paleomagnetic

fields by geodynamo currents instead of the usual representation of

magnetic multipoles that have no physical basis.

As a first approximation, we consider two circular current loops

one at the ICB and the other at the CMB. When the sense of these

electric current loops is the same, we obtain a present day or slightly

higher magnetic field intensity. When the sense of the current loops

oppose one another, the resulting geomagnetic field intensity sig-

nificantly weakens. Reliable paleomagnetic and archaeomagnetic

records support the existence of such levels in paleointensity.

Our natural geodynamo-type currents may also be successfully

used for magnetic tomography of the Earths deep interior as well

as more physical representations of the global geomagnetic field.

WAVELET ANALYSIS OF

ARCHAEOMAGNETIC DATA OVER THE

LAST 4,000 YEARS

K. BURAKOV, D. GALYAGIN, P. FRICK,

I. NACHASOVA, M. RESHETNYAK* and D. SOKOLOFF

*Geophysical Institute, Boèní II/1401, 141 31 Prague 4,

Czech Republic; rm@ig.cas.cz

Although geomagnetic secular variation (SV) has its origin within

the liquid core of the Earth, we can only observe the processes in-

volved beyond the core-mantle boundary. Its source may involve

MAC-waves (10

years timescale) or Alfven waves (10

years ti-

mescale), as well as non-regular flow which may expel toroidal

magnetic field at the boundary. Since SV cannot be considered a

purely periodic process, classical spectral analysis techniques such

as Fourier or MEM cannot be directly applicable. Here we present

the wavelet approach applied to archaeomagnetic secular variation

data from Bulgaria, Georgia (FSU) and Central Asia for the last

4,000 years. The main advantage of this approach is that it may be

applied to non-stationary spectra as well as time series containing

gaps. The obtained wavelet time spectrum is presented in Fig. 1.

Most notable is a 1750-year variation, which is observed in all three

time series. The phase shifts of this variation correspond to wave

propagation from east to west with velocity V

= +0.2

per year.

Processes occurring on with the smaller timescales demonstrate the

lower level of correlation.

THE XITLE - EL PEDREGAL LAVA FIELD,

MEXICO CITY: ENIGMATIC INTRA- AND

INTER-FLOW PALAEOMAGNETIC

VARIATIONS

H. BÖHNEL* and G. McINTOSH

UNICIT-Instituto de Geofisica UNAM, Campus Juriquilla,

POB 11-742, C.P. 76001 Santiago de Queretaro, Mexico;

*harald@tonatiuh.igeofcu.unam.mx

A rock-magnetic and paleomagnetic profile through a flow-unit

of the Xitle-El Pedregal lava field (about 2000 BP) is documented.

The flow shows signs of emplacement via inflation in its internal

structure. Comparison with a previous profile through a Xitle flow

(Böhnel et al. 1997) shows that over the extent of the lava field flow

emplacement histories varied, probably due to topographical fea-

tures. Broad similarities in the magnetic properties of the two pro-

files suggests that such differences do not impact on the magnetic

mineralogy of the flows. Significant intra- and inter-flow differenc-

es in both the characteristic directions and paleointensities, obtained

using Thellier-type methods, are seen, both in the new profile and

previous studies of sites distributed across the lava field. These vari-

ations do not correlate with any of the measured physical or magnet-

ic properties of the flows. At any one site the mean directions are

well-defined and it is only when considered collectively that the in-

consistencies are recognized. Intra-flow and inter-site paleointensity

variations are large: a total of 117 determinations yield answers be-

tween 36.6 and 139.7 mT. Within this range it is difficult to recog-

nize a best estimate on the basis of rock-magnetic criteria. These re-

sults raise questions about the reliability of lavas as paleomagnetic

recorders and highlight the importance of sampling strategy in ob-

taining representative flow-mean parameters.

Fig. 1 Bulgaria, 2 Georgia, 3 Central Asia and T in years.

207

Böhnel H., Morales J., Caballero C., Alva L., McIntosh G., Gonzalez S. &

Sherwood G.J., 1997: Variation of rock magnetic parameters and pa-

leointensities over a single Holocene lava flow. J. Geomag. Geoelec-

tr., 49, 523542.

SECULAR VARIATIONS AND RELATIVE

PALAEOINTENSITY OF THE EARTHS

MAGNETIC FIELD BETWEEN 7,000 BC

AND 500 AD RECORDED BY ANNUALLY

LAMINATED LAKE SEDIMENTS

IN NORTHERN SWEDEN

I. SNOWBALL*, P. SANDGREN and G. PETTERSON

Department of Quaternary Geology, Lund University,

Tornavägen 13, 223 63 Lund, Sweden; *ian.snowball@geol.lu.se

Although continuous records of geomagnetic field variations can

be recovered through the palaeomagnetic analysis of lake sediments,

Holocene palaeomagnetic secular variation (PSV) master-curves have

generally relied upon

C dating to provide a chronology (Thompson

1983). Recently it has been discovered that

C dates obtained on bulk

lacustrine sediments (even organic rich sediments) can include signif-

icant errors due to the inclusion of old carbon in the dated material,

with the result that calibrated

C dates can overestimate the true age

of sediment deposition by up to 2,000 years (e.g. Snyder et al. 1994;

Barnekow et al. 1998). Such errors are likely to be inherent in early

PSV master-curves that relied on the

C dating of bulk sediments.

Saarinen (1998) has shown that calendar year dated PSV and relative

palaeointensity data can be obtained from the analysis of annually

laminated lake sediments in Finland (for the last 3,200 years). We

show here PSV and relative palaeointensity data obtained from an an-

nually laminated lake sediment sequence in northern Sweden, which

covers the time interval between 7,000 BC and 500 AD.

Lake Sarsjön is a small lake (area = ca. 10 ha) that lies at an altitude

of 167 m, approximately 40 km north-west of the city of Umeå in

northern Sweden. Due to isostatic land uplift the basin was isolated

from the Ancylus Lake (now the Baltic Sea), and the deposition of al-

ternating minerogenic and organic laminations on an annual basis has

been continuous since the isolation. Bioturbation is absent from these

anaerobic sediments and the laminations are preserved (see Petterson

1996). Counting of the varves (see Petterson et al. 1993 for method-

ological details) reveals that the isolation occurred at 7,000 ± 200

years BC. Two complete sequences were recovered from the deepest

point of the lake (7.3 m water depth) with a fixed piston corer. The

palaeomagnetic analyses (NRM, ARM and AF demagnetization) of

subsamples taken at 4 cm intervals from each core (with a 2 cm offset

between cores) were undertaken at the Geological Survey of Finland

with a 2G-Enterprises 755R magnetometer. Magnetic hysteresis anal-

ysis were carried out with a PMC MicroMag on contiguous 2 mm

thick samples that had been imbedded with epoxy resin (ca. 3,000 in-

dividual samples from overlapping sections).

AF demagnetization of the NRM and ARM, temperature depen-

dent magnetic behaviour and the magnetic hysteresis measurements

demonstrate that the NRM is carried by stable-single domain (SSD)

magnetite. A positive linear relationship exists between the concen-

tration of SSD magnetite (reflected by SIRM) and the organic car-

bon content of the sediments (r = 0.94). Biological production in the

lake during the summer is responsible for the deposition of organic

carbon. Therefore it is most likely that the SSD magnetite is pro-

duced extracelluraly by dissimilatory iron-reducing bacteria that

live at (or very near) the surface of the sediments and which take

part in the decomposition of organic material. The NRM is therefore

interpreted as a near surface PDRM. The AF cleaned NRM data and

the NRM is plotted against calendar years in Fig. 1. Data for central

Finland, from Saarinen (1998) is also shown.

Fig. 1

208

There is excellent agreement between the palaeomagnetic records

recovered from the two Lake Sarsjön cores (one core is shown for

clarity) and also between the Finnish data which extends back to

1,200 BC. In particular, the estimates of relative palaeointensity

(based on ARM standardization of the NRM) are comparable. This

agreement between the Swedish and Finnish data sets indicates that

annually laminated lake sediments do contain high resolution palae-

omagnetic records of direction and intensity which can be used for

(1) reconstructing the behaviour of Earths magnetic field and (2)

magnetostratigraphic studies, i.e. the indirect dating and correlation

of sediment sequences.

References

Barnekow L., Possnert G. & Sandgren P., 1998: AMS

C chronologies of

Holocene lake sediments in the Abiso area, northern Sweden a

comparison between bulk dated sediment and macrofossil samples.

GFF, 120, 5967.

Petterson G., 1996: Varved sediments in Sweden: a brief review. In: Kemp

A.E.S. (Ed.): Palaeoclimatology and Palaeoceanography from Lami-

nated Sediments. Geol. Soc. Spec. Publ., 116, 7377.

Petterson G., Renberg I., Geladi P., Lindberg A. & Lingren F., 1993: Spatial

uniformity of sediment accumulation in varved lake sediments in

northern Sweden. Journal of Palaeolimnology, 9, 195208.

Saarinen T., 1998: Paleomagnetic study of annually laminated sediments in

Lake Pohjajävri: paleosecular variation and paleointensity in Finland

during the last 3200 years. Physics of the Earth and Planetary interi-

ors. In press.

Snyder J.A., Miller G.H., Werner A., Jull A.J.T. & Stafford Jr., T.W., 1994:

AMS-radiocarbon dating of organic-poor lake sediment, an example

from Linnévatnet, Spitsbergen, Svalbad. The Holocene, 4, 413421.

Thompson R., 1983:

C dating and magnetostratigraphy. Radiocarbon,

25, 229238.

PALEOINTENSITY OF THE EARTHS

MAGNETIC FIELD MEASURED

ON SUBMARINE BASALTIC GLASSES

M.T. JUÁREZ*, L. TAUXE, J.S. GEE and T. PICK

*Paleomagnetic Laboratory, Fort Hoofddijk, University of Utrecht,

Budapestlaan 17, 3584 CD Utrecht, The Netherlands; juarez@geof.ruu.nl

Understanding the Earths magnetic field requires a complete

description of both paleointensity and paleodirectional data. In

contrast with the directional behaviour of the Earths magnetic

field during geological and historic times, which has been relatively

deeply investigated, the paleointensity data are still rather scarce,

inspite of the many efforts made in the last few decades. This is

mainly due to the difficulty of obtaining reliable paleointensity re-

sults, a difficulty that is inherent to the recording materials them-

selves. The best and simplest material found up to now for paleoin-

tensity experiments is Submarine Basaltic Glass (SBG). We focused

on SBG obtained from 30 sites sampled by the DSDP and distribut-

ed throughout the worlds oceans. The ages of the studied sites

range from the present day to the Jurassic. The quality of the data is

supported by parallel rock-magnetic experiments. The rock magnet-

ic investigation suggests that the carrier of the remanence is pre-

dominantly single-domain magnetite. The magnetic properties from

the studied glasses are very similar regardless of age or geographi-

cal location. The obtained results correspond to virtual axial dipole

moment values that range from 1.8

to 9.0

Therefore

suggesting an average dipole moment of approximately half of the

present day field. Finally, comparison of our results with the NRM

record of the sea-floor basalts suggests that changes in the natural

magnetization may also be influenced by the intensity of the dipole

moment and not only by mineralogical alteration with time.

III. Magnetic fabric, magnetic susceptibility anisotropy and paleotectonics

ROCKMAGNETIC INVESTIGATIONS

OF LATE QUATERNARY SEDIMENTS

FROM LAGO DI MEZZANO

(CENTRAL ITALY)

U. BRANDT*, N.R. NOWACZYK, A. RAMRATH

and J.W.F. NEGENDANK

GeoForschungszentrum, Telegrafenberg, Haus C,

14473 Potsdam, Germany; *brandt@gfz-potsdam.de

Two sediment cores recovered from Lago di Mezzano, a maar

lake in central Italy, reaching back to ca. 31,000 years, were subject-

ed to detailed rock magnetic investigations. The cores, LMZ-C and

LMZ-G, 27 and 17 m in length, were continuously subsampled into

plastic boxes (20

20 mm), providing an average time resolution

of about 25 years per sample. The investigations comprised mea-

surements of initial magnetic susceptibility, natural remanent mag-

netization (NRM), anhysteretic remanent magnetization (ARM) and

isothermal remanent magnetization (IRM). Intra-lake core correla-

tion is based on the data of continuous high-resolution susceptibility

measurements, which were carried out using a Bartington M.S.2.F

surface scanning sensor. The bulk susceptibility (

) of the palaeo-

magnetic samples of Cores LMZ-C and LMZ-G was measured with

a Kappabridge KLY-3S (AGICO Brno). Measurement of NRM di-

rections and intensity were performed with a fully automated 2G-

Enterprises 755 SRM (Superconducting Rock Magnetometer). Al-

ternating field (AF) demagnetization of all 1800 subsamples was

carried out in 10 steps of up to 100 mT using the integrated in-line

3-axis AF demagnetizer of the cryogenic-magnetometer. IRM was

imprinted with a 2G-Enterprises 660 pulse magnetizer, every 10th

sample of Core LMZ-C was exposed stepwise to peak fields up to

1.5 T. The saturation isothermal remanence (SIRM) was also deter-

mined for the remaining samples of this core.

All the recorded IRM acquisition curves reached saturation be-

tween 450 and 1000 mT, indicating that there is no significant con-

tribution from high-coercivity minerals like hematite or goethite in

the samples. In order to obtain information about the uniformity of

magnetic mineralogy and grain size the magnetic parameters ARM,

ARM

sus7

SIRM and

were plotted against each other (Creer & Mor-

ris 1996; King et al. 1982). In all plots, except SIRM versus K, the

data of samples belonging to the same lithological units (Ramrath et

al., submitted-1997) clustered in well-defined groups, the elongated

clusters showing different slopes to the origin. In addition, different

magnetic parameters like S-ratio and median destructive field

(MDF) of ARM were compared with the dry density record. Periods

of increased dry density, which are interpreted as periods of cooler

temperatures and dry conditions (Ramrath et al., submitted-1998)

often correspond to decreases in both magnetic parameters men-

tioned above, as well as to increases in ARM, ARM

sus7

SIRM and

209

These results indicate, that the changes in composition and rate of

sedimentary input in Lago di Mezzano, which are related to climatic

changes, are associated with changes in grain size and composition

of the carrier of the magnetic remanences. The associations are

more complicated than they appear to be so I would like to make

only a general statement in the abstract.

References

Creer K.M. & Morris A., 1996: Proxy-climate and geomagnetic palaeoin-

tensity records extending back to ca. 75,000 BP derived from sedi-

ments cored from Lago Grande Di Monticchio, Southern Italy. Quat.

Sci. Rev., 15, 167188.

King J., Banerjee S.K., Marvin J. & Özdemir Ö., 1982: A comparison of

different magnetic methods for determining the relative grain size of

magnetite in natural materials: some results from lake sediments.

Earth Planet. Sci. Lett., 59, 404419.

Ramrath A., Nowaczyk N.R. & Negendank J.F.W., (submitted, 1997): Sedi-

mentological evidence for environmental changes since 34,000 yrs

BP from Lago di Mezzano, central Italy. J. Palaeolim.

Ramrath A., Zolitschka B., Wulf S. & Negendank J.F.W. (submitted, 1998):

Late Pleistocene climatic variations as recorded in two Italian lakes

(Lago di Mezzano, Lago Grande di Monticchio). Quat. Sci. Rew.

ROCK MAGNETISM OF EOCENE MARINE

MARLS FROM THE JACA-PAMPLONA BASIN

(SOUTH CENTRAL PYRENEES,

NORTHERN SPAIN)

J.C. LARRASOAÑA

1-2*

, J.M. PARÉS

1-3

, E.L. PUEYO

1-2

H. MILLÁN

and J. DEL VALLE

Paleomagnetic Laboratory, Institute of Earth Sciences

Jaume Almera, CSIC, c/ Solé i Sabarís s/n, 28080 Barcelona, Spain;

*jclarra@ija.csic.es

Department of Earth Sciences, University of Zaragoza,

c/ Pedro Cervuna 12, 50009 Zaragoza, Spain

Department of Geological Sciences, University of Michigan, 1006 C. C.

Little Building, Ann Arbor, MI 48109, USA

We are conducting a magnetotectonic study on the Eocene marls

of the Pamplona-Arguis Formation, which outcrops along the Jaca-

Pamplona Basin (Southern Pyrenees, Northern Spain). It is made up

of a monotonous sequence (up to 2000 m thick) of bluish marine

marls, deposited during middle-upper Eocene in the distal part of a

deltaic system with ESE-WNW polarity (Puigdefábregas 1975). Pre-

vious magnetostratigraphic and magnetotectonic works have pro-

vided the geochronological and geodynamic constraints of the basin

(Hogan 1993; Hogan & Burbank 1996; Pueyo et al. 1997; Larra-

soaña et al. 1997; among others). We have carried out a rock mag-

netic study on these rocks because their magnetic mineralogy has

still not been well established and up to now little is known about

the origin of the natural remanence.

Specimens of these rocks are weakly magnetized (NRM values

about 10

A/m) and two components are normally revealed by step-

wise thermal demagnetization. At high temperatures the samples

show very weak magnetization and become thermally unstable,

sometimes making the interpretation of paleomagnetic data diffi-

cult. The low temperature component unblocks at temperatures up

to 250350

C and shows maximum clustering prior to tectonic cor-

rection. The high temperature component unblocks between 320

460

C, shows both normal and reverse polarity and, in accordance

with the fold test, is a pre-folding magnetization.

IRM acquisition curves reach almost complete saturation at less

than 300 mT, suggesting the predominance of low-coercivity miner-

als such as (Ti) magnetite and/or ferrimagnetic iron sulfides. Low-

temperature measurements of Jrs evidenced the presence of magne-

tite and pyrrhotite. There is also evidence of ultrafine grained mag-

netite in or close to the SP range, in agreement with the short-term

viscous behaviour of IRM. Thermal demagnetization of a composite

IRM shows a decay in remanence at 340

and 580

in the three com-

ponents (up to 50 mT; 50300 mT and 0.31.5 T) in accordance

with the presence of magnetite and pyrrhotite with a wide grain-size

distribution. There is no evidence of goethite or hematite. Due to the

very low concentration of ferromagnetic minerals in the samples

their magnetic hysteresis loops are mainly controlled by the para-

magnetic matrix. After substraction of the paramagnetic contribu-

tion, a weak ferromagnetic signal is revealed. Any distorted hystere-

sis loop has been observed despite the rock magnetic evidence of

pyrrhotite and ultrafine-grained magnetite. This is somewhat sur-

prising as both components can contribute to distorted loops. Taking

into account that the total magnetization of the samples is the sum of

the weighted contribution of each component, pyrrhotite and SP

magnetite are not likely to be present in large amounts on the stud-

ied rocks, which is consistent with other rock magnetic parameters

such as ARM/IRM ratios. Low-T measurements of magnetic suscep-

tibility and thermomagnetic runs have not provided any conclusive

results due to the very low ferromagnetic content and the new for-

mation of magnetic phases during heating.

We interprete the low temperature component as a recently ac-

quired viscous overprint carried by the finer magnetite fraction rath-

er than pyrrhotite. Ultrafine-grained magnetite, rather than iron sul-

fides, is likely to form due to weathering in the near surface

environment. The higher temperature component is considered as a

primary Eocene magnetization carried by fine-grained magnetite,

and probably in some cases, pyrrhotite. SEM observations show the

presence of framboids of primary iron sulfides but not clear evi-

dence of magnetite. Aditional SEM and TEM analyses are being

conducted to invesigate their possible genetic relation and thus to

establish the origin of the primary magnetization.

References

Hogan P.J., 1993: Geochronologic, tectonic and stratigraphic evolution of

the South-west Pyrenean foreland basin, Northern Spain. Ph. D. The-

sis, Univ. Southern California, 1219.

Hogan P.J. & Burbank D.W., 1996: Evolution of the Jaca piggyback basin

and emergence of the External Sierra, southern Pyrenees. In: Friend

P.F. & Dabrio C.J. (Eds): Tertiary basins of Spain. Cambridge Univ.

Press, 153160.

Larrasoaña J.C., Pueyo E.L., Del Valle J., Millán H., Parés J.M., Pocoví A. &

Dinares J., 1996: Datos magnetotectónicos del Eoceno de la cuenca de

Jaca-Pamplona: resultados preliminares. Geogaceta, 20, 5, 10581061.

Pueyo E.L., Millán H., Pocoví A. & Parés J.M., 1996: Correcciones

geométricas en magnetotectónica: filtrado de rotaciones aparentes debi-

das a pliegues. Geogaceta, 20, 5, 10541057.

Puigdefábregas C., 1975: La sedimentación molásica de la cuenca de Jaca,

Pirineos. 104, 1188.

ROCK MAGNETISM

AND PALEOMAGNETISM

OF PORCELANITES/CLINKERS

FROM THE WESTERN DACIC BASIN

(ROMANIA)

S. C. RÃDAN and M. RÃDAN

Geological Institute of Romania, 1, Caransebes Str.,

79678 Bucharest, Romania; radan@igr.ro

210

1. Introduction

Porcelanites and/or clinkers baked and/or sedimentary rocks

generated by natural spontaneous burning of coal seams (see also

the Dictionary of geological terms, Anchor Press, 1976) are

exposed at different localities in the western Dacic Basin within the

Upper Pliocene lignite-clay sequences.

Firstly, the existence of such deposits in the area was detected by

one of the authors (S.C.R.), in 1969, on the occasion of creating the

geomagnetic field vertical component map over the Romanian terri-

tory in 1:200,000 scale. Importance of the enhanced magnetic prop-

erties due to porcelanites, which represent modified mineral assem-

blages related to the original clays, was analysed in the context of

the regional magnetic mapping interpretation (Roºca et al. 1973).

Since 1987, when magnetostratigraphic investigations of lignite-

clay sequences in the Motru-Jiu area started, a rock magneticpa-

leomagnetic approach of the heated rocks in coal quarries has been

developed. The mineralogy and geochemistry of porcelanites and

clinkers have also been studied, and magnetic anomalies produced

by sediments with magnetic properties of enhanced during post-

depositional thermal perturbation have been measured. The applied

methods and some results were reported in abstracts and research

notes (Rãdan & Rãdan 1989, 1990, 1991, 1993, 1996; Rãdan et al.

1989, 1990, 1992, 1993, 1994a, 1994b, 1995, 1996a, 1996b).

2. Methods and instrumentation

Oriented samples of porcelanites, clinkers and porcelanite-like

clays were collected from two lignite quarries (Lupoaia and Jilt Sud)

and some unoriented core fragments were taken from two explora-

tion boreholes (in Lupoaia-Motru area). Moreover, several oriented

monolith-blocks (of about 25 cm thickness) of porcelanites and por-

celanite-like clays were sampled and cut in 25 oriented subsamples

and, afterwards, to cubic specimens. Magnetic susceptibility (MS)

and the anisotropy of magnetic susceptibility (AMS) were measured

using a KLY-2 Kappabridge, and (total) natural remanent magnetisa-

tion (NRM) by a JR-4 spinner-magnetometer. The Koenigsberger ra-

tio (Q = In/kH) was determined, too.

To determine the characteristic remanent magnetisation (ChRM),

a stepwise thermal demagnetisation (STD) was applied by means of

a TSD-1 Schonstedt demagnetiser. MS was monitored during this

process and, in a few cases, the behaviour of the AMS during mag-

netic cleaning was investigated (see Fig. 1b

A Geometrics G-826 magnetometer was used to examine nine

magnetic profiles in the Lupoaia-Motru area.

3. Results and discussion

3.1. Rock magnetism
Mineral transformations induced by heating during the natural

burning of coal beds prove that the forming temperature of porcela-

nite-like clays, porcelanites and clinkers has usually reached 250

400

C (no more than 800850

C); in some cases temperatures of

1000

C have been exceeded, and sometimes 11001200

C was

reached (Rãdan et al. 1994a).

Consequently, the detrital remanent magnetisation (DRM) ac-

quired by the original clays with low and very low NRM intesities

(often less than 1mA/m, seldomly higher than 30mA/m) has chaged,

and the thermoremanent feature magnetisation (TRM) produced in

the heat-affected rocks is a new and important feature. The NRM

structure could include TRMs (PTRM) and thermochemical rema-

nent magnetisations (TCRM) as well (Jones et al. 1984).

Anyway, total NRM measured on porcelanites and porcelanite-

like clays show high values, usually ranging between 1000

7000 mA/m. The magnetic polarity of total NRM is normal (close

of the present geomagnetic field direction), while the original clays

(thermally non-affected) from the same stratigraphic level (e.g., the

coal seam X; Fig. 1a) usually have reversed NRM (see an example

in Fig. 1c

As regards the anisotropy of magnetic susceptibility (AMS) of

porcelanites, magnetic fabric is defined by characteristics similar to

those of fresh clays (a (primary) depositional/sedimentary fabric):

near vertical minimum susceptibility axes, and near horizontal

maximum and intermediate susceptibility axes (belding plane) (see

also the example in Fig. 1b

; a slight deviation in the porcelanite

case could be noticed). The results can be compared with the data of

Fig. 1. Rock magnetic and palaeomagnetic characteristics for porcelanites and original clays (examples from Jilt Sud quarry, western Dacic Basin).
a) clay, coal. Note: The sampling sites (not shown in the figure) for porcelanites (JL 70-JL 73) are at the level of the coal seam X; b) s maxi-

mum susceptibility; n intermediate susceptibility; l minimum susceptibility; c) l normal polarity;

reverse polarity.

_ _

::::

211

Perarnau & Tarling (1985) on thermal enhancement of magnetic

fabric; they concluded that irrespective of the mineralogical mod-

els, it is clear that the observed fabric can only be a result of mim-

icking of fabrics that must already exist within the rocks.

The enhancement of anisotropy parameters determined for por-

celanites is clearly expresses, especially by magnetic foliation (F)

and the anisotropy degree (P); the values range between 1.101.20,

reaching even 1.301.40, while for the original clays the most fre-

quent F and P values are situated within the interval 1.031.05.

Certainly, the mean magnetic susceptibility measured on porcela-

nite-like clays and porcelanites is stronghly enhanced, the MS val-

ues usually being 10100 times higher than the MS of the original

(thermally non-affected) clays.

Increased magnetic properties (NRM and MS) of porcelanites

clearly results in strong magnetic anomalies (up to 1880 nT) pro-

duced by the respective deposits (the depth, as shown by three bore-

holes, did not exceed 35 m). The results obtained in the western

Dacic Basin are compared with (aero) magnetic data for clinker de-

posits from USA and New Zealand (Hasbrouck & Hadsell; Gay &

Hawley 1991; Lindqvist et al. 1985).

3.2. Paleomagnetism
During thermal demagnetisation, the behaviour of RM and MS

for porcelanites was significantly different from that of the origi-

nal clays (Fig. 1b

, c

The thermal demagnetisation diagrams show the predominance

of thermally distributed components of magnetisation (according

to the terminology of Irving & Opdyke 1965), suggesting some

mineralogical significances as well.

The MS monitoring during STD usually did not reveal ther-

mal alterations as observed in many cases for the original clays,

especially when they are rich in organic/vegetal material.

Stability of the remanent magnetisation direction, generally up

to 500

C, could give some indications on the temperatures that

were reached during the natural baking of clays (see also,

Lindqvist et al. 1985); a correlation with the temperatures shown

by XRD was established.

The ChRM directions, mostly close to the present geomagnetic

field direction, supported the calibration of the Brunhes chron

(0.78 Ma; GPTS, Cande & Kent 1995). The autocombustion pro-

cess of the lignite seam which produced the porcelanites could

have been during the Middle-Upper Pleistocene.

Some comments regarding the normal ChRM of porcelanites

related to the Jaramillo subchron (0.990.07 Ma, GPTS-CK95) or

to the Cobb Mountain subchron (1.2011.211 Ma, GPTS-CK95)

are done, but without arguments for correlation in the case of short

lithostratigraphic segment sampled in the two quarries.

4. Concluding remarks

The presence of porcelanites within the lignite-clay sequences

in the western Dacic Basin could represent taking into account

their magnetochronologic characteristics a noise for the magne-

tostratigraphic scales elaborated in the area (e.g. Rãdan & Rãdan

19951997).

On the other hand, the rock magnetic and paleomagnetic results

obtained for porcelanites and/or clinkers which are pointed out in

the paper demonstrate a clear quality of signal with an interesting

geophysical content and various geological significances.

Besides the obtained data, the porcelanites and/or clinkers may

provide a high resolution record of geomagnetic field (directional

variability and paleointensity) in Quaternary.

References (selection)

Gay S., Parker Jr. & Bronson W.H., 1991: Geophysics, 56, 902913.

Jones A.H., Geissman J.W. & Coates D.A., 1984: Geophys. Res. Lett., 11, 12,

12311234.

Krs M., 1968: Pure Appl. Geophys. (PAGEOPH), 69, 158-167.

Lindqvist J.K., Hatherton T. & Mumme T.C., 1985: New Zeal J. Geol. Geo-

phys., 28, 405412.

Perarnau A. & Tarling D.H., 1985: J. Geol. Soc. London, 142, 10291034.

Rãdan S., Rãdan S.C., Rãdan M. & Vanghelie I., 1994a: Int. Miner. Assoc.,

16th Gen. Meet. Pisa, Italy, Abstracts, 344345.

Rãdan S.C., Rãdan S., Rãdan M. & Vanghelie I., 1995: Rom. J. Miner., 77,

Suppl. 1, 39.

Rãdan S.C., Rãdan S., Rãdan M., Andreescu I. & Vanghelie I., 1996: An.

Inst. Geol. Rom., 69/I, 324331.

Rãdan M. & Rãdan M., 1997: An. Inst. Geol. Rom., 70 (in print).

Rosca Vl., Rãdan S.C. & Rãdan S., 1973: St. Cerc. Geol. Geofiz. Geogr., Se-

ria Geofiz., 11, 2, 303313 (in Romanian, English summary).

Tarling D.H. & Hrouda F., 1994: The magnetic anisotropy of rocks. Chap-

man & Hall, 1217 (Preprint). as determined, too.

THE MAGNETIC PROPERTIES

OF DOLOMITES WITH DIFFERENT

ORIGINS FROM THE ESTONIAN EARLY

PALEOZOIC SEDIMENTARY BASIN

A. SHOGENOVA

Institute of Geology, Tallinn Technical University, 7 Estonia Ave,

0001 Tallin, Estonia; alla@gi.ee

Dolomites with different origins in the Estonian Early Paleozoic

shallow sedimentary basin were sampled and analysed by compari-

son with neighbouring limestones. All Estonian carbonate rocks in-

clude varying amounts of clay impurities and are characterized by

low-field magnetic susceptibility. This usually increases in sedimen-

tary rocks with increasing clay content (Shön 1996). Magnetic prop-

erties of rock samples were studied together with their bulk chemi-

cal composition and iron forms (FeO, Fe

). This permitted

interpretation of differences in the processes leading to changes in

magnetic properties.

Dolomite groups with five different ages and geneses were stud-

ied and compared: 1) samples from the dolomite layer of Pae

Member of Väo Formation, Middle Ordovician (thickness up to 1.5

m); 2) Lower Ordovician widespread dolomite layer with glauco-

nite impurities of Volkhov stage (thickness of stage 020 m); 3)

late diagenetic dolomites associated with fracture zone in Middle

Ordovician (Shogenova & Puura 1997); 4) Upper Ordovician; 5)

Silurian (rocks of both last groups are highly argillaceous and do-

lomites are often cavernous).

During dolomitization the chemical composition of carbonate

rocks was changed. Substitution of Ca

by Mg

was often accom-

panied by alteration in iron and manganese contents. Increases in to-

tal iron content, causing increasing magnetic susceptibility were

identified in all the Estonian dolomites. The largest increase in mag-

netic properties was observed in the widespread dolomite layer of

the Väo Formation, which is hard and relatively free from clay im-

purities. Magnetic susceptibility increased there from (04)

in the limestones to (1722)

SI in the dolomites. It is explained

by (00.9)% of Fe

total measured in the limestones and its in-

creasing to (2.52.8)% in the dolomites. Therefore two distinctive

groups of limestones and dolomites can be clearly observed in the

plot of magnetic susceptibility versus bulk density (Fig. 1a). The

highest values of magnetic susceptibility were determined in the

Lower Ordovician widespread dolomites including different

amounts of clay and glauconite impurities. The magnetic suscepti-

bility increased in these rocks from (216)

SI in limestones to

(846)

SI in the dolomites (Fig. 1b). Magnetic susceptibility of

the dolomites formed in fracture zone was also higher than in the

212

limestones. It increased from (012)

SI to (423)

SI in

the dolomites of different Middle Ordovician formations. Some

rocks were very porous and therefore had low bulk densities (Fig.

1c). The total iron content in the dolomites of the fracture zone var-

ied in the limits of (12.5)% and in the argillaceous in different de-

gree limestones it changed from 0.3 to 1.9 %.

Increases in the magnetic susceptibility of Silurian and Upper Or-

dovician dolomites were not so clearly observed as in the rocks de-

scribed above. These rocks were formed in shallower environments

than the underlying Lower and Middle Ordovician rocks and they

are relatively more argillaceous (Jhrgenson 1988). We could not ob-

serve clearly distinguished groups of rocks in the plots of magnetic

susceptibility versus bulk density for these rocks (Fig.1d, 1e). In or-

der to reveal alterations in iron content only rocks of similar clay

content should be compared. Doing this, we noticed that in Upper

Ordovician and Silurian rocks with clay contents less than 8 % the

total iron content increases from 0.20.5 % in limestones to 0.51.2

% in dolomites, that is to say it more than doubles. This may the

magnetic susceptibility to double as well. In Silurian rocks with clay

contents of 1018 %, the total iron increased from 0.31.0 % in

limestones to 0.81.2 % in dolomites with simultaneous increases

of susceptibility from (04)

SI to (27)

SI. This was

caused by the increase in FeO. Generally the Upper Ordovician and

Silurian strongly argillaceous rocks are characterized by higher

magnetic susceptibilities compared to dolomites.

Remagnetization of Estonian rocks during dolomitization may

be caused by different iron forms and may be explained by the fol-

lowing processes. Fe

may substitute the Mg

in the crystalline

lattice of the late diagenetic dolomites together with Mn

(in the

fracture zone and dolomite layer of the Väo Formation both in

the Middle Ordovician). Fe

may occur in the form of hematite

in the carbonate matrix of dolomites or may be included as glauco-

nite impurities (Volkhov stage of the Lower Ordovician).

References

Johrgenson E., 1988: Deposition of the Silurian beds in the Baltic. Valgus,

Tallin, 1175 (in Russian with English summary).

Schon J.H., 1996: Physical properties of rocks: fundamentals and principles

of petrophysics. Handbook of geophysical exploration. Section I, Seis-

mic exploration. Pergamon Press, V. 18, 1583.

Fig. 1. Magnetic susceptibility vs. bulk density of limestones and dolo-

mites from: a Väo Formation of Middle Ordovician, b Lower Ordovi-

cian, c fracture zone in Middle Ordovician, d Upper Ordovician, e

Silurian.

Bulk density (g/ccm)

Magnetic susceptibility (*10

-5

SI)

2.6

2.65

2.7

2.75

2.8

2.85

Limestones

Dolomite layer

Väo Formation

MIDDLE ORDOVICIAN

Bulk density (g/ccm)

Magnetic susceptibility (*10

-5

SI)

-5

2.1

2.32.5

2.7

2.9

Limestones

Dolomite layer

FRACTURE ZONE

Bulk density (g/ccm)

Magnetic susceptibility (*10

-5

SI)

2.5

2.6

2.7

2.8

2.9

Limestones

Dolomite

LOWER ORDOVICIAN

Bulk density (g/ccm)

agnetic susceptibility (*10

-5

SI)

-2

2.4

2.5

2.6

2.7

2.8

2.9

Limestones

Dolomites

UPPER ORDOVICIAN

Bulk density (g/ccm)

agnetic susceptibility (*10

-5

SI)

-2

2.2

2.32.4

2.5

2.6

2.7

2.8

Limestones

Dolomites

SILURIAN

213

MAGNETIC FABRIC AROUND

FRACTURES IN THE BOHUS GRANITE,

SOUTHWESTERN SWEDEN

BO A. SJÖBERG

and A. KAPIÈKA

Swedish Museum of Natural History, Box 50007,

S-104 05 Stockholm, Sweden; Ritva.Woode@nrm.se

Geophysical Institute, Acad. Sci., Boèní II/1401, Prague, Czech

Republic; kapicka@ig.cas.cz

The Bohus granite of the west coast of Sweden intruded into older

supracrustal rocks about 920 Ma ago (Eliasson & Schöberg 1991). It

occurs as gray, grayish red and red varieties (Asklund 1947). The

rocks are divided by Asklund into different grain size classes, from

one mm to several mm. Our investigation of minor fractures has

been restricted to a grayish red variety with grains of medium size

(27 mm).

Profiles across a few fractures were selected for a study of the

magnetic properties and their modifications and dependence upon

mechanical and thermal modifications. Samples were taken from

cores drilled at systematically selected distances from minor frac-

tures, a few metres or tens of metres long. The magnetic properties

determined from the samples were: the mean magnetic susceptibili-

ty, its degree of anisotropy, the shape factor and its principal axes.

Hysteresis ratios of samples taken along one profile were deter-

mined in order to identify the magnetic carriers.

Rocks cut by fractures show relatively small differences in the

values of the magnetic susceptibility and degree of anisotropy on

both sides of the fractures. The mean principal axes of the magnetic

susceptibility of individual blocks may show minor clockwise and

counterclockwise deviations from the mean value.

On the other hand, rocks cut by curving fractures show a wider

range of variation of the magnetic susceptibility. In the vicinity of

some curving fractures, only slight changes of principal axes of

the magnetic susceptibility have been observed and all other prop-

erties remain constant. In other cases of both curving and straight

fractures, the mean magnetic susceptibility and the degree of

anisotropy increase systematically with increasing distance (on a

decimetre scale) from the fracture; the coercivity increases with

decreasing distance, indicating that magnetic mineral changes

have taken place. The changes in the magnetic fabric close to frac-

tures of this type may include a metamorphic factor.

References

Asklund B., 1947: Svenska stenindustriomr den. Gatsten och kantsten I-II.

Sveriges Geologiska Undersökning, C 479, 1187.

Eliasson & Schöberg H., 1991: U-Pb dating of the post-kinematic Sveconor-

wegian (Grenvillian) Bohus granite, SW Sweden: evidence of restitic

zircon. Precambrian Research, 51, 337350.

MODEL INTERPRETATION

OF PALEOTECTONIC ROTATIONS

IN THE ALPINE AND VARISCAN

COLLISION ZONES

M. KRS, O. MAN and P. PRUNER

Institute of Geology, Academy of Sciences of the Czech Republic,

Rozvojová 135, Prague 6 - Lysolaje, Czech Republic; housa@gli.cas.cz

Paleomagnetic data from the Permian to the Neogene have been

evaluated for the Western Carpathians (Krs et al. 1996). One of the

first papers on this region suggested the presence of paleotectonic

rotations of varying magnitudes and in different senses (Kotásek

& Krs 1965). Subsequent studies resulted in explanation of the ob-

served paleomagnetic declinations in terms of rotational deforma-

tion about vertical axes of nappes and blocks during the Alpine

folding. Next syntheses of paleomagnetic data carried out by dif-

ferent authors resulted in documentation of numerous paleotecton-

ic rotations in different parts of the Alpine-Carpathian-Pannonian

Zone as well as in other regions of the Alpine tectonic belt. The

paleomagnetic data from the Western Carpathians indicate

marked, mostly counterclockwise paleotectonic rotations of larger

rock complexes, nappes and nappe systems yielding a characteris-

tic distribution of pole positions. Recognition of the separation of

components of tectonic rotation may contribute to a better under-

standing of the paleotectonic and dynamic evolution of rock for-

mations affected by Alpine folding. The scatter of pole positions is

explained by means of a model in which the movements are parti-

tioned into two components. The first component is due to rotation

about a distant rotation pole and relates to the rotation of the litho-

spheric plate to which the unit shows paleogeographic affinity.

The second component is due to rotation about a proximal pole of

rotation and relates to rotation during the Alpine collision of

smaller-scale units (blocks, nappes). European paleomagnetic

data from the Devonian to the Triassic accumulated over the last

thirty years for the regions north of the Alpine tectonic belt and

west of the Ural Mts. up to Great Britain were statistically evaluat-

ed, a number of results concerning paleogeography and the paleo-

tectonic deformation of rock complexes from the Hercynian oro-

genic belt were obtained. The Trans-European Suture Zone (TESZ)

played a significant role in the distribution of paleomagnetic pole

positions. The homogeneous grouping of E. Permian paleomagnet-

ic pole positions for all the territories north of the Alpine tectonic

belt is due to consolidation of the European lithospheric plate

without major paleotectonic deformations of its segments during

later geological history. The TESZ represents a plate boundary be-

tween the East-European Platform (cratonic Europe) in the NE,

and the Hercynian mobile belt on the SW. The Variscan and pre-

Variscan formations from the Hercynian belt show different de-

grees of paleotectonic rotations. The changes in paleomagnetic

pole positions due to continental drift of the European plate are

small in comparison to the changes in pole positions due to paleo-

tectonic rotations. The paleotectonic rotations recognized both in

the Alpine and Hercynian tectonic belts are evidently typical fea-

tures of collision zones, as may be shown on models simulating

translation and rotation movements.

References

Kotásek J. & Krs M., 1965: Palaeomagnetic study of tectonic rotation in

the Carpathian Mountains of Czechoslovakia. Palaeogeography,

Palaeoclimatology, Palaeoecology, 1, 3944.

Krs M., Krsová M. & Pruner P., 1996: Palaeomagnetism and palaeogeogra-

phy of the Western Carpathians from the Permian to the Neogene. In:

Morris A. & Tarling D.H. (Eds.): Palaeomagnetism and Tectonics of

the Mediterranean Region. Geol. Soc. Spec. Publ. (London), 105,

175184.

214

TECTONIC AND STRUCTURAL

IMPLICATIONS OF PALEOMAGNETIC

AND AMS STUDY OF PALEOZOIC ROCKS

IN THE GEMERIC SUPERUNIT, SLOVAKIA

(PRELIMINARY RESULTS)

J.KRUCZYK

, M. K¥DZIA£KO-HOFMOKL

M. JELEÑSKA

, I. TÚNYI

and D. NÁVESÒÁK

Institute of Geophysics, Polish Academy of Sciences, Ks. Janusza 64,

01-452 Warsaw, Poland

Institute of Geophysics, SlovakAcademy of Sciences,

Dúbravská cesta 9, 842 28 Bratislava, Slovak Republic

Geocomplex a.s., Geological.Div., Koice Werferova 1, Slovak Republic

The Gemeric Superunit (GEM) is cut by two systems of principal

shear zones that strongly affected its geological setting: the West

system (WS) striking SW-NE that has the character of a sinistral

strike-slip fault and the East system (ES) that has the character of a

dextral strike-slip fault. Both systems cut the Gemeric Superunit

into a mosaic of blocks. The shear zones are dated Lower Creta-

ceousMiddle Miocene (Grecula et al. 1990). The metamorphosed

Paleozoic rocks for this study were sampled within particular blocks

in the following areas: along the most intensive Koice-Margecany

shear zone (KMSZES system), forming the E border of the GEM

(4 exposures KV, CR, J, MA), in the vicinity of the Dobina shear

zone (DSZWS system) in the western part of the GEM close to its

N border (two exposures DO, ML) and one exposure (GP) lying in

the middle part of the GEM. The rocks represent strongly metamor-

phosed and mylonitized sericitic schists (CR, J), cataclasites (MA,

KV), phyllites and schists rich in carbonates (ML, DO, GP). Micro-

scopic, thermomagnetic and hysteresis study revealed the presence

of magnetite sometimes partly altered to martite, hematite automor-

phic grains, tablets, smears, Fe-hydroxides. Magnetite and Fe-hy-

droxides are often of post-pyrite origin. Amount and distribution of

magnetic minerals in the studied rocks inhomogenous bulk sus-

ceptibility and intensity of NRM differ even between specimens

from the same exposure see Table 1.

best fit was obtained between the reference Middle-Upper Jurassic

direction and the following results: in situ data of KV, MA and J;

data obtained for 50% unfolding of ML and DO; data obtained af-

ter full tectonic correction for GP, after individual CW rotation of

each exposure (block) Table 2.

Geographic position of GEM : 20.5E, 48.5N; paleomagnetic reference

direction: D=33, I=61

shear z. /exp

D/I in situ

D/I

50% unfold.

D/I atc

CCW

rotation

KMSZES

305/50

275/65

197/-61

120

DSZWS

300/62

359/59

no shear zone

330/59

shear z/exp shear zone/exposure, D/I declination/inclination,

50% unfold tectonic tilt unfolded in 50%, atc full tectonic correction.

Table 2: Directions of ChRM in appropriate coordination frames, and an-

gles of CCW rotations.

exposure

Kmx10

-4

SI 32 - 220 40 - 80 2

64 - 700 4 - 7

13 - 90 2 - 6

NRM mA/m 14 - 250 2 - 3.5

0.1 - 0.6 27 - 300 0.2 - 3 0.8 - 40 0.1 - 0.4

1.04-1.21 1.08-1.17 1.03-1.10 1.10-1.28 1.06-1.13 1.03-1.06 1.03-1.10

Table 1: Ranges of bulk susceptibility Km, intensity of NRM and anisot-

ropy parameter P.

AMS study shows that the values of anisotropy parameter P re-

main in a broad range (Table 1) and that in the majority of speci-

mens lineation prevails over foliation. The latter is similar to the

mylonitic foliation observed by Návesòák (1993) in the respective

areas: foliation S2 connected with the Transgemeric Shear Zone

(WS system) observed in DO, S3 connected with the Lacember-

Starovod shear zone (WS system) observed in ML and S4 connect-

ed with KMSZ (ES system) observed in KV, CR, J and MA. These

results suggest direct relations between shear zones and anisotropy

of susceptibility of the studied rocks.

Standard paleomagnetic procedures led to isolation of secondary

characteristic components of NRM (ChRM) for six exposures (ex-

posure CR did not give any interpretable results). The directions of

ChRMs revealed some similarity in inclinations with very different

declinations. In order to interpret the results we compared the direc-

tions in situ and after tectonic correction and also after partial cor-

rection for tectonics with the expected data for Slovakia obtained

after the reference Eurasian data of Besse & Courtillot (1991). The

The presented results suggest, that all the rocks became remagne-

tized during the Jurassic . One of them (GP) carries pre-tectonic

remanence, two (DO and ML) syntectonic remanence and three

(KV, MA and J) post tectonic remanence. This implies that fold-

ing processes were extinguished first at the eastern side of the Ge-

meric Superunit and prolonged in its inner parts. The stated various

rotations remain in agreement with the fact, that the Gemeric Supe-

runit is cut by systems of shear zones into a mosaic of blocks which

behaved (rotated) independently. Our results suggest, that studied

units rotated in the same, CCW direction.

References

Besse & Courtillot, 1991: J. Geophys. Res., 96, B3, 40294050.

Grecula P. et al., 1990: Miner. slovaca, 22, 97110.

Návesòák D., 1993: Miner. slovaca, 25, 263273.

PALAEOMAGNETIC AND AMS EVIDENCE

FOR A PROGRESSIVE ROTATION

OF GROIX ISLAND (BRITTANY, FRANCE):

PRELIMINARY RESULTS

J.P. LEFORT

, T. AÏFA

, M. JELEÑSKA

M. K¥DZIA£KO-HOFMOKL

and C. AUDREN

Géosciences-Rennes, CNRS UPR4661, Université de Rennes 1,

Bt 15, Campus de Beaulieu, 35042 Rennes Cedex, France;

*aifa@univ-rennes1.fr

Institute of Geophysics, Polish Academy of Sciences,

Ks. Janusza 64, 01-452 Warsaw, Poland

Groix Island, located south of Brittany (France), is famous for its

blueschist outcrops (Carpenter 1976). It is mainly formed of garnet,

quartzitic micaschists, albitic micaschists, quartzites, glaucophan-

ites, prasinites and few serpentinites. The sediments represent, how-

ever, 90 % of the total outcrop. The blueschist formation, which de-

veloped in high pressure/low temperature conditions has been dated

between 375 and 420 Ma (Peucat & Cogné 1977).

215

The first deformation (D1) which developed in a flat-lying shear

zone, produced sheathed folds. It is sometimes considered that the

shearing event resulted from movements oriented from the South-

east to the Northwest (Cannat 1983). This deformation was first

considered as contemporaneous with the blueschist metamorphism

(Triboulet 1977, 1991), but we now know that the sheathfolds

(Quinquis 1980) developed in a continuous deformation regime

which started in the high pressure, continued in the amphibolite fa-

cies and ended in the greenschist facies (Audren & Triboulet 1993),

well before 320 Ma (Djro et al. 1989). Folds oriented NWSE are

the result of a D2 deformation phase, while the minor D3 deforma-

tion was responsible for an East-West oriented fold axis. The D2 and

D3 deformations are considered to be charaterized by a low green-

schist facies metamorphism. The most representative structure of

the island, is the D2, N300

oriented anticline, which was modeled

at depth by gravity (Lefort & Vigneresse 1991) and magnetics (Aud-

rain & Lefort 1986).

Because the size of the island is small, the full attitude of the geo-

logical structures can only be understood if one incorporates the off-

shore data (Lefort et al. 1982). These data show that the D2 struc-

tures of Groix can be correlated with the D2 deformation of

Belle-Ile Island located 35 km in the South-East. The en-echelon

fold axis which links the two islands exhibits an inverted s shape.

The study of the gravity, magnetic and seismological (Delhaye

1976) data shows the existence of a 300 km long but narrow disrup-

tion, running parallel to the South Brittany shoreline and located 40

km South of Groix Island. This geophysical disruption, character-

ized by well expressed gravity and magnetic highs, is interpreted as

a mafic belt (Poulpiquet & Lefort 1989). It is thought to be a pre-

Variscan suture zone and has been called the South Armorican Su-

ture (SAS) (Lefort 1979). The calculation of the first derivative of

the magnetic data associated with the deep mafic body, and the re-

duction to the actual pole, show the existence of a strong unevenness of

the magnetic contours, just south of Groix Island. The study of the grav-

ity anomaly associated with the offshore extension of Pont-LAbbé

leucogranites, shows that this granite is characterized by a general cur-

vature which mimics in an inverse position the Groix-Belle-ile D2 S

shape. The Pont-LAbbé Granite curvature and the Groix-Belle-Ile D2

S shape follow the SAS unevenness and are broadly symmetrical (Si-

buet 1972). Since Pont-LAbbé granite has been dated at 300 Ma (Ber-

nard-Griffiths et al. 1985) and because of the apparent continuity be-

tween the two structures, it can be suggested that the

Pont-LAbbé-Groix-Belle-Ile micaschist and granitic formations were

bent during the same period of time. It has been suggested that this

structural arch resulted from an impingement of the suture.

Paleomagnetic and magnetic studies have been undertaken on the

L1 and D2 structures of Groix in order to better understand the di-

rection changes recorded at Groix and between Groix and Belle-Ile

islands. The rock magnetic experiments show the occurrence of a

weak coercivity mineral, probably of magnetite type, often associat-

ed with sulphides of pyrrhotite type. In some specimens, mineralog-

ical changes controled by susceptibility increase, may occur before

450

C. This is corroborated by the isothermal remanent magnetiza-

tion curves and by their thermal demagnetization. The natural rema-

nent magnetization, measured with a cryogenic magnetometer,

shows an unimodal distribution of the intensities around 3

m. Two components of magnetization have been isolated. One be-

fore 300

C, is close to the present dipole field, the other up to 620

C, is not far from the Hercynian directions already published for

Armorica (Van der Voo 1993). The fold test is not significant but the

great circle analysis gives a consistant direction of magnetization

which can be attributed to a Late Hercynian event. These results

show that the North-West linear direction of the D2 axis known

West of Groix Island and which begins to veer towards a North-

South direction in the Southeasternmost part of the island (Pointe

des Chats) results from a clockwise rotation.

Although there are only 8 calculated rotation sites, we can clearly

see that the amount of rotation is not constant all over the island.

Broadly speaking, East of the D2 axis, the measured rotations tend

to be smaller as one moves away from the anticlinal. This is not the

case when one moves towards the West. For a better understanding

of the phenomenon, these results have been compared with the L1

lineations and with the AMS measurements.

Comparisons between the maximum of the magnetic anisotropy

(Kmax) and the L1 lineations show good similarities between the

structural and magnetic directions of lineation. This result suggests

that the L1 lineations are the maximum anisotropy markers.

If one incorporates the L1 geological lineations measured in the

field, the Kmax magnetic markers and the results of the paleomag-

netic rotations, it is possible to divide the island into three zones: 1

Zone II, corresponds with a zone where the L1 lineations and

Kmax directions are almost superimposed and parallel with the D2

axis. In this zone the clockwise amount of rotation is maximum. 2

Zone III, corresponds with a zone where the L1 lineations fit

closely with the Kmax directions but the paleomagnetic clockwise

rotation is smaller than in the West. The L1 and Kmax lineations are

no more parallel with the D2 fold axis, which suggests an original

fanning ditribution of the L1 lineations. 3 Zone I is notable, be-

cause it shows, at the Westernmost tip of the island, a place where

the L1 lineations make an angle of 40

(Audren et al. 1993) with re-

spect to the D2 axis. Elsewhere, in the same zone, the L1 lineations

are nearly parallel with the D2 axis. This particular area can be con-

sidered as a zone which was not reoriented, which apparently fits

with the lack of rotation observed by paleomagnetism. However, the

lack of rotation recorded at PMN and PSN sites is in contradiction

with the Northwest-Southeast orientation of the L1 and Kmax linea-

tions since these lineations are homothetic with all the other linea-

tions of the island which suffered a rotation.

A detailed study of this particular area, joining together magnetic

mineralogy, microstructural analysis and thermobarometry gives in-

teresting information on the local appearance and thermal control of

hematites and magnetites (Aïfa et al. 1998). The whole zone is char-

acterized by the presence of two generations of magnetites. One is

related to the D1 phase and the other to the D2/D3 phase. It is clear

that it existed a post-D1 remagnetization. The oldest of the two com-

ponents of magnetization which has been isolated (one being close

to the present day magnetic field) is not far from the Hercynian di-

rections already published for Armorica. This suggests that the giv-

en area has been locally remagnetized at the time of the appearance

of the second magnetite generation. This remagnetization explain

why the Kmax and L1 lineations are not superimposed on clockwise

rotations like in the other studied sites.

The curvature of the D2 fold axis observed between Groix and

Belle-Ile islands which was first interpreted as the result of an im-

pingement by an unevenness of the SAS is confirmed by the clock-

wise rotations measured at all the sites bordering the D2 axis. The

new paleomagnetic data show that the rotation of the D2 Groix-

Belle-Ile axis is a reality and that it developed during late Variscan

times. This rotation was produced by the Northeastern corner of the

impinging uneveness. We can consequently infer that Groix Island

and Belle-Ile may have been in line before Variscan times.

References

Aïfa T., Audren C. & Triboulet C., 1998: Thermal control of magnetite in

Palaeozoic blueschists rocks from île de Groix (Southern Brittany,

France), through microstructural thermobarometry. Geophys. J. Int.

(submitted).

Audrain J. & Lefort J.P., 1986: Le levé magnétique de Groix (Massif Armor-

icain) : une aide pour linterpretation des structures profondes de lîle.

Hercynica, II,1, 6570.

Audren C. & Triboulet C., 1993a: Les chemins pression-température enregis-

trés au cours de la formation de plis non cylindriques dans les schistes

216

bleus de lîle de Groix (Bretagne méridionale, France). C.R. Acad. Sci.

Paris, t. 317, II, 259265.

Audren C., Triboulet C., Chauris L., Lefort J.P., Vigneresse J.L., Audrain J.,

Thiéblemont D., Goyallon J., Jégouzo P., Guennoc P., Augris C. & Carn

A., 1993: Notice explicative. Carte Géol. France (1/25000), feuille île

de Groix (415), Orléans: BRGM, 1101.

Bernard-Griffiths J., Peucat J.J., Sheppard S. & Vidal Ph., 1985: Petrogenesis of

Hercynian leucogranites from the southern Armorican Massif: contribution

of REE and isotopic (Sr, Nd, Pb and O) geochemical data to the study of

source rock characteristics and ages. Earth Planet. Sc. Lett., 74, 235250.

Cannat M., 1983: Cinématique de charriages ophiolitiques (Klamath mountains,

Semail, Groix) et convergence océanique. Ph. D. Thesis, Nantes, 1125.

Carpenter M.S.N., 1976: Petrogenetic study of the glaucophane schists and as-

sociated rocks from île de Groix, France. Ph.D. Thesis, Oxford,1150.

Delhaye A., 1976: Etude de la sismicité récente de la région dOléron. The-

sis 3rd cycle, Univ. Paris VI, 161.

Djro C.S., Triboulet C. & Audren C., 1989: Les chemins pression-tempera-

ture-temps-déformation-espace (chemins P-T-t-d-e) dans les mic-

aschistes associés aux schistes bleus de lîle de Groix, Bretagne

méridionale, France. Schweiz. Mineral. Petrogr. Mitt., 69, 7188.

Lefort J.P., 1979: Iberian-Armorican arc and Hercynian orogeny in Western

Europe. Geology, 7, 384388.

Lefort J.P., Audren C. & Max M.D., 1982: The southern part of the Armori-

can orogeny: a result of crustal shortening related to reactivation of a

pre-Hercynian mafic belt during Carboniferous time. Tectonophysics,

89, 359377.

Lefort J.P. & Vigneresse J.L., 1991: Le lever magnétique et gravimétrique de

Groix: une aide pour comprendre les structures profondes de lile et son

mode de mise en place. Bull. Soc. géol. Fr., 163, I, 311.

Peucat J.J. & Cogné J., 1977: Geochronology of some bluschists from île de

Groix, France. Geol. Soc. America, Mem., 164, 229238.

Poulpiquet J. & Lefort J.P., 1989: Modelling of structures representing the

South Armorican suture. Tectonophysics, 165, 93203.

Quinquis H., 1980: Schistes bleus et déformation progressive: lexemple de

lîle de Groix (Massif Armoricain). Ph. D.Thesis, Rennes 1145.

Sibuet J.C., 1972: Contribution de la gravimétrie à létude de la Bretagne et

du plateau adjacent. Soc. géol. France. C.R. Somm., 3,124129.

Triboulet C., 1977: Les glaucophanites et roches associées de lile de Groix

(Morbihan, France): étude minéralogique et pétrogénétique. Contr.

Mineral. Petrology, 45, 6590.

Triboulet C., 1991: Etude géothermo-barométrique comparée des schistes bleus

paléozoïques de lOuest de la France (Île de Groix, Bretagne méridionale et

Bois de Céné, Vendée). C.R. Acad. Sci. Paris, 312 II, 11631168.

Van der Voo R., 1993: Paleomagnetism of the Atlantic, Tethys and Iapetus

oceans. Cambridge Univ. Press, 1411.

KINEMATIC RELATIONS BETWEEN

MAGNETIC (AMS, AARM)

AND TECTONIC FABRICS

IN THE NIEMCZA FAULT ZONE,

SUDETES FORELAND, SW POLAND

PRELIMINARY RESULTS

T. WERNER

Institute of Geophysics P.A.Sc., Warsaw, Poland; twerner@igf.edu.pl

Anisotropy of magnetic susceptibility (AMS) and anisotropy of an-

hysteretic remanence (AARM) studies are being performed for 17

sites with mylonites, gneisses, gabbro, serpentinite and granitoids

within the Niemcza Fault Zone located in the Sudetic foreland in SW

Poland. Previous petrographic, structural and microtectonic studies in

sites within the zone support the process of mylonitization of the

Sowie Mountains gneisses in the strike-slip sinistral shear zone devel-

oped along the eastern margin of the Sowie Mountains block. The re-

lations between magnetic foliations (AMS kmin and AARM kmin

axes) and tectonic foliation planes as well as magnetic vs. tectonic

lineation are recorded on sample and site scale. The preliminary re-

sults indicate that they may be used as the kinematic indicators of

noncoaxial deformation with a dominant sinistral shear component

within the area. The regional variability of the magnetic fabric (both

direction and anisotropy parameters) will also be correlated with the

postulated Sowie Mountains gneissic origin of the Niemcza zone my-

lonites. The relation of the magnetic fabric of the gabbro, serpentinite

and granitic bodies with those of the mylonites is also tested.

MAGNETIC ANISOTROPIES FOR TOLUCA

AND GIBEON IRON METEORITES

T. FUKUHARA

, M. FUNAKI

1**

and H. NAGAI

National Institute of Poler Researh, 9-10 Kaga 1,

Itabashi Tokyo, Japan; *fukuhara@nipr.ac.jp **funaki@nipr.ac.jp

Department of physics, Faculty of Science, Shinshu University,

3-1-1 Asahi Matsumoto, Nagano, Japan

It is known that there are lunar magnetic anomalies of magnitude

10 nT to 50 nT. These anomalies are found in Apollo 15 and 16 sub

satellite magnetometer data which were collected at altitudes of be-

tween 15 km and 60 km above the surface of the moon (Hood et al.

1981). If there are iron meteorites below the lunar surface, these

would cause magnetic anomalies. To understand the role of magnet-

ic meteorites in magnetizing planetary surfaces we must have infor-

mation about the basic magnetic properties of meteorites. It has

been reported by Albertsen et al. (1978) that tetrataenite is ordered

FeNi with a tetragonal super lattice. The properties of tetrataenite

have been reported by Clarke & Scott (1980). It is important to

know the properties of tetrataenite because the NRM of a meteorite

is due to the uniaxial character of the meteorites structure and the

large magnetocrystalline anisotropy intensity (Néel et al. 1964). We

have examined the relationship between magnetic anisotropies and

octahedron crystalline which is called Widmanstätten structure for

Toluca and Gibeon meteorites to study the cause of NRM.

We performed three different experiments to examine magnetic

anisotropies. 1) Microscopic observation with bitter pattern. We can

find tetrataenite from this observation. 2) The measurements of

NRM. We cut 21 small subsamples from Toluca and 32 from Gibeon

for NRM measurement. 3) Measurements of magnetic anisotropies.

We use a coil magnet and Vibrating Sample Magnetometer for mea-

surements.

We could find tetrataenite in Toluca by microscopic observation

with bitter pattern, but could not find it in Gibeon. The average

NRM intensity for Toluca was 1.17

-3

/kg and for Gibeon it

was 17.2

/kg. The directions of NRM for Toluca and

Gibeon looked along Widmanstätten structure. The magnetic

anisotropies along Widmanstätten structure were measured from

Toluca and Gibeon. Toluca had stronger magnetic anisotropy than

Gibeon. This is probably due to the tetrataenite in Toluca.

References

Albertsen J.F., Jensen G.B. & Knudsen J.M., 1978: Structure of taenite in

two iron meteorites. Nature, 273, 453454.

Clarke R.S. & Scott E.R.D., 1980: Tetrataenite-ordered FeNi, C a new min-

eral in meteorites. Amer. Mineralogist, 65, 624630.

Hood L.L., Russel C.T. & Coleman P.J., 1981: Counter maps of Luner Re-

manent Magnetic Fields. J.G.G., 86, 10551069.

Néel L., Pauleve J., Pauthenet R., Langier J. & Dautreppe D., 1964: Proper-

ties of an iron-nickel single crystal ordered by neutron bombardment.

J. Appl. Phys., 35, 873876.

218

About 100 oriented rock samples from different stratigraphic

level were collected along 23 m of the Margon section. It records

the boundary between Fassanian and Longobardian (lower-upper

Ladinian) and is palaeontologically well controlled by ammonoids

and conodonts. The ammonoid biozones recognized in the studied

section are Serpianensis, Chiesense, Curionii, Recubarensis, Mar-

garitosum (Fig. l).

Laboratory measurements

The natural remanent magnetization (NRM) has been measured

with a three-axes cryogenic magnetometer. Rock magnetic investi-

gations of isothermal remanent magnetization (IRM) acquisition

curves and progressive thermal demagnetization (PTD) of three or-

thogonal IRM components show the presence of soft components

(unblocking temperature of 575

C) and hard components (unblock-

ing up to 675

C) interpreted as magnetite and hematite, respective-

ly. In some samples there is a contribution from goethite (unblock-

ing temperature around 120

C).

All samples were subjected to PTD. In order to check possible

mineralogical alteration during heating, the low field magnetic sus-

ceptibility was measured after each heating step using a KLY-2

bridge. Susceptibility changes above 500

C prevented from defini-

tion of the high temperature NRM component in a few specimens

only. These specimens were discarded.

Discussion and Conclusion

Nearly all specimens carry two NRM components. One compo-

nent, removed up to 300

C, represents the present day field; the

high temperature components, isolated between 350-575

C, are

characterized by both polarities: northerly declinations with positive

inclination and southerly declination with negative inclination. For a

few specimens the definition of a primary component was inhibited

by a strong overprint of the present day field component.

The high temperature component plotted as a function of the

stratigraphic depht provides a clear magnetic zonation in the

Margon section as evidenced by the virtual geomagnetic pole lat-

itude (VGP) distribution below. Four main magnetozones are dis-

tinguished. The Margon magnetostratigraphic pattern reflects the

one defined in a coeval pelagic section in the Southern Alps

(Muttoni et al. 1977). The close between-site agreement of the

two Middle Triassic sections underlines the high value of magne-

tostratigraphic studies for correlating sedimentary sequences in

the Tethyan realm.

References

Gallet Y., Krystyn L. & Bess J., 1998: Upper Anisian to Lower Carnian

magnetostratigraphy from the Northern Calcareous Apls (Austria). J.

Geophys. Res., 103, 605621.

Mietto P. & Manfrin S., 1995: A high resolution middle Triassic ammonoid

standard scale in the Tethys realm. A preliminary report. Bull. Soc.

Géol. Fr., 166, 539563.

Muttoni G. & Kent D.W., 1994: Paleomagnetism of latest Anisian (middle

Triassic) sections of Prezzo Limestone and Buchenstein formation,

sourthern Alps, Italy. Earth Planet. Sci. Lett., 122, 118.

Muttoni G., Kent D.W., Brack P., Nicora A. & Balini M., 1997: Middle Tri-

assic magnetostratigraphy and biostratigraphy from the Dolomites

and Greece. Earth Planet. Sci. Lett., 146, 107120.

LOWER TOARCIAN MAGNETO-

STRATIGRAPHY FROM THE IBERIAN

RANGE (SPAIN)

P.R. GIALANELLA

, M.L. OSETE

, A. GOY

J.J. VILLALAÍN

, J.J. GOMEZ

and F. HELLER

Dept. of Earth Science, University of Naples Federico II,

Lgo S. Marcellino 10, I-80128 Naples, Italy

Dept. of Geophysics, Universitad Complutense Madrid,

Ciudad Universitaria, E-28040 Madrid, Spain

Dpt. of Palaeontology, Universitad Complutense Madrid,

E-28040 Madrid, Spain

Escuela Universitaria Politecnica, Universitad de Burgos,

E-09006 Burgos, Spain

Dpt. of Stratigraphy, Universitad Complutense Madrid,

E-28040 Madrid, Spain

Institut für Geophysik, ETH Hönggerberg,

CH-8093 Zürich, Switzerland

Introduction

Two well-dated sections covering the lower Toarcian (Lower Ju-

rassic) time interval have been investigated palaeomagnetically. The

sections named Sierra Palomera and Ariòo, are located in the central

part of the Iberian Range (Spain). The collected specimens are char-

acterized by multicomponent magnetizations. The magnetic polarity

stratigraphy as defined by the characteristic component of natural

remanent magnetization allows us to establish a good correlation

between the two sections.

Geological setting, biochronology and sampling

The Iberian Range, a fold belt formed during the Alpine orogeny

from the Cantabrian Range to the Mediterranean coast. It is divited

into two branches: the Aragonian and the Castillian, which are sepa-

rated by a Tertiary graben structure. The Sierra Palomera and the

Ariòo sections belong to the Castillian and Aragonian branches, re-

spectively. The lithology at both sections is represented by rhythmi-

cally alternating marls and marly limestones (Goy et al. 1997)

which are well dated palaeontologically by means of ammonite bio-

zonation. The lower Toarcian Tenuicostatum, Serpentinus and Bi-

frons subzones and part of the Variabilis subzones have been sam-

pled in both sections using a gasoline-powered drilling machine.

About 100 oriented rock samples were collected along 60 m of the

Sierra Palomera section and 80 oriented specimens across 35 m of

the Ariòo section.

Rock magnetic properties and palaeomagnetic

measurements

The magnetic behaviour of the specimens from the two sections

is similar. Acquisition of isothermal remanent magnetization

(IRM) and its progressive thermal demagnetization indicate the

presence of hard coercivity components residing in hematite (un-

blocking temperatures of 675

C), coexisting with low coercivity

minerals interpreted as magnetite (unblocking temperature

575

C). The natural remanent magnetization (NRM) has been

measured using a three axes cryogenic magnetometer (2G Enter-

prises). All samples were thermally demagnetized. In order to

check possible mineralogical changes during heating, the low field

susceptibility was measured with a KLY-2 bridge after each heat-

ing step. Mineralogical changes occurred above 450500

sometimes above 575

219

Discussion and conclusion

Nearly all specimens are characterized by a multicomponent

NRM. After initial removal of a viscous component, an intermediate

component (A component) was isolated between 220

C and 300-

350

C. This component has always normal polarity and is interpret-

ed on the basis of previous palaeomagnetic studies in this region

(Juarez et al. 1994) to represent a Cretaceous remagnetization event.

The B component isolated between 350450

C and sometimes up

to 575

C is interpreted as the characteristic remanence component

(ChRM) and carries both positive and negative polarity. Above

575

C, a high temperature unblocking component of uncertain ori-

gin is sometimes observed, if mineralogical changes due to heating

are negligible.

The virtual geomagnetic pole (VGP) latitudes of the ChRM plot-

ted as a function of the stratigraphic level define clear magnetic zo-

nations in both sections. These polarity patterns are similar and can

easily be correlated. They will also be compared with earlier results

from Sierra Palomera (Galbrun et al. 1988) and with Toarcian sites

from the Southern Alps (Horner & Heller 1983).

References

Galbrun B., Baudin F., Comas-Rengifo M.J., Foucault A., Fourcade E.,

Goy A., Mouterde R. & Ruget C., 1988: Résultats magnétostrati-

graphiques préliminaires sur le Toarcien de la Sierra Palomera

(Chaîne ibérique, Espagne). Bull. Soc. géol. France, IV, l, 193198.

Goy A., Comas-Rengifo M.J., Arias C., García Joral F., Gómez J.J., Herrero

C., Martínez G. & Rodrigo A., 1997: El tránsito Pliensbachiense/

Toarciense en el sector central dela Rama Aragonesa de la Cordillera

Ibérica (Espaòa). Cahiers Univ. Catho Lyon, 10, 159179.

Horner F. & Heller F., 1983: Lower Jurassic magnetostratigraphy at the

Breggia Gorge (Ticino, Switzerland) and Alpe Turati (Como, Italy).

Geophys. J.R. astr. Soc., 73, 705718.

Juarez T., Osete M.L., Melendez G., Langereis C.G. & Zijderveld J.D.A., 1994:

Oxfordian magnetostratigraphy of the Aguilon and Tosos sections (Iberi-

an Range, Spain) and evidence of a pre-Oligocene Overprint. Earth Plan-

et. Sci. Lett., 85, 195211.

HIGH-RESOLUTION

MAGNETOSTRATIGRAPHY ACROSS

THE J/C BOUNDARY STRATA AT BRODNO

NEAR ILINA, W. SLOVAKIA: DEFINITION

OF TWO SUBZONES IN M19 AND M20

V. HOUA*, M. KRS, M. KRSOVÁ, O. MAN,

P. PRUNER and D. VENHODOVÁ

Institute of Geology, Academy of Sciences of the Czech Republic,

Rozvojová 135, Prague 6 - Lysolaje, Czech Republic; *housa@gli.cas.cz

Magnetostratigraphic studies of Jurassic-Cretaceous (J/C) bound-

ary strata were mostly carried out in the Tethyan realm with the aim

of setting up a synoptic scheme of N and R polarity magnetozones

or possibly magnetosubzones, but not aspiring on their detailed def-

inition. The aim of joint efforts of the Paleomagnetic and Paleonto-

logical Groups of the Inst. of Geol., Acad. Sci. Prague was to elabo-

rate high-resolution magnetostratigraphic scales applicable to

correlation of chronostratigraphic units of J/C boundary strata in the

Tethyan and later also in the Boreal realms. The Tithonian-Berria-

sian profile at Brodno, near ilina, W. Slovakia, was selected from

five localities in the Western Carpathians for detailed investigations

due to favourable sedimentation setting, favourable physical proper-

ties of limestone samples and rich calpionellid associations. Up to

the present time, a total of 360 oriented samples have been paleo-

magnetically analyzed, with the maximum sampling density be-

tween the base of M21r and the base of M18r. In the cross-section,

this interval represents only 10 m of the true thickness of strata. An

innovation of the hitherto methods of data processing and graphic

presentation of results was introduced comprising interpolation,

smoothing techniques and presentation of total angular deflection of

paleomagnetic directions. Two magnetosubzones with reverse pale-

omagnetic directions were detected, the Brodno Subzone (see Fig.)

in the late part of M19n and the Kysuca Subzone in the middle part

of M20n. The Kysuca Subzone is located at the beginning of the

Crassicolaria Standard Zone, i.e. in the early Upper Tithonian. The

last Chitinoidella disappears immediately before the beginning of

the Kysuca Subzone. The beginning of the magnetozone M19 is

characterized by the appearance of Calpionella grandalpina, which

appears immediately before it and indicates the beginning of the In-

termedia Subzone (late Upper Tithonian). The J/C boundary defined

on calpionellids (base of Calpionella Standard Zone) is located in

the early 35% part of the local thickness of the magnetozone M19n.

The Brodno Subzone is located in the early Berriasian, in the pale-

ontologically monotonous part of the Alpina Subzone. The short

acme of Crassicolaria parvula occurs approx. in the middle be-

tween the J/C boundary and the base of the Brodno Subzone.

HIGH-RESOLUTION

MAGNETOSTRATIGRAPHY ACROSS THE

J/C BOUNDARY STRATA AT THE BOSSO

VALLEY, UMBRIA, CENTRAL ITALY:

CORRELATION WITH THE BRODNO DATA

V. HOUA

, M. KRS

, P. PRUNER

, O. MAN

,D. VENHO-

DOVÁ

, F. CECCA

, G. NARDI

and M. PISCITELLO

Institute of Geology, Academy of Sciences of the Czech Republic,

Rozvojová 135, Prague 6 - Lysolaje, Czech Republic; *housa@gli.cas.cz

Universita di Urbino, Italy,

Dipartemento Sci. d. Terra, Universita di Napoli, Italy

Prior to the magnetostratigraphic investigations of the profile at

the Bosso Valley, the profile at Brodno was investigated and consid-

ered unique among all the sections studied in the Tethyan realm. Al-

though the next profile at tramberk, N. Moravia, is based on the

analyses of 342 drill samples, it did not reach the accuracy and reli-

ability of the Brodno profile. Consequently, the next profile was

needed for high-resolution magnetostratigraphy across the Jurasic/

Cretaceous (J/C) boundary strata in the Tethyan realm. The profile

at the Bosso Valley was selected, this 110-m section of the Early

220

Cretaceous Maiolica pelagic limestones was synoptically investigat-

ed by W. Lowrie and J. E. T. Channell (1983). In 1996 and 1997, a

total of 197 hand samples were collected from the basal 40-m part

of the Bosso profile. The studied section of the Bosso profile com-

prises the magnetozones M20n to M17r. Moreover, two subzones

with reverse paleomagnetic polarity were delineated, in the upper

part of M19n (Brodno Subzone, see Fig.) and in the middle part of

M20 (Kysuca Subzone). Both the subzones detected in the Brodno

and Bosso profiles are located in the same position in relation to the

surrounding magnetozones and they may be well correlated with

similar subzones in the sequence of marine magnetic anomalies. Pri-

or to our investigations, these subzones were known from the ma-

rine sequence only and have been detected only sporadically on

continental profiles. The J/C boundary defined on calpionellids

(base of the Calpionella Standard Zone) lies in the early 37% part of

the local thickness of the magnetozone M19n, i.e. approx. at the

same level as in Brodno. The position of the Kysuca Subzone could

not be defined in terms of calpionellid biostratigraphy, because of

the complete absence of calpionellids in this part of the profile. The

first stratigraphically usable calpionellid associations occur in the

late magnetozone M20n, i.e. stratigraphically above the Kysuca

Subzone. Exactly as in Brodno, Calpionella grandalpina appears

closely before the beginning of the magnetozone M19r. The Brodno

Subzone lies in the Early Berriasian, stratigraphically above a short

acme of Crassicolaria parvula, which here, as in the Brodno profile

is located in the middle between the J/C boundary and the base of

the Brodno Subzone.

ceptibility was 50.6

-6

u.SI and the maximum of NRMP was

4.9 nT. The IRM acquisition curves as well as the demagnetizing

characteristics showed that hematite was the dominant magnetic

mineral in the investigated rocks. All the samples were subjected to

thermal demagnetization. Only normal magnetic polarization was

obtained on both profiles (Figs. 1, 2). This means that according to

biostratigraphical and magnetostratigraphical scale, presented by

Mutterlose (1996), the limestones studied belong to uppermost

Hauterivian normal magnetic subzone CM 4. The measurements of

anisotropy of magnetic susceptibility, performed on all samples be-

fore thermal demagnetization, showed sedimentary character with-

out more important deformation.

MAGNETOSTRATIGRAPHY OF THE

CRETACEOUS LIMESTONES FROM THE

KRÍNA NAPPE (WESTERN SLOVAKIA)

J. MICHALÍK

, I. TÚNYI

and Z. DOBIAOVÁ

Geological Institute SAS, Dúbravská cesta 9, 842 26 Bratislava,

Slovak Republic; geolmich@savba.sk

Geophysical Institute SAS, Dúbravská cesta 9, 842 28 Bratislava,

Slovak Republic; *geoftuny@savba.sk

Magnetostratigraphic study of the Cretaceous limestones from the

Krína Nappe in the North-Western part of Slovakia was carried out

with the aim of precisely dating the sedimentary rocks from the Hau-

terivian/Barremian boundary. According to Reháková & Michalík

(1997), the age of investigated rocks is l28 mil. years. The lime-

stones were dated by the Borzai and Mortilleti ammonite zones (lat-

est Hauterivian) and Hugii ammonite Zone (earliest Barremian)

(Vaíèek et al. 1997). Althogether 332 oriented samples were paleo-

magneticaly analyzed. The samples came from two profiles:

Daïová Ryha (27 layers) and Na Laze (11 layers) sections. The

rocks were very weakly magnetized. The maximum of magnetic sus-

Fig. 2. Na Laze Section.

Fig. 1. Daïová Ryha Section.

References

Mutterlose J., 1996: The Hauterivian Stage. Bulletin de LInstitut Royal

des Sciences Naturalles de Belgique. Procced. Second Int. Symp. on

Cretaceous Stage Boundaries. Brussel, 1924.

Reháková D. & Michalík J., 1997: Calpionellid associations versus Late

Jurassic and Early Cretaceous sea-level fluctuations. Miner. slovaca,

45, 306307.

Vaíèek Z., Michalík J. &Reháková D., 1997: Polomec Quary. Excursion

221

sal attempt not involving even a short-lived interval of normal po-

larity. Thus, there is reason to believe that the geodynamo attempted

reversal (regardless of success)no less than 7, and more likely at

least 10 times during this 400 kyr interval of time. This more-ac-

tive account of the reversing geodynamo during the upper part of

the Matuyama reverse epoch is consistent with and extends findings

principally from the Brunhes normal epoch (see Champion et al.

1988; Lund et al. 1997), suggesting this level of activity may in fact

represent the geodynamo over the past several millions of years.

quid book. Final Meeting of the Project No. 362 Tethyan/Boreal
Cretaceous Correlation. Miner. slovaca, 45, 363366.

HIGH-RESOLUTION

MAGNETOSTRATIGRAPHY

OF THE UPPER MATUYAMA:

TEMPO OF THE GEODYNAMO

FROM

AR/

AR GEOCHRONOLOGY

B.S. SINGER and K.A. HOFFMAN

Department of Mineralogy, University of Geneva,

1211 Geneva 4, Switzerland

(K.A.H.: permanent address: Physics Department, Cal Poly State

University, San Luis Obispo, CA 93407 USA)

One important application of magnetostratigraphic data concerns

the problem of the geodynamo; specifically, the frequency at which

polarity reversals are attempted over geologic time. In this regard,

high-precision, complete magnetostratigraphic datasets are re-

quired. Indeed, Merrill et al. (1997) demonstrate that even one

missed short polarity interval can have a significant effect on analy-

ses of reversal frequency and the two polarity states. Yet, even the

most comprehensive geomagnetic polarity timescales (GPTSs) pos-

sess limited fidelity. For nearly two decades the GPTS for the final

400 kyr of the Matuyama reverse chron has contained, at most, 5

polarity reversals corresponding to the Cobb Mountain event, the

Jaramillo subchron, and the Matuyama-Brunhes transition (see Op-

dyke & Channell 1996).

Newly-conducted

Ar/

Ar age determinations at the University

of Geneva on 17 transitionally-magnetized basaltic lavas from flow

sequences at Punaruu Valley, Tahiti, and Haleakala volcano, Maui

when considered with other published upper Matuyama

Ar/

dataindicate that this picture is both largely incomplete and in

need of significant revision. The revised GPTS for the upper

Matuyama (Fig. 1) includes a total of 5 subchrons and less-under-

stood events (2 of which are newly named the Punaruu and Santa

Rosa), in addition to the Matuyama-Brunhes transition (Table 1).

Table 1:

Matuyama (T)

0.779 ± 0.002 Ma

Kamikatsura Event

0.886 ± 0.003 Ma

Santa Rosa Event

0.920 0.930 Ma

Jaramillo Subchron (T)

0.986 ± 0.005 Ma

Jaramillo Subchron (O)

1.053 ± 0.005 Ma

Punaruu Event

1.105 ± 0.005 Ma

Cobb Mountain Event

1.186 ± 0.006 Ma

signifies that normal polarity paleomagnetic data have been reported elsewhere

The fact that normal polarity has been observed during 3 of the 4

events listed in Table 1 suggests that they are short-lived subchrons.

Indeed, the Cobb Mountain is already considered by some to be a

normal polarity subchron. However, we choose to denote any geo-

magnetic activity an event until a set of precise non-overlapping

age determinations are available from transitionally magnetized ma-

terials defining both onset and termination; at such time the term

subchron will be invoked. Nonetheless, there is sufficient evi-

dence to assume for this discussion that the Kamikatsura, Santa

Rosa, and Cobb Mountain are short-lived normal polarity sub-

chrons. Although the same situation may apply to the Punaruu as

well, we cannot yet argue that two close back-to-back reversals are

involved. On the other hand, the Punaruu may be an aborted rever-

Fig. 1. Revised Geomagnetic po-

larity timescale for the upper

Matuyama from 40Ar/39Ar age

determinations of transitionally

magnetized flows.

References:

Champion D.E., Lamphere M.A. & Kunz M.A., 1988: Evidence for new

geomagnetic polarity reversal from lava flows in Idaho: Discussion of

short polarity reversals in the Brunhes and late Matuyama polarity

chrons, J. Geophys. Res., 93, 11,6711,680.

Lund S., Acton G., Clement B., Hastadt M., Okada M., Williams T., and

ODP Leg 172 Scientific Party, 1997: Initial paleomagnetic results

from ODP leg 172: Evidence for at least 14 geomagnetic field excur-

sions during the Brunhes Epoch, EOS Trans. Am. Geophys. Union,

78, 181.

Merrill R.T., McElhinny M.W. & McFadden P.L., 1996: The Magnetic Field

of the Earth. Academic Press, 1527,

Opdyke N.D. & Channell J.E.T., 1996: Magnetic Stratigraphy. Academic

Press, 1341.

ULTRA-FINE MAGNETOSTRATIGRAPHY

IN SHALLOW WATER CARBONATES:

SECULAR VARIATIONS IN THE LOWER

CRETACEOUS

D.H. TARLING

, M. IORIO

and B. DARGENIO

Department of Geological Sciences, University, Plymouth, PL4 8AA,

U.K.; d.tarling@plymouth.ac.uk

Geomare sud Research Institute, C.N.R., Via Vespucci 9, Napoli, Italy

Dip. Scienze della Terra, Univ. di Napoli, Largo San Marcellino 10, Napoli, Italy

Shallow-water carbonates undergo diagenesis and cementation

very rapidly after deposition. This process involves the formation of

222

biogenetic magnetite within the anoxic zone immediately below

the depositing carbonates. These grains are free to align with the

geomagnetic direction until cemented by largely authigenic car-

bonate-saturated fluids. The cementation preserves the sediment,

with its associated remanence, against most later changes, unless

exposed to prolonged meteoritic diagenetic processes, such as can

occur at the top of Milankovich-driven (planetary orbital varia-

tions) sedimentary thickness cycles. In southern Italy during most

of the Lower Cretaceous, the gradual subsidence of the basin re-

stricted such meteoritic diagenesis to the prolonged emersions at

the top of the major sedimentary cycles (superbundles) while those

at the tops of lesser cycles were apparently at or near sea-level for

only brief intervals. Consequently a largely continuous record of

geomagnetic field behaviour is preserved throughout most of the

sequence (DArgenio et al. in press). Bore cores, with a total

length of 120 m, have been obtained in the Monte Raggeto area of

the Southern Apennines, N of Naples, where the sequences are

well exposed in quarries and have been shown to be largely free

from tectonic activity, other than a uniform tilt occurring as a re-

sult of nappe development during the Late Miocene. Combined

sedimentological and palaeomagnetic studies at 12 cm intervals

(Iorio 1995; Iorio et al. 1995, 1996a,b, in press; Tarling et al.

1997) have shown that the magnetic polarity zonations can be cor-

related with those identified in oceanic marine anomalies of the

Pacific and that a fine-scale geomagnetic structure can also be iso-

lated which can be ascribed to ca. 700 year smoothed geomagnetic

secular variations (Tarling et al. submitted).

Spectral analyses of the sedimentological and palaeomagnetic

parameters enable the duration of each of the sedimentary cycles

to be determined. Since identical cycles can also be determined in

the palaeomagnetic parameters, but these are not controlled by the

sedimentary features (linear correlation coefficients between the

magnetic and sedimentary characteristics are all close to zero),

these enable precision of the duration of the sedimentary and

palaeomagnetic cycles with 1 cm = 360±16 years on average.

Stratigraphic correlations can be undertaken, at this level, over

several kilometres. Current studies are of the overlap sections be-

tween the two bore cores (300 m apart), as well as to coeval sur-

face exposures, to elucidate and date the time scale of diagenetic

changes within these sediments on the basis of this smoothed geo-

magnetic secular variation record. The data also provide major

time constraints on the polarity zones used for coarser scale mag-

netostratigraphic dating and have implication for geomagnetic

field behaviour.

References

DArgenio B., Ferreri V., Iorio M., Raspini A. & Tarling D.H., in press: Di-

agenesis and remanence acquisition in the Cretaceous carbonates of

Monte Raggeto, southern Italy. Geol. Soc. (London), Spec Publ.

Iorio M., 1995: High Resultion Palaeomagnetic Analyses of Cretaceous Shal-

low-water Carbonates. Ph.D. thesis, University of Plymouth, 1393.

Iorio M., Tarling D., DArgenio B., Nardi G. & Hailwood A.E., 1995: Mi-

lankovitch cyclicity of magnetic directions in Cretaceous shallow-

water carbonate rocks, Southern Italy. Boll. Geof. Teor. Appl. 37,

109118.

Iorio M., Nardi G., Pierattini D. & Tarling D.H., 1996a: Palaeomagnetic evi-

dence of block rotations in the Matese Mountains, Southern Apennines,

Italy. In: A. Morris & D.H. Tarling (Eds.): Palaeomagnetism of the Med-

iterranean. Geological Society, Special Publication, 105, 133139.

Iorio M., Tarling D.H., DArgenio B. & Nardi G., 1996b: Ultra-fine magneto-

stratigraphy of Cretaceous shallow water carbonates, Monte Raggeto,

southern Italy. In: A. Morris & D.H. Tarling, D.H. (Eds.): Palaeomagnetism

of the Mediterranean. Geol. Soc. (London), Spec. Publ., 105, 195203.

Tarling D.H., DArgenio B. & Iorio M., 1997: Astronomical influences on

biomagnetic activity some 120 Ma ago: The potential for estimating

the evolution of ancient planetary orbits within the Solar System. In:

Cosmovici C.B., Bowyer. S. & Werthimer D. (Eds.): Astronomical and

Biochemical Origins amd the Search for Life in the Universe. Editrice

Compositori, Bologna, 245252.

Tarling D.H., Iorio M. & DArgenio B., (submitted): Geomagnetic Secular

Variations and Polarity Transitions in Italian Lower Cretaceous Shal-

low-Water Carbonates. Geophys. J. Internat.

V. Rock magnetic methods and experiments

THERMOREMANENT MAGNETIZATION:

THEORIES AND EXPERIMENTS

D. J. DUNLOP

Physics Department, Erindale College, University of Toronto,

3359 Mississauga Road North, Mississauga, Ontario, Canada L5L 1C6;

dunlop@physics.utoronto.ca

An elegant theory of thermoremanent magnetization (TRM) in

single-domain (SD) grains was given by Néel (1949) but the ac-

quisition of TRM in larger, non-uniformly magnetized grains is

more difficult to explain. SD TRM is a frozen high-temperature

partition between two microstates: spins parallel or antiparallel to

an applied magnetic field. Non-uniformly magnetized grains have

a much greater choice of microstates (local energy minimum or

LEM states) and partitioning among various LEM states continues

to change during cooling. These changes may involve Barkhausen

jumps of domain walls between positions of minimum local ener-

gy or nucleation of new domains and walls.

Because of the lower remanence capacity of non-uniform mi-

crostates compared to the uniform SD state, TRM intensity de-

creases as grain size increases, although certain microstates, e.g.

single-vortex states, seem to contribute little to TRM. Thermal de-

magnetization of TRM begins just above room temperature and

continues to the Curie point, quite unlike the sharp unblocking of

SD TRM. This continuous demagnetization, resulting from chang-

es in microstates driven by the changing internal demagnetizing

field during heating, profoundly affects the separation of different

components of natural remanent magnetization and the determina-

tion of paleomagnetic field intensity.

A peak in the intensity ratio TRM/ARM (anhysteretic remanence)

around 100200 nm in magnetite appears to pinpoint the appearance

of vortex states. To explain the peak, it is necessary to hypothesize

that vortex structures are responsible for ARM but structures with

much higher remanence, possibly metastable SD states, carry TRM.

If this is true, the LEM state in magnetite grains just above SD size

is strongly dependent on magnetic history, being quite different for

weak-field thermal processes like TRM and thermal demagnetiza-

tion than for strong-field isothermal processes like ARM, IRM, and

AF demagnetization.

Recent experiments concerning partial TRMs of large multidomain

magnetites and their thermal demagnetization will also be discussed.

223

TEMPERATURE VARIATIONS IN DOMAIN

STATE OF MAGNETITE-BEARING

VOLCANIC ROCKS AS REVEALED

BY HYSTERESIS MEASUREMENTS

A. A. KOSTEROV* and V. A. SHASHKANOV

Institute of Physics, St. Petersburg University, 1, Uljanovskaya, St. Petergoff,

198904 St. Petersburg, Russia; *kosterov@snoopy.phys.spbu.ru

Magnetic hysteresis loops for magnetite-bearing volcanic rocks

of different origin and age ranging from about 1 million years to 180

million years have been measured in as a function of temperature

between 90 K and 873 K, using the vibrating sample magnetometer

at the Institute for Rock Magnetism, Minneapolis. All measurements

above room temperature have been done in a helium atmosphere.

Below and at room temperature hysteresis parameters for all sam-

ples are in the pseudo-single-domain (PSD) range. Moreover, this

PSD state remains stable up to 750800 K, depending on a sample.

Between 250 K and about 780800 K, coercive force (H

) is propor-

tional to J

, the n factor being in a narrow range from 1.2 to 1.6.

This can be interpreted in favor of mainly magnetostrictive control

of coercivity, since the magnetostriction constant

of magnetite is

known to vary with temperature as J

2.3

(Moskowitz 1993) and H

/ J

(Hodych 1986). Our findings are in general agreement with

previous studies (Hodych 1986, 1990, 1996); however it is worth

noticing that these latter studies were carried out on samples with

probably coarser grain sizes than ours and below room temperature

only. It appears that this magnetostrictive zone does not extend

up to the Curie point: above 780800 K the coercive force exhibits

much slower decrease with temperature, probably indicating the

transition from PSD into SD state occurs.

In most of our samples over 70 per cent of TRM is blocked above

500

C, which is a common situation for magnetite-bearing basaltic

rocks. Since nucleation of new domain walls in weak fields is

strongly inhibited (Dunlop et al. 1994), TRM is likely to be initially

acquired through a single-domain-like mechanism. On further cool-

ing this high-temperature SD state becomes metastable, and, on ap-

plying magnetic fields at room temperature or on moderate heating,

it can easily evolve develop into a non-SD state with significantly

lower remanence. Such process would result in hysteresis and A.F.

demagnetization characteristics like those of the pseudo-single-do-

main range, and a demagnetization of TRM would occur at tempera-

tures lower than the true blocking temperatures.

References

Dunlop D.J., Newell A.J. & Enkin R.J., 1994: Transdomain thermoremanent

magnetization. J. Geophys. Res., 99, 19,74119,755.

Hodych J.P., 1986: Evidence for magnetostrictive control of intrinsic sus-

ceptibility and coercive force of multidomain magnetite in rocks.

Phys. Earth Planet. Inter., 42, 184194.

Hodych J.P., 1990: Magnetic hysteresis as a function of low temperature in

rocks: evidence for internal stress control of remanence in multi-do-

main and pseudo-single-domain magnetite. Phys. Earth Planet. Inter.,

64, 2136.

Hodych J.P., 1996: Inferring domain state from magnetic hysteresis in high

coercivity dolerites bearing magnetite with ilmenite lamellae. Earth

Plan. Sci. Lett., 142, 523533.

Moskowitz B.M., 1993: High-temperature magnetostriction of magnetite and

titanomagnetites. J. Geophys. Res., 98, 359371.

SIGNIFICANCE OF HIGH-COERCIVITY

RHOMBOHEDRAL OXIDES AND FINE-

GRAINED MAGNETITE FOR THE ORIGIN

OF STRONG REMANENCE-DOMINATED

AEROMAGNETIC ANOMALIES,

SOUTH ROGALAND, NORWAY

S.A. McENROE

, P. ROBINSON

1,2

, S. RONNING

and P.T. PANISH

1,2

Geological Survey of Norway, PO Box 3006-Lade, N-7002 Trondheim,

Norway; *suzanne.mcenroe@ngu.no

Dept. of Geosciences, University of Massachusetts, Amherst,

MA 01003, USA

A case study from ilmenite-rich norites and their anorthosite host

rocks from the Proterozoic basement in southern Norway shows the

importance of rock-magnetic, petrophysical and petrological data

needed for accurate geological interpretation of aeromagnetic sur-

veys, especially high-resolution surveys. A closely spaced helicop-

ter aeromagnetic survey in the South Rogaland area (Rønning 1995)

showed strong induced- and remanence-dominated anomalies. The

variations found in compositions and textures of the magnetic ox-

ides and in oxide exsolution in the silicate grains can help explain

the different magnetic responses found (McEnroe et al. 1996). In

this region there is a large variation in remanent intensities from <1

to >100 A/m, susceptibility from 10

to 10

SI, and Koeningsberg-

er ratios ranging from 0.1 to 237. The active and historical ilmenite

ore deposits in general have remanence-dominated anomalies which

we believe are controlled by the ilmenite mineralogy (McEnroe

1997) and oxide exsolution in pyroxenes.

Rock-magnetic tests combined with reflected-light petrography

aided in the identification of the high-coervicity phases responsible

for the remanence-dominated anomalies in the norites. The major

phases were identified by reflected-light microscopy and composi-

tions were determined by microprobe analyses. Abundant ilmenite

with multiple generations of hematite exsolution is the common

high-coercivity phase. MPMS measurements were used to help

identify phases that were not visible optically. A Verwey transition

was measured in samples where magnetite was not optically ob-

served, indicating the presence of fine-grained magnetite. Surpris-

ingly, a Morin transition was not identified in any sample though

abundant hematite is present as Ti-hematite to almost pure hematite.

The hematite is present in discrete ilmenite grains as exsolution

lamellae and as an exsolution product in ilmenite needles in py-

roxenes. Even samples from almost pure hemo-ilmenite dikes (up to

97 % hemo-ilmenite) with abundant hematite exsolution, did not

show a Morin transition. The lack of the Morin transition may be

due to the Ti-content of the hematite.

In thin section many pyroxene grains have abundant highly elon-

gate blades of ilmenite and magnetite. The blades are a few microns

or less in thickness and up to 20 microns long. Magnetite in the cli-

nopyroxenes has exsolved in elongated blades parallel to (010) with

long axes parallel to either a- or c-crystallographic axes. Ilmenite

has exsolved in similar elongated blades but parallel to (100) with

long axes parallel to the b-crystallographic axes as described by

Morse (1970) in Laborador. Ilmenite oxidation-exsolution lamellae

were observed in many of the magnetite blades. Mineral separates

of clinopyroxene and orthopyroxene were analyzed by MPMS and

the Verwey transition was consistently found, verifying that magne-

tite is a common exsolution phase in the clinopyroxene. Hematite

exsolution in the ilmenite was observed in many of the clinopyrox-

ene samples but not in the orthopyroxene grains.

224

In addition to the high coercivity phases, up to 5 % of co-existing

large MD magnetite grains with ilmenite oxidation-exsolution were

common in many samples from localities having remanence-dominat-

ed anomalies. Norites with low Q values have magnetite as the domi-

nant oxide and are from areas of induced anomalies. Though magne-

tite-rich, these rocks have a range in Q values that may be explained

by the different oxidation-exsolution textures in the magnetites.

Most anorthosite regions have remanence-dominated anomalies.

Anorthosite samples have low susceptibility and high remanence

with corresponding high Q values, up to 237. Hemo-ilmenite is the

predominant oxide in the anorthosites, occurring as fine discrete

grains and as extremely fine exsolution, along with sulphides, in

the plagioclase.

Acknowledgments: The first author gratefully acknowledges the

Institute of Rock Magnetism in Minnesota, USA where the rock-

magnetic experiments were performed.

References

McEnroe S.A., 1997: Ilmenite mineral magnetism: implications for geo-

physical exploration for ilmenite deposits. Norges Geologiske Under-

søkelse Bulletin, 433, 3637.

McEnroe S.A., Robinson P. & Panish P.T., 1996: Rock-magnetic properties,

oxide mineralogy, and mineral chemistry in relation to aeromagnetic in-

terpretation and the search for ilmenite reserves. Norges Geologiske Un-

dersøkelse Report, 96,060, 1148.

Morse S.A., 1970: Preliminary chemical data on the augite series of the Kigla-

pait layered intrusion (abstract). Amer. Mineralogist, 55, 303304.

Rønning S., 1995: Helikoptermålinger over kartblad 1311IV Sokndal.

Norges Geologiske Undersøkelse Report, 95,120.

A MODERNIZED COERCIVITY

SPECTROMETER

P.G. JASONOV, D.K. NURGALIEV, B.V. BUROV,

F. HELLER*

Institut für Geophysik, ETH Hoenggerberg, 8093 Zürich, Switzerland;

*frieder@mag.geo.phys.ethz.ch

A coercivity spectrometer which had already been designed at the

Palaeomagnetic Laboratory of Kazan University, Russia some years

ago (Burov et al. 1986), has been reconstructed, modernized and in-

terfaced to an IBM compatible personal computer. The instrument

which is fully controlled by the computer, consists of a non-magnet-

ic disk, housing a 1cc volume sample and rotating with a speed of

about 22 Hz between the pole tips of an electromagnet. The magnet-

ic field is continuously changed and monitored whereby the sample

is measured outside and inside the applied field. Thus, isothermal

remanent magnetization (IRM) and saturation magnetization (M

)

as well as coercivity (B

)

and coercivity of remanence (B

)

can be

measured easily, quickly and with high sensivity. At least 10000

magnetization values may be recorded in one run in less than 6

minutes up to the maximum field (B

max

=300 mT in the present

configuration) and to the corresponding back field. This contrasts

the usual palaeomagnetic IRM acquisition experiment where only

10 to 15 steps are taken up to the maximum field in a much more

time consuming manner (about 15 to 20 minutes per sample). IRM

Fig. 1. (Abstract Jasonov et al.). Explanations see next page.

225

coercivity spectra are extremely well documented allowing for

spectral analysis and modelling (Eyre 1996) as well as for interpre-

tation according to Preisach-Néel theory. All hysteresis parameters

are measured which are necessary for analysing ferrimagnetic grain

size using the Day-method (Day et al. 1977). This low-cost instru-

ment may be of great value for many palaeomagnetic laboratories.

References

Burov B.V., Nurgaliev D.K. & Jasonov P.G., 1986: Paleomagnetic Analy-

sis. Kazan University Press, 1176 (in Russian).

Eyre J.K., 1996: The application of high resolution IRM acquisition to the

discrimination of remanence carriers in Chinese loess.Studia Geo-

physica et Geodetica, 40, 234-242.

Day R., Fuller M. & Schmidt V.A., 1977: Hysteresis properties of titanomag-

netites: grain-size and compositional dependence.Physics of the Earth

and Planetary Interiors, 13, 260-267.

CHEMICAL REMANENT MAGNETIZATION

DURING THE TRANSFORMATION

OF GOETHITE TO HEMATITE:

POSSIBLE FORMATION OF

INTERMEDIATE SPINEL PRODUCT

Ö. ÖZDEMIR

Department of Physics, Erindale College, University of Toronto, 3359

Mississauga Road N., L5L IC6 Mississauga, Ontario, Canada;

ozdemir@physics.utoronto.ca

Chemical remanent magnetization (CRM) was studied in the go-

ethite-hematite transformation, starting from synthetic, acicular go-

ethite crystals. Mössbauer spectra, X-ray analysis and low-tempera-

ture demagnetization of saturation remanence (SIRM) confirmed

that the starting material was pure goethite. In the CRM experi-

ments, samples were heated in zero field to temperatures between

150 and 600

C, held for 2.5 hours in a 1 Oe field, and cooled in

zero field to room temperature. The CRM intensity reached a maxi-

mum after the 250

C run, which is close to the spontaneous dehy-

dration temperature of goethite, then decreased in the range 275

500

C. The resulting CRMs were always in the direction of the ap-

plied field. The dehydration of goethite to hematite around 250

was marked by a weight loss in thermogravimetric analysis (TGA),

and was also indicated by sharp increases in coercive force, coerciv-

ity of remanence and saturation remanence ratios which are sensi-

tive to magnetic grain size.

Low temperature induced magnetization was measured as a func-

tion of temperature from 10 K to 300 K with the MPMS to identify

the mixture of phases in the dehydration product. Goethite and he-

matite are the magnetically dominant phases after all runs except

500 and 600

C at which hematite is the only remanence carrier. The

partially dehydrated goethites after the 240400

C runs show broad

susceptibility peaks or inflections around 120 K, probably indicat-

ing formation of an intermediate spinel phase. These samples were

next given an SIRM in a field of 2 T at 10 K and the remanence was

measured continuously during zero field warming to 300 K. A de-

crease in remanence was observed at the Verwey transition (120 K)

indicating the presence of magnetite.

The possible formation of even a small amount of magnetite is a

matter of serious concern in studies of goethite-bearing sediments

and rocks. CRM of this strongly magnetic spinel phase would sig-

nificantly modify or even overwhelm the original CRM of the goet-

hite. It would also be a new source of paleomagnetic noise as far as

primary NRMs carried by other mineral phases are concerned.

EXPERIMENTAL MODELLING OF THE

FERRIMAGNETIC CRYSTALLIZATION

PROCESS UNDER VARIOUS

P-T -pO

CONDITIONS

Y. GENSHAFT, V. TSELMOVITCH* and A. GAPEEV

Geophysical Observatory Borok, Nekouzskij region, Yaroslavskaya

oblast, 152742 Russia; *otselm@ibiw.yar.ru

The study of the picrobasalt-ilmenite system, started in 1996, is

continued in a range of pressure up to 50 kbar and temperature up to

Fig. 1. 1. row IRM and induced magnetization versus the field applied in two antiparallel directions (forward an backward) of a loess and a palaeo-

sol sample from the Chinese Loess Plateau, as measured using the modernized coercivity spectrometer. 2. row Forward IRM (J

r,a

) and difference of

backwardforward IRM (J

r,b-a

) versus applied field. 3. row Derivatives of 2. row parameters.

Continuation of Fig. 1

226

1450

C. The study of the crystallization of the system <alkaline ba-

saltilmenite> is done under the same conditions. The influence of

firm buffers FeFeO, MtHem, NiNiO on crystallization Fe-Ti ox-

ides in ilmenite-containing systems is investigated. It is shown, that

crystallization of ferrimagnetic phases (titanomagnetites) occurs by

the use of the buffer MtHem and in oxidizing conditions with the

addition of K

: in a basalt system with variable contents of Fe,

Ti, K at lgfO

> 9, temperature 1150

C and P < 20 kbar titanomag-

netite crystallizes. In basalt systems with increased concentration of

Fe and Ti at pressure 2050 kbar picroilmenite containing more than

40 % of geikielite in a solid solution crystallizes in association with

silicate phases. In the condition of the experiment lgfO2 > 9 picro-

ilmenite crystallizing at P = 2037 kbar contains less hematite. Ex-

pansion of rutile field at increasing P was observed. At P = 50 kbar

rutile can be a near-liquids phase. Basalt with a high concentration

of K is characterized by the expansion of the field of crystallization

of olivine (up to P = 35 kbar). Under increased contents of fluid

phase (H

O, CO

) in near-liquids conditions crystallize phlogopite

up to pressure 35 kbar. The phase diagram of the picrobasaltil-

menite system was studied at pressures of up to 50 kbar and tem-

peratures up to 1450

C. Crystallization in the system alkaline ba-

saltilmenite is done under the same conditions from the melt. The

compositions are substantially different: in picrobasalt >Mg and Na,

K, while in the alkaline basalt- >alkalis and <Mg. The influence of

the buffers FeFeO, MtHem, NiNiO on the crystallization of Fe-

Ti oxides in ilmenite-containing systems is investigated. Ferrimag-

netic phases (titanomagnetites) crystallize under P, T, fO

conditions

dictated by the buffer MtHem. Oxidizing conditions created by ad-

dition of K

(at the conditions of our experiments is liberated

out at the decomposition of potash CO

acts as oxidizer) in a basalt

system with variable contents of Fe, Ti, K at logfO

> 9, also yield

titanomagnetite at 1150

C and P < 20 kbar. In basalt systems with

higher concentrations of Fe and Ti at pressures of 2050 kbar picro-

ilmenite containing more than 40 % of geikielite in a solid solution

crystallizes, along with silicate phases. In the experimental condi-

tions (logfO

> 9, P = 2037 kbar (the of temperature interval near

200

C below liquidus)), picroilmenite contains less Fe. The struc-

ture of crystallizing picroilmenite depends on the PT of the basalt

system, and results in the expansion of the field of rutile crystalliza-

tion. Rutile begins to crystallize in a rich titanium silicate system at

pressures over 25 kbar and under certain fO

. The field of rutile

crystallization is increased at the expense of the field of crystalliza-

tion of ilmenite. Probably, ilmenite does not crystallize at pressures

>60 kbar, but rutile crystallizes instead of ilmenite in the silicate

systems in the field of existence of the melt. At P = 50 kbar rutile

can be a near-liquidus phase. Basalts with a high concentration of K

are characterized by an expansion of the olivine crystallization field

(up to P = 35 kbar). Non-magnetic flogopite (silicate phases, which

existing in association with picroilmenite) crystallizes under in-

creased contents of fluid phase (H

O, CO

) in near-liquids condi-

tions up to pressure 25 kbar.

THE INFLUENCE OF LOW-TEMPERATURE

OXIDATION OF MAGNETITE GRAINS

ON THE INTERPRETATION OF ROCK

MAGNETIC PARAMETERS

A.J. VAN VELZEN

Paleomagnetic Laboratory Fort Hoofddijk, University of Utrecht,

Budapestlaan 17, 3584 CD Utrecht, The Netherlands; velzen@geof.ruu.nl

The magnetic properties of magnetite grains change under the in-

fluence of low-temperature oxidation, giving rise to erroneous inter-

pretation of some of the widely used rock magnetic parameters. Sur-

face oxidation of magnetite may occur under atmospheric condi-

tions at ambient temperature. Weathering is the most common ex-

ample of low-temperature oxidation. This study shows how to

recognize the presence of low-temperature oxidation, investigates

whether it is related to weathering and proposes a method to esti-

mate the original magnetic properties of the rock.

Oxidation of magnetite grains at ambient temperature under mild-

ly oxidizing conditions is initially confined to the surface layer of

the grains. Considerable internal stress results and causing an in-

crease of coercivities. Related rock magnetic properties are modi-

fied accordingly (van Velzen & Zijderveld 1995). Heating to moder-

ate temperatures (150

C) reduces the coercivities to values

expected for unoxidized magnetite. These are the same values found

in samples that are demonstrably less affected by weathering. The

changes after heating to 150

C can serve as a test for surface oxida-

tion of magnetite grains.

The 150

C heating effect was initially discovered in only slightly

weathered outcrops of marine sediments (van Velzen & Zijderveld

1992), but was also recognized in samples from the surface of vol-

canic rocks. So far, it was only found in pure magnetite, not in tita-

nomagnetite. Loess is an interesting study object in this respect.

During the history of formation of a loess-paleosol sequence, there

are many opportunities for low-temperature oxidation. Recent

weathering in the outcrop is the most likely cause, but oxidation

might also occur during wind transport before deposition, during

soil formation or in a later stage due to exposure to groundwater.

In loesses from several areas, the reduction of coercivities after

heating to 150

C has been observed. The effect of the enhanced co-

ercivities on other rock magnetic parameters depends on the grain

size distribution of the magnetite in each case. The most common

effects are an increase of IRM, a decrease of ARM and a decrease of

magnetic susceptibility. Often used parameters such as S(100 mT)

or S(300 mT) (back field remanences of saturation IRM) will indi-

cate an erroneously large amount of high-coercivity minerals such

as goethite or hematite in the rock, while the high-coercivities are in

fact due to slightly oxidized magnetite.

The first aim is now to separate the effects of recent weathering

from possible earlier low-temperature oxidation. This can be done

by comparing the properties of samples with different degrees of

weathering. The timing of the oxidation can have important conse-

quences for the interpretation of variations in rock magnetic param-

eters. In the simple case of recent weathering, only a correction of

rock magnetic parameters is required. If the low-temperature oxida-

tion is not recent, but took place before or shortly after deposition of

the loess, the degree of oxidation can possibly be a direct indicator

of paleoclimatic circumstances.

References

Van Velzen A.J. & Zijderveld J.D.A., 1992: A method to study alterations of

magnetic minerals during thermal demagnetization applied to a fine-

grained marine marl (Trubi formation, Sicily). Geophys. J. Int., 110, 7990.

Van Velzen A.J. & Zijderveld J.D.A., 1995: The effects of weathering on single domain

magnetite in Early Pliocene marine marls. Geophys. J. Int., 121, 267278.

SHOCK-INDUCED MAGNETIZATION

(SIM) AT 10 AND 20 GPa

ON GIBEON IRON METEORITE

M. FUNAKI

, Y. SHONO

and T. YAMAUCHI

National Institute of Polar Research, Tokyo; funaki@nipr.ac.jp

Tohoku University, Sendai Japan

Shinshu University, Matsumoto Japan

227

Introduction

All meteorites have experienced shocks when their parent bodies

were crushed by hypervelocity-collisions among asteroids. We have

almost no information that the natural remanent magnetization ac-

quired on the primordial asteroid can survived the shocks. In order

to estimate a possibility shock loading magnetization for meteorite,

two disks prepared from a Gibeon iron meteorite (octahedrite) were

examined at 10 and 20 GPa produced by explosive gun.

A projectile of aluminum or stainless steel 1cm in diameter col-

lided with the target (10 mm in diameter and 2 mm thick) of Gibeon

at 10 GPa or 20 GPa respectively. The magnetic field was 37.5 T (I

= 55.1, D = 271.4) at the sample holder made of iron steel and cop-

per, although it could not be measured in the holder due to too nar-

row space. Shock was loaded toward the perpendicular with the

disks. The samples were removed from the holder using a lathe with

magnetization of less than 5 Oe.

Experimental results

Two disk samples of A and B were demagnetized up to 100 mT

by AF demagnetization. Their NRM intensity decayed from

7.365

to 3.830

/kg for sample A and from 1.480

to 1.922

/kg for sample B. Shocks of 10 and 20 GPa were

loaded to the samples A and B respectively. The magnetization in-

creased consequently to 8.871

/kg (I = 4.8, D = 222.6) for

sample A and to 3.349

/kg (I = 5.2, D = 347.4) for sample

B. When the sample A was cut into 4 subsamples, their directions of

remanence scattered maintaining low inclination. This characteristic

was also observed in the sample B. In general, the intensities of re-

manence of the subsamples acquired at 20 GPa were larger than

those acquired at 10 GPa.

Discussion

If the sample acquired some magnetization referred to the ambient

magnetic field triggered shock, the remanence should be acquired to

the field direction. The scattered directions of remanence among the

subsamples may suggest that the remanence is acquired disregarding

the ambient magnetic field. Although we have estimated that these

samples acquired magnetization as a shock-induced magnetization by

hypervelocity collision, the shape anisotropy of samples should be

considered to elucidate the acquisition mechanism.

ON THE SUSCEPTIBILITY OF AN

ASSEMBLE OF INTERACTING SD GRAINS

B.E. LAMASH*, V.P. SHCHERBAKOV and N.K. SYCHEVA

Dept. of Physics, Far East State University, 8 Suhanova str.,

690600 Vladivostock, Russia; *lamash@phys.dvgu.ru

Monte Carlo method (MCM) is used last years for analysis of

TRM, CRM and VRM of SD interacting grains randomly distributed

within a specimen. As is well known this method should be used

with care to obtain reliable results. An independent way to testify

the results of the MCM is comparison of the results with those ob-

tained by more rigorous approach.

According to the Boltzmanns distribution the susceptibility:

exp(-E

/kT)/

exp(-E

/kT) (1)

where m

is a total magnetic moment, E

is the energy of the ensem-

ble where the summation is done over all possible configurations

numbered by symbol

. Let the number of the particles in the en-

semble to be N. Because of number of the possible states

increases

in geometric progression N

, use of the relation (1) in practice is

restricted by relatively small N. For example for N=30 we have

≈

which set a limit of calculations of

for N >30 by

means of the eq. (1) even for powerful computers. Additional com-

plexities in direct calculations of

result from the necessity of aver-

aging the result over the possible space distribution of grains which

strongly reduces a possibility to carry out rigorous analysis.

Let consider first a system consisting from only two oriented SD

grains with equal magnetic moments m. The result of their mutual

interaction on

depends on their relative space disposition: f.e. if

the second particle is placed near the pole of the first one, the sus-

ceptibility increases as compared with the susceptibility of non-

interacting grains

. On the other hand if the second particle is

placed near the equator of the first one, the susceptibility decreases.

Integration over the space gives 0.84<

χ/χ

<1 depending on the

strength of the interaction. In this simple example the magnetostatic

interaction leads to reduction of the susceptibility though the value

of the reduction is very modest.

Next numerical calculations of

according to eq. (1) for N=(2

20) of non-oriented particles with size d=40 nm at T =460 °C were

carried out. The space averaging of the values of

was done by ran-

dom selection of 10,000 space configurations of the particles. The

results showed a tendency of decreasing the susceptibility with in-

creasing N, the stronger are the interactions, more significant is the

decrease. However the tendency is rather gentle: even for strong in-

teractions at the volume concentration

=5% the decrease is no

more than 50%. For moderate concentrations c=(0.52)% the de-

pendence

(N) shows saturation; hence we can suggest that for big

N, representing a macro-ensemble, the relation

χ/χ

would be close

to one. If so, then the magnetostatic interactions do not significantly

change the thermodynamic value of the susceptibility as compared

with the susceptibility of the non-interacting systems. This deduc-

tion agrees with the conclusion of Shcherbakov et al. (1995) when

modelling the TRM acquisition by the MCM: the only important re-

sult of the interactions is the increase in the blocking temperatures

of the grains compared with the non-interacting case.

Fig. 1.

Fig. 2.

228

The blocking temperatures are determined by the kinetics of the

attainment of the steady state by the susceptibility. For the non-in-

teracting system with identical grains

(t) =

(1-exp(-t/

)) where t

is time and

is the relaxation time for reaching equilibrium. For the

example given above

= 1s at the condition that the potential barri-

er is determined by shape anisotropy with the demagnetising factor

=1. However if the interactions are taken into account, a number

of grains with

exceed

by many orders of value and the depen-

dence

(t) approaches log(t).

The general conclusion is that the magnetostatic interactions do

not affect the value of the susceptibility significantly but drastical-

ly change the spectrum of relaxation times which leads to quasi-

logarithmic increase of VRM with time even for the ensemble of

identical grains.

References

Shcherbakov V.P., Lamash B.E. & N.K. Sycheva, 1995: Monte-Carlo mod-

elling of thermoremanence acquisition in interacting single-domain

grains. Phys. Earth Planet. Inter., 87, 197211.

THE CONCEPT OF

PARTIAL SUSCEPTIBILITIES

T. VON DOBENECK

University of Bremen, Germany; dobeneck@uni-bremen.de

Environmental magnetic studies of marine and terrestrial sus-

ceptibility records usually draw additional information from min-

eral and grain-size selective rock magnetic parameters such as an-

hysteretic (M

), isothermal (M

) and high field (M

hir

) remanence

and frequency dependent susceptibility (

). A new multivariate

method for the quantitative analysis of the carriers of susceptibili-

ty is proposed, which has many advantages over traditional verbal

interpretations.

Susceptibility is carried by discrete rock magnetic fractions, e.g.

SP, SD, MD magnetite, high-coercive, para- and diamagnetic min-

erals. Consequently, the experimentally observed volume suscepti-

bility

obs

can be expressed as a sum of (fictive) partial suscepti-

bilities:

obs

para

dia

The above cited selective rock magnetic parameters

, M

hir

are uncalibrated linear concentration measures for four of these

fractions. Their individual contributions to susceptibility can be ap-

proximated by multiplication with suitable, but a priori unknown

calibration factors

4. The sum of all contributions should then

be a predicted susceptibility:

pre

hir

The five unknown coefficients

4 can be determined from

the studied sample set itself, by solving the second equation for

obs

instead of

pre

, using a multiple regression algorithm. Obvi-

ously, most terms of the above equations can be identified:

≅

hir

≅

, and

≅

dia

. The values

obtained as calibration coefficients vary with magnetic lithology

and experimental settings (acquisition fields etc.). They should

therefore be neither generalized nor applied to different sample

sets, but hold diagnostic value.

As the given example, a pelagic sediment core (GeoB 1505-2)

from the central equatorial Atlantic, illustrates, it is possible to

Fig. Compared to SPECMAP, a global climate reference (fat line), the ob-

served susceptibility signal

obs

(circles) of central equatorial Atlantic core

GeoB 1505-2 shows similar as well as dissimilar pattern sections. To ana-

lyze this complex record quantitatively, partial susceptibilities of four mag-

netic fractions (SP, SD, MD magnetite, hematite) were determined by cali-

brating (formulas given in graph) their corresponding magnetic parameters.

By summation, they yield the predicted susceptibility

pre

(gray), which co-

incides very well with

obs

. Transformed into percentages of

pre

, relative

partial susceptibilities document internal mineral and grain-size shifts of

the magnetic mineral assemblage. In this specific case, the visible loss of

the finer magnetite fractions in several core sections deposited within cold

oxygen isotope stages (gray bands and numbers) is indicative for partial

magnetite dissolution by reductive diagenesis and related to enhanced or-

ganic deposition. Particular lithologies such as intercalated sand and tephra

layers independently modify the susceptibility signal.

predict and thereby explain a complex susceptibility signal in

amazing detail from cumulative partial susceptibilitites, if the un-

derlying parameters represent all magnetic fractions of the sample.

A diamagnetic value of 12.4

SI, typical for calcite and wa-

ter, was found for

. To equally quantify

para

, a selective parame-

ter for paramagnetism such as non-ferromagnetic susceptibility

, the asymptotic incline of the outer hysteresis branch would

have to be included in the regression. If this laborious measure-

ment is omitted, the regression algorithm will still correct for the

paramagnetic fraction to make

pre

as close to

obs

as possible. In

consequence, the coefficients of other magnetic fractions geologi-

cally associated with paramagnetism will be augmented ac-

cordingly.

100

SP Magnetite

SD Magnetite

Hematite

PSD-MD Magnetite

= 3.97

= 0.35 M

= 0.16 M

hir

= 0.09 M

! "

100

125

150

Sand

Layer

Tephra

Layers

100

125

150

175

200

225

250

[

l S

ilit

-6

100

150

200

250

300

350

400

Age [ka]

O Stage

200

400

500

1000

1500

[

all

-3

]

100

-6

-2

-1

SPECM

[st

and.
]

100

pre

]

obs

pre

GeoB 1505-2

Central Equatorial Atlantic

100

150

200

250

300

350

400

art

l S

ptib

ilit

cent
ages

229

The transformation of concentration-dependent magnetic pa-

rameters into partial susceptibilities is linear and leaves signal pat-

terns unchanged. It enables us to compare and add individual sedi-

mentary magnetic fractions and determine their absolute

contribution to susceptibility.

ARTIFICIAL MAGNETITE-BEARING

SEDIMENTS WITH DIFFERENT INITIAL

MAGNETIC STATES OF MAGNETIC

PARTICLES: FIELD DEPENDENCIES

OF THEIR ORIENTATIONAL

MAGNETIZATION AND ANISOTROPY

V.A. SHASHKANOV and A.A. KOSTEROV*

Institute of Physics, St. Petersburg University, 1, Uljanovskaya, St. Petergoff,

198904 St. Petersburg, Russia; *kosterov@snoopy.phys.spbu.ru

To investigate a relationship between the acquisition of detrital

remanent magnetization (DRM) and the magnetic state of precipitat-

ing ferrimagnetic grains, eight sets of artificial clayey sediments (8

sediments in each set) were deposited in horizontal magnetic fields

ranging from 0 to 48 Oe. The initial states of the magnetic parti-

cles were either absolute zero state (AZS) created by heating a mag-

netic above its Curie point in zero field, or various TRMs acquired

in the magnetic fields H

from 0.5 to 300 Oe. These states were ex-

pected to cover the whole range of possible particles magnetic mo-

ments. The magnetic phase was represented by submicron magnetite

grains extracted from the crushed iron ore deposit of Kostomuksha

(Karelia). Concentration of the magnetic phase in the sediments was

taken about 0.7 weight per cent.

Analysis of the experimental results has led to a conclusion that

the field dependencies of detrital remanence magnetization (DRM)

cannot generally be treated as emerging from a single Langevin-

type deposition process. Instead, it was shown that all the data on

DRM (H

, H

) field dependencies could be considered as a set of bi-

modal Langevin processes as follows:

DRM(H

) = I

Lang (H

/ H

chaot,1

) + I

· Lang (H

/ H

chaot,2

);

Lang (X) = coth (X) 1 / X.

Here I

and I

are the limit magnetizations of the first and second

modes, and H

chaot,1

and H

chaot,2

are their respective chaotization pa-

rameters. The values of these parameters in function of the thermo-

magnetization field H

are given in the Table.

suggesting that some PSD-like mechanism is at the origin of the

magnetization of this mode. The second mode, whose moment I

shows a regular increase with thermomagnetization field H

, would

then be a result of the true, i.e. taking place by a displacement of

domain walls, thermoremanent magnetization process.

The orientational magnetic anisotropy, defined as the ratio of an-

hysteretic remanences acquired parallel and orthogonally to the sed-

imentation field respectively (Kosterov & Shashkanov 1996), for

these sediments approached values of about four in sedimentation

field of 48 Oe. In terms of the above model this can be readily inter-

preted as a direct evidence for the predominantly uniaxial intrinsic

anisotropy of magnetite grains belonging to the second (highly

aligned) mode.

References

Kosterov A.A. & Shashkanov V.A., 1996: A phenomenological model of ori-

entational magnetic anisotropy of sediments. Geophys. J. Int., 125,

149162.

A LARGE GYROMAGNETIC EFFECT

IN GREIGITE

A. STEPHENSON

and I.F. SNOWBALL

Dept. of Physics, University of Newcastle, Newcastle upon Tyne,

NE1 7RU, England; alan.stephenson@ncl.ac.uk

Dept. of Quaternary Geology, Lund University, Tornavagen 13,

S-223 63 Lund, Sweden; ian.snowball@geol.lu.se

The mineral greigite is often found in marine and freshwater

sediments and contributes to the NRM of such material since it ac-

quires a CRM. Two sediment samples used here were from (a)

Bjorkerods Mosse and (b) Bare Mosse in Sweden and were taken

from horizons known to contain high concentrations of greigite.

The samples were made by embedding the sediment in resin and

cutting out cubes of side 1 cm.

RRM and ARM acquisition curves were measured up to peak

fields of 80 mT at a rotation frequency of 95 revolutions per second

(rps). While the ARM (0.07 mT direct field) increased almost linear-

ly with peak field, the RRM increased approximately exponentially.

The effective field Bg, defined in this case as 0.07

RRM/ARM, was

about 1.2 mT for the two samples after the application of an AF of

80 mT peak. These values are about 10 times higher than those pre-

viously observed for fine-grained magnetite of about 1 micron in

size. It should be noted that although greigite is the dominant ferri-

magnetic mineral present in these samples, other studies have

shown that low concentrations of detrital multidomain magnetite are

also present. Magnetite has always been found to have a lower value

of Bg than this and thus the high value quoted here must be regarded

as a lower limit for greigite.

Measurements of RRM and ARM were then carried out at differ-

ent rotation rates using an AF of 80 mT peak and the usual 0.07 mT

direct field for the ARM. The results were compared with those for a

2.24.4 micron fraction of natural magnetite which had been mea-

sured some years ago but was remeasured here, partly as a calibra-

tion check. As expected, the magnetite fraction gave a low negative

RRM at low rotation rates. (RRM antiparallel to the rotation vector

of the spinning sample is defined as negative). At about 5 rps the

RRM reversed sign and at about 35 rps it once more became nega-

tive. Above 50 rps (AF frequency 50 Hz), the RRM became positive

and much stronger. These results are similar to previous measure-

ments on magnetite and magnetite-bearing rocks. The ARM, which

was measured simultaneously, was almost constant as expected,

Oe 0 (AZS) 0.5 1.0 2.0 5.0

15 100 300

,a.u.

143

94.5 39 36.5 93.5 42.5 93.5 90

chaot,1

,Oe

,a.u.

1.5

34 49.5 89

chaot,2

, Oe 0.8

0.6 0.7 1.55 0.9 1.25 1.3 2.05

The two modes that can be identified in the DRM field dependen-

cies might be related to peculiarities of the initial thermomagnetic

states of magnetite grains contained in our sediments. The magnetic

moment of the first mode, characterized by a strong H

-independent

chaotization field, reaches its maximum in the absolute zero state,

230

showing no dependence on either rotation frequency or RRM. Bg

for this magnetite sample under the above conditions was 0.028 mT,

in agreement with values obtained previously.

Unlike the magnetite sample, both greigite samples had a nega-

tive RRM at all rotation frequencies below 50 rps but, like the mag-

netite sample, there was a large increase in RRM from low negative

to large positive values above 50 rps. In addition unlike the magne-

tite, the ARM was not constant but approximately halved as the ro-

tation frequency increased from below to above 50 rps and the RRM

not well known. Therefore, we plan to further investigate the mag-

netic properties of natural and synthetic greigite and pyrrhotite.

Schoonen & Barnes (1991) designed a method to synthesize py-

rite hydrothermally at easily controlled reaction conditions. Dekkers

& Schoonen (1994, 1996) adapted this method to synthesize greigite

at 140

C and pyrrhotite from 190

C upward. This method yields

precipitates that can easily be handled during subsequent rock mag-

netic experiments. We plan to adjust this method to investigate the

dependency of mineral magnetic properties on formation conditions

and grain-size. For example, the high temperatures and fairly alka-

line conditions of the Dekkers & Schoonen (1994, 1996) hydrother-

mal synthesis may not represent natural formation conditions, but

synthesis at low pH can be done at a lower temperature (e.g.,

Yamaguchi & Wada 1970).

References

Dekkers M.J., 1988: Magnetic properties of natural pyrrhotite Part I: Behav-

iour of initial susceptibility and saturation-magnetization-related rock-

magnetic parameters in a grain-size dependent framework. Phys. Earth

Planet. Inter., 52, 376393.

Dekkers M.J., 1989: Magnetic properties of natural pyrrhotite. II. High- and

low-temperature behaviour of J

and TRM as function of grain size.

Phys. Earth Planer. Inter., 57, 266283.

Dekkers M.J. & Schoonen M.A.A., 1994: An electrokinetic study of synthetic

greigite and pyrrhotite. Geochim. Cosmochim. Acta, 58, 41474153.

Dekkers M.J. & Schoonen M.A.A., 1996: Magnetic properties of hydrother-

mally synthesised greigite (Fe

)-I. Rock magnetic parameters at room

temperature. Geophys. J. Int., 126, 360368.

Canfield D.E. & Berner R.A., 1987: Dissolution and pyritization of magnetite in

anoxic marine sediments. Geochim. Cosmochim. Acta, 51, 645659.

Menyeh A. & OReilly W., 1991: The magnetization process in monoclinic

pyrrhotite (Fe

) particles containing few domains.

Morse J.W., Millero F.J., Cornwell J.C. & Rickard D., 1987: The chemistry

of the hydrogen sulfide and the iron sulfide systems in natural waters.

Earth-Science Reviews, 24, 142.

Passier H.F, Middelburg J.J., van Os B.J.H. & de Lange G.J., 1996: Diagenetic

pyritisation under eastern Mediterranean sapropels caused by downward

sulphide diffusion. Geochim. Cosmochim. Acta, 60, 751763.

Passier H.F., Middelburg J.J., de Lange G.J. & Böttcher M.E., 1998: Modes

of sapropel formation in the eastern Mediterranean: some constraints

based on pyrite properties. Mar. Geol., in press.

Roberts A.P. & Turner G.M., 1993: Diagenetic formation of ferrimagnetic

iron sulphide minerals in rapidly deposited marine sediments, South Is-

land, New Zealand. Earth Planet. Sci. Lett., 115, 257273.

Roberts A.P., Stoner J. & Richter, C., 1998: Diagenetic magnetic enhancement of

sapropels from the Eastern Mediterranean Sea, Mar. Geol., in press.

Schoonen M.A.A. & Barnes H.L., 1991: Mechanisms of pyrite and marcasite

formation from solution: III. Hydrothermal processes. Geochim. Cos-

mochim. Acta, 55, 34913504.

Snowball I.A., 1997: The detection of single-domain greigite (Fe

) using rotational

remanent magnetisation (RRM) and the effective gyro field (B

): mineral

magnetic and palaeomagnetic applications. Geophys. J. Int., 130, 704716.

Snowball I. & Thompson R., 1990: A stable chemical remanence in Ho-

locene sediments. J. Geophys. Res., 95, 44714479.

Yamaguchi S. & Wada H., 1970: Zur Synthese von Greigite. Neues Jahrb.

Mineral. Monatsh., 139140.

became strong and positive. Thus there appeared to be an interac-

tion between the ARM and the RRM.

At present it is not clear why gyromagnetic effects (e.g. the ac-

quisition of RRM) are so strong in greigite. Such high values of Bg

have never been observed before except in the very special case of

a self reversing lithium chromium ferrite near its compensation

temperature. Such high values of Bg might enable RRM /ARM to

be used as an indicator for greigite.

VI. Environmental magnetism

MARINE GEOCHEMISTRY AND

ROCK MAGNETISM

H. F. PASSIER* and M. J. DEKKERS

Paleomagnetic Laboratory Fort Hoofddijk, Budapestlaan 17,

3584 CD Utrecht, The Netherlands; *hpassier@geof.ruu.nl

Diagenetic processes in sediments may change and even destroy

the magnetic mineralogy during or after deposition. The presence of

sedimentary organic matter and the availability of sulphate in pore

water and seawater sustain bacterial sulphate reduction in sedi-

ments. The main product of sulphate reduction is dissolved sul-

phide. Sulphide may react with dissolved reduced iron and particu-

late iron (hydr)oxides including the important NRM-carrier

magnetite (reductive dissolution; Canfield & Berner 1987). The

products of the reaction between sulphide and iron are iron sulphide

minerals such as mackinawite, pyrrhotite, greigite, and pyrite (e.g.,

Morse et al. 1987).

An example of destructive diagenesis is the reductive dissolution

of iron oxides within and below sapropels in the eastern Mediterra-

nean (Passier et al. 1996, 1998). Sapropels are recurrent organic-

rich layers centimetres to decimetres in thickness in the Neogene

sedimentary record of the eastern Mediterranean. These layers were

deposited as a result of orbitally controlled climatic and oceano-

graphic variations. Within sapropels bacterial sulphate reduction oc-

curred during and shortly after deposition of the layers. The pre-

dominant iron sulphide associated with the sapropels is pyrite.

Pyrite formation was limited by iron availability within sapropels.

This resulted in the downward diffusion of sulphide out of the or-

ganic-rich layers into the sediments directly below, causing reduc-

tive dissolution of iron oxides and pyrite formation there as well

(Passier et al. 1998).

Whereas sulphate reduction and subsequent reductive dissolution

of iron (hydr)oxides by sulphide stopped soon after burial of most

sapropels, sulphate reduction presumably still occurs at very low

rates in an extremely organic-rich Pliocene sapropel recovered dur-

ing ODP Leg 160. In this sapropel other iron sulphide compounds

than pyrite were found, and were magnetic (Roberts et al. 1998;

Passier et al. 1998).

Although pyrite is the predominant iron sulphide mineral in ma-

rine sediments, magnetic sulphides (greigite, pyrrhotite), which are

intermediates in pyrite formation, have been reported to accumulate

in marine sediments as well (e.g., Roberts & Turner 1993). This is

partly due to the recent development of rock-magnetic criteria for

their detection (e.g., for pyrrhotite: Dekkers 1988, 1989; Menyeh &

OReilly 1991; for greigite: Snowball & Thompson 1990; Dekkers

& Schoonen 1996; Snowball 1997). It is not yet clear, however,

what the exact contribution of magnetic iron sulphides to rock mag-

netic signals in sediments is. In addition, the exact influence of for-

mation conditions and grain-size on mineral magnetic properties is

231

PALEOWINDS AND MAGNETO-

CLIMATOLOGY IN SIBERIA

T. EVANS

, N. RUTTER

and J. CHLACHULA

Institute of Geophysics, Meteorology & Space Physics, University of Alberta,

Edmonton, Alberta, Canada T6G 2J1; evans@phys.ualberta.ca

Department of Earth & Atmospheric Sciences, University of Alberta,

Edmonton, Alberta, Canada T6G 2E3

Laboratory of Paleoecology, Technical University Brno, 762 72 Zlín, Czech

Republic

Our published magnetic susceptibility results (GJI, 132, 128132,

1998) indicate that the Kurtak section in southern Siberia contains a

climate proxy record that is in the opposite sence to that found in the

classic sites of the Chinese Loess Plateau. In China cold periods

produce low susceptibilities (<3

), whereas intergla-

cials are characterized by pedogenically-driven magnetic enhance-

ment. In Siberia, on the other hand, cold periods yield high suscepti-

bilities (>40

). The Kurtak susceptibility pattern (549

samples spanning a 34 m section) implies a magnetite volume frac-

tion of ~0.2% at the peak of glacial intervals (oxygen isotope stages

2 and 4), which decreases to ~0.05% in stages 1, 3 and 5. We at-

tribute the lowering of magnetic content to density sorting and de-

creased average wind vigour during interglacials, in the manner sug-

gested by Begt et al. (Geology, 18, 403, 1990) for loess in Alaska.

This suggestion is supported by sympathetic variations in the dust-

flux record from core V21146 in the north Pacific (Hovan et al.,

Nature, 340, 296298, 1989). To test this notion further, we sought a

well-defined natural analogue as a model of atmospheric dispersal.

To this end, we have initiated a study of the 1980 Mount St. Helens

ash (samples of which are archived in the Department of Earth &

Atmospheric Sciences at the University of Alberta). Preliminary re-

sults indicate that susceptibility drops by more than a factor of two

between sites situated 150 to 300 km from the vent; this agrees with

the qualitative predictions of the wind-vigour model.

Although the paleosols at Kurtak have low susceptibility values

(~10

), they do show increased frequency-dependence

of susceptibility (as determined by measurements at 0.47 and 4.7

kHz). This implies the presence of ultrafine magnetite particles near

the stable single-domain/superparamagnetic threshold (~30 nm),

which, in turn, indicates that magnetite was produced pedogenical-
ly during isotope stages 1, 3, 5 and 7.

PALEOMAGNETIC AND ENVIRONMENTAL

MAGNETIC RECORD FROM SAANICH INLET,

VANCOUVER ISLAND, BRITISH COLUMBIA

K.L. VEROSUB* and R. KARLIN

Dept. of Geology, University of California, One Shields Ave.,

CA 95616 Davis, USA; *verosub@geology.ucdavis.edu

Saanich Inlet is a fjord on Vancouver Island, British Columbia,

with a very high rate of sedimentation. In 1996, ODP Leg 169S

cored the sediments of Saanich Inlet in seven holes at two sites. The

deepest hole at each site penetrated approximately 60 meters of Ho-

locene varved sediment and 50 meters of Late Pleistocene glacioma-

rine muds. A complete suite of U-channel samples was collected

from two overlapped holes at each site. Each U-channel was sub-

jected to a ten-level alternating field demagnetization of natural re-

manent magnetization (NRM), anhysteretic remanent magnetization

(ARM) and saturation isothermal remanent magnetization (SIRM).

The measurements were done at a one-centimeter sampling interval

and resulted in a database that contains over one million paleomag-

netic vector determinations. Measurements were also made of the

magnetic susceptibility and backfield IRM ratios. All paleomagnetic

measurements were done on the automated, long-core cryogenic

magnetometer at the University of California, Davis.

High-resolution correlation between the Holocene portions of

each pair of holes was achieved by X-radiography of the varves in

the U-channels. An initial correlation with centimeter to decimeter

precision was achieved by visual inspection of homogeneous, faint-

ly laminated and well-laminated intervals seen in the X-radiographs.

The X-radiographs were then digitized with an automated densitometer,

and time series analyses of varve counts, varve widths, and systematic

lithologic variations were used to refine the correlations further. A

detailed chronology was then constructed from the varve series and

was calibrated with 35

C dates on wood and shell fragments.

After removal of a very soft drilling overprint, the sediments of

Saanich Inlet appeared to yield an excellent record of Holocene and

Late Pleistocene geomagnetic field behaviour. The Holocene por-

tion of the record contains the same declination and inclination fea-

tures that are found in the paleosecular variation record from Fish

Lake, Oregon. In addition, the ARM-normalized record of NRM in-

tensities from Saanich Inlet is similar to the relative paleointensity

record from Fish Lake as well as to the absolute paleointensity

record from Holocene lava flows in the western United States. The

Late Pleistocene to Holocene transition is clearly marked by a more

than ten-fold decrease in magnetic susceptibility and in the ARM

and SIRM intensities. Other environmental magnetic parameters

also change dramatically at this boundary. These changes in a ma-

rine sequence are similar to changes previously reported across sim-

ilar boundaries in lacustrine sequences from northern California.

The homogenous intervals in the Holocene portion of the Saanich

Inlet records are 0.1 m to 1 m thick and appear to indicate intervals

when anoxic conditions were interrupted. The environmental mag-

netic record also provides evidence for the occurrence of massive

floods during the Late Pleistocene.

PALEOCLIMATIC CORRELATIONS

BETWEEN ROCK MAGNETIC PROPERTIES

AND OXYGEN ISOTOPES OF SEDIMENTS

FROM THE TANNER BASIN, CALIFORNIA

BORDERLAND (ODP LEG 167 SITE 1014)

F. HEIDER

, J.M. BOCK

, J.P. KENNETT

and I. HENDY

Institut fuer Allgemeine und Angewandte Geophysik,

Ludwig-Maximilians-Universitaet, Theresienstr. 41,

80333 München, Germany;

*fheider@rockmag.geophysik.uni-muenchen.de

Department of Geological Sciences, University of California,

Santa Barbara, CA 93106, USA

The top 20 meters of the sediments at Site 1014 in the Tanner Ba-

sin (California Borderlands) were sampled at 511 cm resolution for

rock magnetic and stable isotope investigations. The Brunhes-

Matuyama boundary lies 65 meters below the seafloor. AF demag-

netization of single specimens every 10 cm did not show any excur-

sions during the Brunhes chron. Remanence acquisition over an

extended time period or a strong paleoclimatic influence on the

magnetic properties of the sediment could be the reason. A complete

oxygen isotope record was determined with 10 cm resolution on

benthic foraminifera (Uvigerina) between stage 6 (170 ka) and the

232

Holocene. Each of the 200 paleomagnetic samples from the top 20

meters was given an anhysteretic remanent magnetization (ARM)

which was subsequently demagnetized in an alternating field of 15

mT. The ratio of ARM/ARM (15 mT) was used as a measure of mag-

netic stability. A comparison of this ARM ratio with the oxygen iso-

tope record shows a clear correlation between the two. The magneti-

cally harder minerals occur 0.5 m to 1 m below the interglacial

stages. The magnetically softer grains are found in high concentra-

tion at the transition from warmer to colder intervals (e.g. stage 5.e

to 5.d). The increased magnetic input at the transition from intergla-

cial to glacial intervals may be due to enhanced erosion when sea-

level decreases. Stepwise thermal demagnetization of isothermal re-

manent magnetization shows the presence of titanomaghemite and

magnetite in varying proportions along the cores. Therefore the rock

magnetic properties of the sediments from the Tanner Basin seem to

be controlled by two climatically controlled fluxes. Larger size tita-

nomaghemites dominate during dropping sealevel, while small size

magnetite grains control the magnetic properties during warm peri-

ods. Most of the 24 Dansgaard-Oeschger cycles, first observed in

the Greenland ice-core GRIP, appear to be resolved by the rock

magnetic ARM ratio.

MAGNETIC CHARACTERISTICS

OF A LOESS/PALAEOSOL SECTION

IN NORTH-EASTERN BULGARIA

D. JORDANOVA* and G. YANCHEVA

Geophysical Institute BAN, Acad. Bonchev str., bl. 3, 1113 Sofia,

Bulgaria; *vanedi@geophys.acad.bg

Part of the rock-magnetic data in the present contribution are de-

scribed in a paper submitted for publication (Jordanova & Petersen,

submitted). This part will be only briefly discussed here.

The studied loess/palaeosol section is situated in NE Bulgaria

(near the village of Koriten), where the thickest loess profiles on

Bulgarian territory are found. It covers the complete sequence of

seven loess beds and six interbedded palaeosols with a total thick-

ness of 34 m. Using a high-resolution susceptibility curve, obtained

from measurements on loose bore-hole material, complementary

samples of solid non-oriented pieces were cut, in order to study fur-

ther stability and viscous behaviour of undisturbed samples.

The magnetic mineralogy inferred from the carried out thermal

demagnetization of induced magnetization is dominated by the pres-

ence of magnetite/maghemite. The particular behaviour, observed in

the Holocene soil So is characterized by clear indication about

(characteristic of a significant) maghemite contribution. Upon heat-

ing, a new strongly magnetic phase is created, promoted by the pres-

ence of organic matter. The first three palaeosols (S1, S2, S3) which

are of chernozem type, experienced significant low-temperature ox-

idation, resulting in almost complete transformation of in situ pro-

duced magnetite to maghemite. In older palaeosols low-temperature

oxidation probably causes a development of stress-controlled

grains, which leads to a decrease in the effective grain-size (Van

Velzen & Zijderveld 1995).

The first three palaeosol units are characterized by higher coer-

civities (H

, H

) in comparison with older palaeosols and lower con-

tent of SP particles (deduced from Xfd(%) and X/J

variations) (For-

ster et al. 1994; Banerjee 1994; Heller & Evans 1995). Viscous

decay experiments, conducted for a period of 24 days storage in a

mu-metal space, reveal an increasingly higher portion of NRM de-

stroyed, reaching 100 % for units S4, L5, S5, L6 and S6. In order to

specify the possible influences of different coercivity spectra of fer-

rimagnetic carriers in each unit on the one hand, and different expo-

sure time of samples, belonging to units of different age (up to M/B

age of L7) on the other, we have obtained a viscosity index, using

the Thellier methodology (reference: Thellier E. & Thellier O. 1959;

Sholpo 1964) (equal exposure time along and opposite to the local

magnetic field). Thus, excluding the time factor, representative sam-

ple of each unit acquire laboratory VRM. According to the results

obtained so far, samples from S2 and S3 show maximum viscosity,

while S4, S5 and S6, in spite of their enhanced SP content (high

Xfd(%) and X/J

values) are not capable of significant viscous ac-

quisition (Sv in the range 020 %). AF-demagnetization of acquired

laboratory VRM gives additional data about the coercivity of the

viscous fraction. The lowest stability (expressed by MDF values)

are inherent to the old loesses and pedocomplexes (S4L7) and the

highest to L1, L2 and S1. As could be deduced from the distribu-

tion of data points on the SvMDF plot, the lowest stability is asso-

ciated with the lowest viscosity coefficient, which suggests that in

older palaeosols only a small part of viscous SD/SP grains have re-

laxation times, corresponding to the critical volumes for remanence

acquisition.

The following conclusions could be drawn from the carried out

investigations:1) The younger palaeosols (SoS3) show effectively

coarser magnetic grain sizes, which is expressed by their higher

ability for viscous acquisition, lower values of Xfd and X/J

parame-

ters, accompanied by high coercive forces.

2) The main part of the pedogenic ferrimagnetic fraction in the

old palaeosols (S4, S5, S6) is in the SP domain state, while viscous

acquisition is very restricted. One possible reason for that could be

the presence of internal stresses, due to maghemitization processes,

which cause stabilization of primary remanence.

References

Banerjee S.K., 1994: Contributions of fine-particle magnetism to reading the

global paleoclimate record (invited). J. Appl. Phys., 75, 10, 59255930.

Forster Th., Evans M. & Heller F., 1994: The frequency dependence of low

field susceptibility in loess sediments. Geophys. J. Int., 118, 636642.

Heller F. & Evans M.E., 1995: Loess Magnetism. Reviews of Geophysics, 33,

2, 211240.

Jordanova D. & Petersen N., submitted: Palaeoclimatic record from loess-soil

section in NE Bulgaria. Part I: Rock-magnetic properties. Geophys. J. Int.

Sholpo L., 1964: Role of viscous magnetization in natural remanence of

rocks. Geophys. Explorations, Leningrad, 100117 (in Russian).

Thellier E. & Thellier O., 1959: The intensity of earths magnetic field dur-

ing historical and geological time. Izv. AN USSR, No. 9 (in Russian).

Van Velzen A.J. & Zijderveld J.D., 1995: Effects of weathering on single-domain

magnetite in Early Pliocene marine marls. Geophys. J. Int., 121, 267278.

MAGNETOSTRATIGRAPHY

AND ENVIRONMENTAL MAGNETIC

RECORD OF THE CIROS-1 CORE,

ROSS SEA, ANTARCTICA

K.L. VEROSUB*, G.S. WILSON, A.P. ROBERTS,

L. SAGNOTTI and F. FLORINDO

Dept. of Geology, University of California, One Shields Ave,

CA 95616 Davis, USA; *verosub@geology.ucdavis.edu

In 1986, cores were obtained to a depth of 702 m from the CIROS-

1 drill hole beneath the Ross Sea in Antarctica. Glaciogene sediments

identified near the base of the hole mark the earliest known record of

Antarctic glaciation. Initial biostratigraphic analysis indicated that the

lower 336 m of the core was early Oligocene in age, and that the up-

per 366 m was of late Oligoceneearly Miocene age. Recently, the

233

chronology of the CIROS-1 core has been questioned. We have de-

veloped a magnetostratigraphy for the lower 400 m of the CIROS-1

core to clarify the chronology. Our magnetobiostratigraphic results

indicate that the base of the CIROS-1 core is early late Eocene in age.

We identify the Eocene/Oligocene boundary at about 410420 m,

which makes the CIROS-1 core the highest latitude site from which

this datum event has been recognized. We recognize three major cli-

matic episodes in the CIROS-1 core: 1) the late Eocene (430702 m),

when relatively warm conditions dominated with high sedimentation

rates and some glacial activity; 2) the late Eocene/early Oligocene

boundary interval (340430 m), which was a transition from relative-

ly warm to cooler conditions that coincided with glacial intensifica-

tion, sea level fall, and subaerial erosion of the shelf, and; 3) the late

Oligocene-early Miocene (50340 m), when large-scale glaciation

dominated the region, with glaciers grounding across the continental

shelf. From correlation with global oxygen isotope and sea level

records, we infer that the Antarctic climate and surrounding oceans

cooled after separation of Australia and Antarctica and development

of deep-water circulation between them.

The Eocene-Oligocene boundary has recently been recognized as

a critical time for Antarctic climatic evolution. In conjunction with

the magnetostratigraphic study, we conducted an environmental

magnetic study of the lower half of the CIROS-1 core. Measured

magnetic properties included magnetic susceptibility, intensity of

natural and artificial remanences, hysteresis parameters and mag-

netic anisotropy. The data show a clear magnetic signature, with an

alternation of intervals of high and low concentration of magnetic

minerals. The boundaries of these intervals do not correspond to the

lithostratigraphic zonation of the core. Pseudo-single domain mag-

netite is the main magnetic mineral throughout the sequence. Sharp

decreases in magnetite concentration correspond to changes in the

clay mineralogy close to the Eocene/Oligocene boundary, reflecting

a transition to physical weathering under a cooler climate. We con-

clude that large amounts of detrital magnetite were shed from the

continent into the basin during periods of intense weathering in a

relatively warm and humid climate, whereas small amounts of mag-

netite were deposited in a relatively cold and dry climate. By this in-

terpretation, the rock magnetic properties may be used to trace the

alternation of gross and small scale fluctuations in the Antarctic pa-

leoclimatic regime.

ENVIRONMENTAL/CLIMATIC STUDY

ON CLASTIC SEDIMENTS DEPOSITED

IN KULNA CAVE, CZECH REPUBLIC

DURING THE LAST GLACIAL

AND INTERGLACIAL

P. SROUBEK*, J. DIEHL, J. KADLEC, K. VALOCH

and F. HELLER

Dept. of Geology, Michigan Technol. Univ., MI 49931 Houghton,

MI, USA; *pasroube@mtu.edu

Mineral magnetic properties of loess/paleosol sequences are very

sensitive indicators of climatic change. Cave entrances offer thick

deposits of loess or loess like sediments. Their stratigraphy, trans-

port and mode of deposition may be more complex than in surficial

deposits, nevertheless they are intriguing to investigate. We present

the results of a study of sediments deposited in the entrance of Kul-

na Cave, Czech Republic. The goal of this study is to determine the

source of the cave sediments as well as the mode of transport, and to

decipher the climatic/environmental signal which they carry. We

have been using mineral magnetic methods supported by X-ray dif-

fraction, heavy mineral analysis and quartz exoscopy.

Kulna Cave is located near the northern margin of the Moravian

Karst. It has a tunnel like shape and is situated on the side of a river

valley. Its large entrance contains a 15 m thick sequence of stratified

clastic sediments. These sediments consist of loess-like deposits

with varying amount of limestone debris which overlie fluvial silts

and gravels. Accumulation occurred between the Last Interglacial

and the onset of the Holocene. Previous work (Valoch 1988), based

primarily on paleontological and archaeological findings led to a

climatic/environmental reconstruction that has recently been corre-

lated to the stable oxygen isotope record (Valoch 1992). The results

of preliminary mineral magnetic measurement are described in

Sroubek et al. (1997).

We visited Kulna Cave several times in the last 4 years and col-

lected approximately 600 samples throughout 6 major profiles lo-

cated in the cave entrance.

Magnetic susceptibility (MS) was measured in all the samples.

Other mineral magnetic parameters (FDMS, ARM/MS, ARM/SIRM,

S-ratio) were measured throughout one composite profile including

all the major layers. Paramagnetic susceptibility, thermal behavior

of SIRM, anisotropy of MS (AMS), directions of remanence and

mineral composition (X-ray diffraction) were measured in several

samples from each layer.

MS shows in general the same trends in all profiles. In the upper

layers (Last Glacial), MS varies between 20 and 50

/kg. The

interglacial sediments in the lower part of the section have fairly

constant MS around 10

/kg. Paramagnetic susceptibility

contributes in the upper layers 1025 % and in the lower more than

40 %. In the upper part of the profile FDMS and SIRM show a very

similar trend to MS. Magnetic grain-size (ARM/MS and ARM/

SIRM) is fairly constant in the upper part of the profile, but an in-

crease in grain-size can be seen in the interglacial sediments. The S-

ratio varies between 0.850.90 in the upper part of the profile and

drops to values of 0.650.80 in the interglacial sediments.

Thermal demagnetization of SIRM indicates the presence of goet-

hite, magnetite and/or maghemite and hematite. X-ray diffraction

analysis on one magnetic separate confirmed the presence of goet-

hite and magnetite/maghemite.

Fig. 1. Magnetic susceptibility record throughout loess-like sediments depos-

ited in Kulna Cave during the Last Glacial Period. Full and dash line show

results from two different profiles. The sediments are mostly silt to sandy silt

(layer 7d). Layers 7a, 7d and 11b are rich in limestone debris. Layer labelling
after Valoch (1988).

234

The shape of AMS ellipsoids does suggest that in most cases the

sediments have a depositional style magnetic fabric. Maximum MS

axis in nearly all the layers have a preferred orientation which varies

within the NW-SE quadrants.

The characteristic remanent magnetization has normal directions

in all layers except for one layer near the Glacial/Interglacial bound-

ary. Both samples from this layer show excursional directions. Re-

sults of X-ray diffraction show that all the dominant minerals

(quartz, calcite, plagioclase, kaolinite and muscovite) with the ex-

ception of calcite are present in all the layers. SEM analysis of

quartz grains confirms eolian transport.

Our results suggest that variations in magnetic susceptibility char-

acterize changing source material. The originally windblown material

has undergone weathering on the surface probably in the vicinity of

the cave. Depending on the intensity of the weathering process mag-

netic enhancement and dissolution of calcite occurred. The weathered

material was then redeposited by fluvial and slope processes into the

cave. The variations in AMS may indicate a changing mode and di-

rection of transport into the cave.

References

Valoch K., 1988: Die Erforschung der Kulna Höhle 1961-1976. Anthropos

(MM Brno) 24, N.S., 16, 1204 (in German).

Valoch K., 1992: Contribution to the Stratigraphy of the Upper Pleistocene

in Moravia. Scripta (Brno), 22, 7779.

Sroubek P., Diehl J., Kadlec J. & Valoch K., 1996: Preliminary study on the

mineral magnetic properties of sediments in the Kulna Cave (Moravi-

an Karst), Czech Republic. Studia Geoph. et Geod., 40, 301312.

SEABED SUSCEPTIBILITY VARIABILITY IN

COASTAL ESTUARINE-LIKE SEDIMENTS

ANDIRON OXIDES FATE DURING EARLY

DIAGENESIS

A case study in littoral sediments from NW Spain

D. REY*, O. PAZOS, B. RUBIO, N. LOPEZ-RODRIGUEZ,

M.A. NOMBELA and F. VILAS

Dep. Xeociencias Marinas e Ordenacian do Territorio, Universidade de Vigo,

36200 VIGO, Spain; *danirey@uvigo.es

This study intends to evaluate the reliability of magnetic suscepti-

bility measurements to assess the marine influence and heavy-metal

adsorption capabilities of coastal sediments. We present a double

approach using geographically and vertically distributed data. Mea-

surement of low-field susceptibility of over 200 samples of surficial

(top 10 cm) seabed sediments of the Rias of Vigo and Pontevedra

(Fig. 1) in NW Spain (1 per square km) showed a significant in-

crease towards the open sea and away from polluted continental in-

fluenced areas. Vertical variability of the susceptibility was evaluat-

ed in 80 samples (1 every 3 cm) obtained from three 60 to 80 cm

long gravity corers in the Ria de Pontevedra. These samples showed

a very strong decrease in susceptibility with depth (Fig. 2).

The surficial susceptibility value correlated well with existing

data on their grain-size distribution, organic matter and carbonate

content. Samples were then split into their coarse and fine fractions

and susceptibility measured separately for each fraction. This result-

ed in new and very revealing spatial variability patterns suggesting

a link between sediment provenance and origin of the magnetic sig-

nal. To further evaluate these relationships, the available sedimen-

tological data were completed with elemental (ICP-AES) and min-

eralogical analysis of the clay (XRD) and sand (SEM-EDX)

505000

510000

515000

520000

525000

530000

UTM longitude

670000

675000

680000

685000

690000

695000

700000

tit

TOTAL SAMPLE SUSCEPTIBILITY

(in SI per mass units

0E+000

1E-007

2E-007

3E-007

4E-007

5E-007

6E-007

7E-007

Ria de Vigo

Ria de

Pontevedra

5 km

1.0E-8

1.0E-7

1.0E-6

1.0E-5

5 1

Core 1

Core 2
Core 3

depth (cm)

Fig. 1: Geographical location of the study area. The Galician Rias are deep

elongated embayments of about 30 km long and 12 km wide at their mouth.

Sedimentologically they function as estuarine-like environments and are rel-

atively well protected from Atlantic storms. The isolines map on the right

shows the geographical variability of the total sample susceptibility along

the Rias. In general terms, the susceptibility increases towards the open sea

and decreases towards the coastline. Mean values for both Rias varied be-

tween cca 1 and 11 E-07 SI (mass units). Sampling sites marked with a dot.

Fig. 2: Cores 1, 2 and 3 are located respectively in the external, middle and

central part of the longitudinal axis of the Ria de Pontevedra. Cores 2 and 3

showed a quick loss in susceptibility of nearly two orders of magnitude at their

upper part. The bottom part is characterized in both cores by a sharp disruption

of the precedent decreasing trend. Core 1 is characterized by a moderate vari-

ability around a central value similar to the lower part of cores 2 and 3, resem-

bling the tendency observed in the lower part of cores 2 and 3.

235

contributing to contaminants in various environments through abra-

sives which contain iron/iron-oxides and REEs. As these flint-de-

rived particles are ubiquitous in laboratories and in the environment

they should not be neglected in studies of the kinds mentioned be-

low. Our investigation should help to understand the nature of such

contaminants, which are of concern in the fields of:

(1) Magnetism (magnetic contaminations),

(2) Environmental magnetism (carriers of magnetic informa-

tion in environmental samples),

(3) Environmental Chemistry (heavy metals),

(4) REE-geochemistry (REE-distribution patterns in the geo-

sciences),

(5) Public Health (Health-Risk, toxicology).

Magneto-mineralogy and REE

By using a cigarette or gas lighter a profusion of abrasives from

the flint and the drive wheel (made of steel) is created which con-

sists of a mixture of metal shavings and spheres. The flints are man-

ufactured from pyrophoric alloys which have excellent sparking

properties. In general, they contain a high weight proportion (about

75 %) of rare earth elements (so-called mischmetal, consisting

mainly of Ce and La), about 2025 % Fe or SiFe and smaller

amounts of Cu, Mg and other elements. Due to the strong reducing

power of mischmetal, some of the particles are ignited by the heat of

friction, and they may reach fairly high temperatures and even melt

during the oxidation process. Some of the particles subsequently so-

lidify from the liquid state as nearly perfect spheres. SEM studies

revealed that many of the highly magnetic particles which formed

during sparking have spherical shapes with a large grain size distri-

bution (about 1100 m). However, very sharp and sometimes nee-

dle-like metal-shavings are also observed. EDAX and EMP analyses

reveal the presence of iron and iron-oxides with Ce- and La-con-

tents depending on the particle size. Micron-sized particles are al-

most Ce- and La-free. Thermomagnetic runs revealed a T

of about

615

C which is ascribed to a maghemite-like phase, and a smaller

portion of a magnetic phase with a T

> 700

C. This could be due to

an iron-like phase. These data are supported by IRM-, hysteresis-,

and backfield-measurements as well as X-ray analyses and micro-

scopic observations. The findings provide a new explanation for the

ubiquitous presence of spherical magnetic grains which very often

disturb high precision magnetic measurements and geochemical in-

vestigations (REE) in the laboratory.

Health effects of REEs

Distribution and some uses of REEs

REEs can be found in many products and production processes.

They are present in emissions from coal burning, electrowelding,

metallurgy and certain alloys, in Neodym-Iron-Boron magnets, in

dental instruments and orthopedics. Organocerium compounds

are used in fluid-cracking catalysts in petroleum refining and are

also considered as replacements for organolead antiknock agents for

internal combustion engine fuels. REEs also find wide applications

in fireworks, and as Ce rich alloys in gas/cigarette lighter flints.

Uptake and metabolism

REE uptake is principally possible by inhalation, orally, through

the skin, and by in vivo (i.v.) injection (Seiler et al. 1988). The most

important route of uptake is by inhalation, e.g. of fumes at the work-

place, and possibly by smoking. Aerosol particles can travel down

the respiratory tract to the alveoli. This is followed by a slow inges-

tion by macrophages, with lymphogenic transportation into other or-

gans. After i.v.-application in the animal model most of the REE are

deposited in the liver and the spleen if basic and the bones if acidic

solutions were used.

fractions. In addition some basic magnetic parameters associated

with the magnetization of saturation (M

, M

) and coercivity (H

) were obtained for selected specimens. Additional magnetic

data on representative samples comprised the study of the tempera-

ture and frequency dependence of the susceptibility. A similar pro-

cedure was applied to the gravity cores, but no granulometric frac-

tioning was carried out attending to their high content in clays. The

total fraction showed a characteristic and considerably higher (two

orders of magnitude) variation of the susceptibility with depth relat-

ed to the early diagenetic evolution of iron oxides and hydroxides.

The combined analysis of these data showed that the spatial vari-

ability of the susceptibility observed in the different granulometric

fractions can be spatially related to: (a) sediment provenance and

origin, (b) hydrodynamic regime established between the Rias and

the adjacent shelf, (c) antropogenic solid particulate pollution.

There is a strong negative correlation between elemental contami-

nants and susceptibility but no direct causality could be established.

Finally, it can also be concluded that the evolution of the magnetom-

ineralogical phases during the early stages of burial and diagenesis

is controlled by the organic matter content which in turn controls the

redox potential.

(This abstract is a contribution to projects MAR95-1953 and

MAR97-0627 of programe CYTMAR of the Spanish research agen-

cy C.Y.C.I.T.).

CIGARETTE LIGHTERS: A SIGNIFICANT

SOURCE OF ANTHROPOGENIC MAGNETIC

AND REE-BEARING AEROSOL PARTICLES

A POTENTIAL HEALTH RISK?

V. HOFFMANN

, M. HANZLIK

, M. WILDNER

P. HORN

and K.TH. FEHR

Institüt für Geologie und Palaontologie, Arbeitsbereich Geophysik,

Universitat Tübingen, Sigwartstr. 10, D-72076 Tübingen;

viktor.hoffmann@uni-tuebingen.de

*also at: Physics Department, University of Toronto/Erindale,

3359 Mississauga Road North, Mississauga L5L 1C6, Ontario/Canada

Institut für Geophysik, Universitat München, Theresienstr. 41,

80333 München

Bayrischer Forschungsverbund Public Health,

Pettenkoferstr. 35, 80799 München

Institut für Mineralogie, Petrologie und Geochemie,

Universitat München, Theresienstr. 41, 80333 München

Introduction

We report the finding of nearly perfect spheres and metal shav-

ings which contain Rare Earth Elements (REE: Ce, La, Nd, Sm, etc.)

and originate from the use of mechanical cigarette and gas lighters.

Furthermore, and of concern for those working in the field of (envi-

ronmental-) magnetism, it has been recognized that strongly mag-

netic particles may stem from the very same sources. In our ultra-

clean laboratories, aimed at magnetic and geochemical studies, such

particles are frequently observed. This kind of sphere was even

found in geological samples during laboratory studies, and was be-

lieved to be of extraterrestrial origin. This leads to profoundly mis-

interpreted REE abundance patterns (Nyquist et al. 1987). The nega-

tive health-effects of cigarette-smoking are well known, as well as

frequent disturbances in the performance of sensitive electronic in-

struments and computers as a result of cigarette smoke particles.

Less known is the fact that cigarette lighters are also significantly

236

Toxicity

Generally, the toxicity of compounds of the REEs are low, and

unlikely for uptake by mouth. The most important route of uptake

however is by inhalation as airborn pollutants, as mentioned above

(Seiler et al. 1988). Cases of rare earth pneumoconiosis and pulmo-

nary fibrosis have been reported following industrial exposure (Su-

lotto et al. 1986). Whether lanthanides can act as component carcin-

ogens by facilitating malignant transformation is unknown. Possible

pathogenic mechanisms could be related to contamination with

strongly fibroplastic Thorium, and to ionising irradiation. Whether

uptake of REEs via the respiratory tract on cigarette lighting, or dur-

ing occupational exposure, is contributing to common serious dis-

eases like lung cancer is unknown at the moment. Certainly, the as-

sociation of exposure to REEs from lighter flints with cigarette

smoke introduces a strong confounder, but may also obscure the ef-

fects of REE as a contributing cause. The synergistic effect of Ra-

don exposure together with smoking on lung cancer is well known,

and radioactive byproducts of REE could well have similar effects,

if uptake is of a significant amount. Quantification of REEs deposits

in the lung macrophages could serve as a proxy for this specific ex-

posure.

References

Nyquist L., Bansal B., Wiesmann H., Shih C. & McKay G., 1987: Isotopic

studies of shergottite chronology: II. Possible effect of contamination

on the Sm-Nd system. Lunar Planet. Sci,. XVIII, 730731.

Seiler H. G., Sigel H. & Siegel A.,1988: Handbook on toxicity of inorganic

compounds. Marcel Dekke, Inc., 76985.

Sulotto F., Romano C., Berra A., Botta G.C., Rubino G.F., Sabbioni E. &

Pierta R., 1986: Rare-earth pneumoconiosis: A new case. Amer. J. In-

dustr. Med., 9, 567575.

PECULIARITIES IN THE MAGNETIC

PROPERTIES OF THREE DIFFERENT SOIL

TYPES FROM BULGARIA

D. JORDANOVA and N. JORDANOVA

Geophysical Institute BAN, Acad. Bonchev str., bl. 3, 1113 Sofia,

Bulgaria; vanedi@geophys.acad.bg

Studying the magnetic characteristics of different soil types can

give valuable information about iron diagenesis and its depth distri-

bution. Such knowledge can highlight directions and peculiarities of

pedogenic processes in each particular case. Specific combination

of the main soil-forming factors (Jenny 1941) results in a wide vari-

ety of soil types. General occurrence of iron oxides in different soil

environments (Schwertmann 1988) as well as the obtained magnetic

properties (e.g. Maher 1986) show their high sensitivity to the

changes in soil environment. Three different soil types are presented

here: Meadow Chernozem, Leached Meadow Cinnamonic Soil and

Pellic Chernozem-like Vertisoil. Each of them is characterized by

specific behaviour of magnetic characteristics, influenced by the

type of parent material, as well as by climatic factors.

Meadow Chernozem

The section is situated on the first non-flooded terrace of Russen-

ski Lom River and formed on delluvial loess material. Magnetic sus-

ceptibility variations show significant magnetic enhancement, ob-

served also for other chernozem soil profiles (Jordanova et al.

1997). The behaviour of the hysteresis parameters (coercive force

, coercivity of remanence H

, ratios J

, H

) measured for a

few representative samples from different soil horizons suggest that

the uppermost humic horizon is enriched with more stable ferrimag-

netic minerals, while downward relative contribution of softer carri-

ers (probably in PSD state) is higher. Following the variations of

viscosity coefficient Sv, it appears that in conjunction with stable

SD ferriminerals, highly viscous SP/SD grains are also present. At

the same time, the ratio of saturation remanence to susceptibility

(SIRM/X) shows relatively uniform values, thus suggesting an in-

variable MD fraction (Thompson & Oldfield 1986). Consequently,

the main factor, determining magnetic properties, is the concentra-

tion of in situ formed pedogenic ferrimagnetic particles.

Leached Meadow Cinnamonic Soil

The second soil profile, presented in this study, is a Leached

Meadow Cinnamonic Soil, developed on the alluvial deposits of the

Maritza River in Central South Bulgaria. In contrast to the more

widely occurring case of magnetic enhancement in the soil horizon,

compared to lower values in the parent material, we observe the op-

posite behaviour. Thermal demagnetization of composite isothermal

remanence point to the presence of magnetite as a main carrier,

which is significantly affected by low temperature oxidation. The

maximum susceptibility values, found in the parent material, are

probably connected with an enhanced MD content, as far as they are

accompanied by high SIRM/X values. As a result of an increased

oxidation process towards the top of the soil, effective grain sizes

decrease, which lead to the lower susceptibility observed. More-

over, the authigenic production of the ultrafine grained fraction is

obviously very limited, as could be concluded from the low Xfd(%)

values (with a maximum of 3.9 %). In spite of this, Xfd(%) varia-

tions very successfully distinguish this part of the section, affected

by pedogenic processes.

Pellic Chernozem-like Vertisoil

Pellic Vertisoils are quite specific soil type, found at the Balkan

Peninsula, as well as at some sites in Africa, India and Indochina.

The present section is situated in the Sofia Valley. Like the previous-

ly described soil profile, the Pellic Vertisoil presented here shows

lower susceptibility values along the soil height and higher values

in the parent Pliocene clays. Having in mind the initial water

logged phase in its history, we could suppose that exactly this cir-

cumstance predetermines the soils magnetic characteristics. Lower

X-enhancement, accompanied by an increased organic content,

bringing about Fe-organic complexation, is a logical result (Schw-

ertmann 1988). The experiments carried out to investigate the coer-

civity of the remanence spectra reveal an interesting evolution.

From the top to the bottom, first a wide high-coercivity spectra ap-

pears (humic horizon), followed by a low-coercivity peak for sam-

ples of illuvial horizon, and downward a two-peak spectra are estab-

lished, obviously reflecting both pedogenic and lithogenic fractions.

The viscosity coefficient Sv also points to a maximum SP/SD contri-

bution at the bottom of the illuvial horizon. The higher SIRM/X val-

ues, permanently found in the Pliocene clays, suggest that their en-

hanced susceptibility is of MD origin. The strong visual enrichment

with Mn-concretions in the carbonate horizon could also influence

the magnetic properties.

Conclusions

1. Two equally important factors could bring about the presence

of soil profiles, where magnetic susceptibilities are lower than that

of the parent material: a) coarse grained parent material with a high

content of minerals with high weathering resistivity and b) a water

logged phase, involved in soil development.

2. The humic horizons of all three profiles studied are characterized

by the presence of a pedogenic fraction with higher coercivities.

237

References

Jenny H., 1941: Factors of soil formation. A system of quantitative pedol-

ogy. Foreword by R. Amundson, Dover Publ. Inc., New York (1994).

Jordanova D., Petrovsky E., Jordanova N., Evlogiev J. & Butchvarova V.,

1997: Rock magnetic properties of recent soils from northeastern Bul-

garia. Geophys. J. Int., 128, 474488.

Maher B.A., 1986: Characterization of soils by mineral magnetic measure-

ments. Phys. Earth Plan. Inter., 42, 7692.

Thompson R. & Oldfield F., 1986: Environmental magnetism. Allen and

Unwin, Winchester, Mass.

Schwertmann U., 1988: Occurrence and formation of iron oxides in various

pedoenvironments. In: Stucki J., Goodman B. & Schwertmann U.

(Eds.): Iron in Soils and Clay Minerals. NATO ASI Series, Series C:

Mathematical and Physical Sciences, vol. 217., Reidel Publ. Compa-

ny, 267308.

MAGNETIC MAPPING OF ROAD-SIDE

POLLUTION AND CORRELATION

WITH HEAVY METALS

M. KNAB

, V. HOFFMANN

, E. APPEL

N. JORDANOVA

and R. BECK

Institut für Geologie und Palaontologie, Arbeitsbereich Geophysik,

Universitat Tübingen, Sigwartstr. 10, D-72076 Tübingen;

*mathis.knab@uni-tuebingen.de

(V.H. also at: Physics Department, University of Toronto/Erindale,

3359 Mississauga Road North, Mississauga L5L 1C6, Ontario/Canada)

Geophysical Institute, Acad. Sci. Czech Rep., Boèní II/1401,

14131 Prague 4, Czech Republic

Geographisches Institut, Universität Tübingen, Hölderlinstr. 12,

D-72074 Tübingen

Introduction

Only few investigations are known which focus on the link be-

tween magnetism and pollution due to traffic (Flanders 1994; Hoff-

mann et al. 1989; Hopke et al. 1980; Unger & Prinz 1992). Distin-

guishing between natural magnetic minerals and man made

magnetic phases is of basic importance for such studies (Dekkers

1997; Flanders 1994). We report magnetic susceptibility (

) map-

ping along highways and detailed analyses of magnetic phases re-

sponsible for the susceptibility signal. Correlations between the

magnetic signal and the content of certain heavy metals were calcu-

lated to detect possible links between them.

Results and interpretation

Our results of susceptibility mapping along roadsides can be

summarized as follows (Hoffmann et al. 1997):

A Bartington susceptibility bridge MS2 and a D-Sensor were used.

Generally, an exponential decay of susceptibility (

) with in-

creasing distance from the highway and increasing depth from the

soil-surface on the roadsides was detected.

The factors which influence the

distribution are traffic density,

weather conditions, wind direction, topography on the roadsides and

the geological background.

Depending on the traffic density

is enhanced up to a distance of

about 25 m from the highway.

Repeated measurements within 3 weeks without any precipita-

tion (rain. snow, etc.) revealed a significant increase of the

signal

on a selected profile.

For isolated emission sources and a low background signal,

map-

ping allows a quick and cheap mapping of potentially polluted areas.

A representative profile was sampled after

mapping up to a depth

of 0.5 m. A series of mineralogical and magnetic parameters were de-

termined on these samples (hysteresis-data, IRM, ARM,

-T, M

-T).

With optical microscope at least 5 ferri(o)magnetic components were

recognized, partly revealing spherical shapes and showing dentritic-

or skeleton-structures. The latter particles are thought to be quenched

from a melting state and may be abrasion products from car brakes. In

general, an a-magnetite like phase with a large grain size range was

found in all samples. An enrichment of this phase is found at shorter

distances from the highway (<1 m), also the grain sizes depend on the

distance from the tar surface (the shorter the distances the larger the

sizes are). Consequently, air transport seems to dominate the distribu-

tion of the magnetic particles (Hoffmann et al. 1989; Lygren et al.

1984; Unger & Prinz 1992). Besides magnetite, we have also detected

hematite and goethite-like phases. In addition, a ferri(o)magnetic

phase with a Curie-temperature of about 250300

C was found

which is unknown in natural systems. We expect that this phase is of

anthropogenic origin.

The content of certain heavy metals in the same samples was

analysed quantitatively using atomic absorption spectroscopy. Val-

ues of

(determined in the lab by using a KLY2 bridge) were com-

pared with the contents of lead, cadmium, chromium, zinc, copper,

nickel, iron and manganese. Pb, Cu, Cd and Zn revealed similar

trends like susceptibility with regard to distance- and depth-depen-

dence. A different and even reverse behaviour was found for Ni, Cr

and Fe or Mn. From statistical calculations a high degree of linear

correlation with

was found for Pb, Cu, Cd and Zn, whereas low or

anticorrelation was obtained for the other heavy metals and surpris-

ingly for Fe.

Estimating or predicting the content of certain heavy metals in

roadside soils and sediments by using magnetic proxies, here sus-

ceptibility, seems to be possible under certain conditions.

References

Dekkers M.J., 1997: Environmental magnetism: an introduction. Geol.

Mijnbouw, 76, 163182.

Flanders P.J., 1994: Collection, measurement, and analysis of airborne

magnetic particulates from pollution in the environment. J. Appl.

Phys., 75, 59315936.

Hoffmann G., Scholl W. & Trenkle A., 1989: Schadstoffbelastung von

Böden durch Kraftfahrzeugverkehr; Blei, Cadmium, Auftausalze und

Kohlenwasserstoffe. Agrar- und Umwelforschung in Baden-Württem-

berg, 19, Ulmer, Stuttgart, 1103.

Hoffmann V., Knab M. & Appel E., 1997: Magnetic susceptibility mapping

of roadside pollution. J. Geochem. Int., submitted.

Hopke P.K., Lamb R.E. & Natusch D.F.S., 1980: Multielemental characteri-

sation of the urban roadway dust. Environ. Sci. Technol., 14, 164172.

Lygren E., Gjessing E. & Berglind L., 1984: Pollution transport from a

highway. Sci. Tot. Environ., 33, 147159.

Unger H.J. & Prinz D., 1992: Verkehrsbedingte Immissionen in Baden-

WürttembergSchwermetalle und organische Fremdstoffe in

stra·ennahen Böden und Aufwuchs. Umweltministerium und LFU,

Baden-Württemberg, 1191.

238

PGE (PLATINUM GROUP ELEMENTS)

CONTAMINATION OF ROADSIDE SOILS:

MAGNETIC PARAMETERS USEFUL

AS A PROXY?

C. LEVEN

, V. HOFFMANN

, M. KNAB

, E. APPEL

J. SCHÄFER

and R. BECK

Institut für Geologie und Palaontologie, Arbeitsbereich Geophysik,

Universität Tübingen, Sigwartstr. 10, D-72076 Tübingen;

*viktor.hoffmann@uni-tuebingen.de

Institut für Petrographie und Geochemie, Universität Karlsruhe,

Kaiserstr. 12, D-76128 Karlsruhe

Geographisches Institut, Univ. Tübingen, Hölderlinstr. 12, D-72074 Tübingen

Introduction

During the last 10 years, the development of the automobile-cata-

lyst technique increased permanently the percentage of catalyst-

equipped cars in Germany and Western Europe due to more restric-

tive rules concerning car exhaust fumes. The present state of the

emission control is the three-way catalyst which consists of a ce-

ramic body coated inside with PGEs (Platinum Group Elements)

such as Platinum (Pt), Rhodium (Rh) and Palladium (Pd) in various

compositions (Eckhard & Schäffer 1997). Recently, the PGEs have

represented a significant contribution to the classical traffic pol-

lutants such as heavy metals and organic contaminants (Eckhard &

Schäffer 1997; Hopke et al. 1980;Unger & Prinz 1992). Beside

some heavy metals, the contents and distribution of PGEs in soils

caused by automobile emissions were investigated at several

transects perpendicular to highways at various depths below the soil

surface in the area of KarlsruheRhine-valleyStuttgart by Cubelic

et al. (1997). They analyzed the concentrations of Pt, Rh and Pa by

using ICP-MS after separation and preconcentration by NiS-fire-as-

say. The aim of our study is to test whether magnetic susceptibility

(

) or other magnetic parameters can be used as proxies for tracing

these relatively new roadside pollutants (Hopke et al. 1980). In

addition, the concentration of certain heavy metals and possible

links with magnetic parameters are investigated.

Results and interpretation

Magnetic susceptibility mapping, using the Bartington MS2 with

the D-sensor, was performed on areas where the PGE studies were

carried out (Hoffmann et al. 1997).

data were also determined in

the lab by using Kappabridge KLY2/3 (Agico) on the PGE-sample

set as well as on new soil samples taken on parallel profiles after the

field mapping.

Generally, a strong and nearly exponential decay of the magnetic

susceptibility with distance from the highway was detected.

found to be controlled by traffic density, wind direction, road-side

maintenance and topography. The distribution of the

mapping is

very similar to the pattern which was obtained from the soil samples

measured in the lab. Resampling in certain timespans established a

clear increase in the concentration of the ferri(o)magnetic phases.

A comparison of PGE (Pt, Rh, Pd) and heavy metal (Pb, Cd, Zn,

Fe, Cu) concentration data together with

(measured by using KLY-

2) with regard to dependence of distance from the highway and

depth from the soil surface reveal the following trends:

Pt, Pa and Rh as well as Pb, Cd, Zn, Cu show a similar distribution

pattern to

in contrast to Fe and Ni. By performing statistical calcu-

lations, high degrees of correlation are found for the PGEs, Pb, Cd,

Zn, Cu and

. A very low or even negative correlation coefficient is

calculated in the case of the other heavy metals. It is interesting to

note that by using non-linear relationships, such as quadratic or ex-

ponential ratios, better correlation coefficients can be obtained for

the PGE and most of the heavy metals.

A series of magnetic and mineralogical analyses were performed

on both sample sets in order to investigate the ferri(o)magnetic

phases which contribute to the magnetic signal of the

mapping.

Susceptibility versus temperature runs on the KLY3/CS3 Kappa-

bridge reveal the presence of a magnetite-like phase as indicated by

the Verwey-transition. From high temperature runs (up to 700

C),

magnetite- and a hematite-like phases are detected. A still unidenti-

fied ferri(o)magnetic phase revealing a T

of at about 250320

was found, but it is unknown in nature. These results are supported

by X-ray and microscopic analyses which were done on magnetic

extracts.

Conclusions

Magnetic parameters such as susceptibility should not be used

directly for the detection of PGE in roadside soils inspite of the ap-

parently high degree of correlation. This is prevented by the very

low content of PGE (in the order of ppb often near the limit of

detection) and the errors of the magnetic susceptibility data. Simi-

lar transport mechanisms and paths are thought to be responsible

for the high correlation between both the PGE and

. The only

source of PGEs are catalysts (exhaust fumes), whereas the main

sources of magnetic minerals are brakes, exhausts, tires and other

parts of the vehicles as well as the asphalt. Therefore a high corre-

lation apparently or indirectly may be controlled by similar trans-

port and deposition processes (mainly air transport). Consequent-

ly, magnetic parameters could represent useful indicators for

possibly higher levels of PGEs in potentially polluted roadside

soils under certain conditions.

References

Cubelic M., Pecoroni R., Schäfer J., Eckhardt J.D., Berner Z. & Stuben D.,

1997: Verteilung verkehrsbedingter Edelmetallimmissionen in

Böden. Z. Umweltchem. ekotox., 9, 249258.

Eckhard J.D. & Schäfer J., 1997: PGE-Emissionen aus KFZ-Katalysatoren.

In: Matschullat J., Tobschall H.J. & Voigt H.J. (Eds.): Geochemie und

Umwelt. Springer, Berlin, New York.

Hoffmann V., Knab M. & Appel E., 1997: Magnetic susceptibility mapping

of roadside pollution. J. Geochem. Int., submitted.

Hopke P.K., Lamb R.E. & Natusch D.F.S., 1980: Multielemental characteri-

sation of the urban roadway dust. Environ. Sci. Technol., 14, 164172.

Unger H.J. & Prinz D., 1992: Verkehrsbedingte Immissionen in Baden-

Württemberg Schwermetalle und organische Fremdstoffe in

stra·ennahen Böden und Aufwuchs. Umweltministerium und LFU,

Baden-Württemberg, 1191.

FERRIMAGNETIC MINERALS

AND HEAVY METAL DISTRIBUTION

WITHIN DIFFERENT GRANULOMETRIC

FRACTION OF FLY ASHES

T. MAGIERA

, Z. STRZYSZCZ

, E. PETROVSKY

A. KAPIÈKA

and D. SMO£KA

Institute of Environmental Engineering PAS, Zabrze, Poland

Geophysical Institut, Academy of Sciences, Prague, Czech Republic

Department of Earth Sciences, Silesian University, Sosnowiec, Poland

Fly ashed derived from fossil-fuel combustion are the main

source of anthropogenic magnetic particles responsible for an in-

crease in magnetic susceptibility in topsoils over large areas. Foss-

239

sil-fuels are basically non-magnetic but ferrimagnetic iron oxides

are produced during combustion processes and by fly ashes are

magnetically enriched. In counties Suchas Poland, where the coal

combustion is the main source of energy, deposition of magnetic

particles occurring in fly ashes into soil is very significant. It is a re-

sult of great amount of coal barned Poland and great dust emission.

Except for power plant emission (in spite of their attennation by

electrofilters) there are also hundreds of small industrial boilers and

thousands of home chimneys emitting dusts and ashes without any

limitation. Topsoil magnetic enhancement can be observed with

more or less intensity almost over the whole territory of Poland.

Studies of fly ashes have shown that their magnetic susceptibility

varies between 350 and 3500

. These values result from

a large number of magnetic particles varying in wide range of grain

size between about 0.02

m to over 200

m (Dekkers & Petersen

1992; Strzyszcz et al. 1996; Magiera 1996). Contributions of indi-

vidual magnetic fractions in fly ashes to the magnetic susceptibility

values and distribution of some trace elements within individual fly

ash fractions were the subject of this study. Fly ashes were collected

from 6 sections of electrofilters installed in 2 thermal power plants

(Bedzin and Huta Katowice, which is a part of a great metallur-

gical plant). A samples were separated into 5 granulometric frac-

tions: 200160

m, 16090

m, 9050

m, 5020

m and < 20

The sample of fly ashe above the electrofilter system was also col-

lected but because of small amount of material it was impossible to

separate individual grain-size fractions. Average percentage contri-

butions of individual fractions are given in Table 1. Magnetic sus-

ceptibility (

) of samples is betweeen 760 and 6150

In each electrofilter section, the

value was highest in the 160

m fraction, what suggests that this fraction is the most rich in

ferrimagnetic minerals.

Concentration of elements as Cu, Zn, Mn, Ni, Co, Cr, V, Pb and

Mo was also measured for each studied fraction. The highest enrich-

ment in heavy metals was observed the finest fraction. It is connect-

ed with the large active surface of this fraction where probably most

of these elements are adsorbed. In some cases the enrichment of Cr,

Cd and Ni is observed also within the magnetically most enhanced

fraction (16090

m), which represent only 16.0 % in case of the

Bedzin power play and 3.3 % in case of the Huta Katowice

power plant. This suggests that within fly ashes these elements are

connected with magnetic minerals.

References

Dekkers M.J. & Pietersen H.S., 1992: Magnetic properties of low-Ca fly ash:

a rapid tool for Fe-assessment and a survey for potentially hazardous el-

ements. In: Glasser F.P., McCarthy G.J., Young J.F., Masson T.O. &

Pratt P.I. (Eds.): Advanced Cementitious Systems: Mechanism and

Properties. Materials Research Society Symposium Proceedings, Ma-

terials Research Society, Pittsburg, 245, 3747.

Magiera T., 1996: Ferromagnetic minerals of anthropogenic origin in soils of

some Polish National Parks. PhD thesis Institute of Environmental En-

gineering, Polish Academy of Aciences, Zabrze.

Strzyszcz Z., Magiera T. & Heller F., 1996: The influence of industrial im-

missions on the magnetic susceptibility of soils in Upper Silesia. Studia

Geoph. et Geol., 40, 276286.

MAGNETIC SUSCEPTIBILITY

MONITORING IN THE DANUBE

RIVERDANUBE DELTAWESTERN BLACK

SEA SYSTEM; ENVIRONMENTAL

SIGNIFICANCES

S.C. RÃDAN*, M. RÃDAN, S. RÃDAN, A. GANCIU,

GH. OAIE and ST. SZOBOTKA

Geological Institute of Romania, Caransebes 1, 79678 Bucharest, Romania

1. Introduction

Anthropic polluting activities in a vast hydrogeographic basin of

the Danube River (D), major hydrotechnic works along its course and

human interventions on the Danube Delta (DD) area increased the en-

vironmental degradation of the DD, and even of the Black Sea (BS).

Since 1992, when the geoecological monitoring (GEM) was intro-

duced in the DDDBS system, the magnetic susceptibility monitor-

Power plant

Size grade (mm)

200160 16090

9050

5020

< 20

Bedzin

6.60

16.00

15.80

27.60

34.00

Huta Katowice

2.80

3.30

16.90

19.00

58.00

Table 1: Average percentage contribution of individual fractions in two

Polish power plants.

Thermomagnetic analysis suggest that only magnetite is present.

However, slight change in the slope of Jrs vs. T curve indicates the

presence of two remanence carries, interacting with each other

through exchange coupling. Basic magnetic parameters are listed in

Table 2, Day plot places the samplex in PSD region with clear ten-

dency towards the MD range with decreasing grain size (Fig. 1).

These findings require futher studies on samples from different

electrofilters and different power plants.

Grain size

(mm)

(Oe) H

(mAm

/kg) J

(mAm

/kg) J

TB Fractions (first electrofilter)

200160

368

159

2.31

180

0.24

16090

328

126

2.60

412

0.18

9050

329

107

3.07

106

883

0.12

5020

285

3.10

137

1047

0.13

< 20

313

3.23

135

1078

0.13

TK Fractions (fifth electrofilter)

200160

266

3.24

740

0.12

16090

245

3.95

582

3445

0.17

9050

251

3.92

464

3380

0.14

5020

283

4.29

332

3500

0.09

< 20

325

4.22

191

3054

0.06

Table 2:

Fig. 1.

240

ing (MSM) has been applied within the Danube Delta and along the

Danube River Romanian course. In 1995 and 1997 the MSM was

tested in the north-western Black Sea as well.

This paper is dealing especially with the MSM results obtained in

the 19951998 period, the previous data were presented elsewhere

(Rãdan et al. 19931997; Oaie et al. 1994; Szobotka et al. 1995).

2. Methods

Research cruises conducted by GeoEcoMar, consisting of 12

legs each year, were carried out in the DDDBS system in order to

collect samples of water, biota, suspended and bottom sediments.

To investigate the MS, subsamples were taken from bottom sedi-

ments sampled in stations supporting the GEM network. Several

cores from DD and D were also sampled for MS study. Another two

collections of subsamples originates from the cores taken by a box-

corer and/or a multicorer in the north-western Black Sea area.

The MS measurements on various types of sediments (Muds,

sometimes rich in vegetal detritus or shells, siltu muds, clayey

muds, silts, sands) were performed by a KLY-2 Kappabridge

SIu. sensitivity).

3. Results and discussion

3.1. Danube River

Two legs in 1995 and 1996 and one leg in 1997 were leamined.

The MS measurements were performed on bottom sediments consist-

ing mainly of silts and sands sampled just below the water/sediment

interface, as well as on subsamples taken from the sediment cores.

The MS data were processed for 3 sectors along the Danube River

Romanian course, another 2 were related to the Danube Delta distrib-

utaries and to the left Danube tributary month zones, respectively.

The sand and silts sampled in the MSM stations have usually

shown k values lower than 1000

Siu. And even lower than

500

Siu.

A strong MS anomaly is detected in the first sector of the Danube,

the highest k values being recorded each year in a zone located be-

tween km 1048.7 and km 1040 (e.g. 10,465

Siu. In the MSM

phase 1995). The MSM data reflect metallic contamination of bot-

tom sediments as a consequence of mining industrial activities de-

veloped on the Danube left bank.

The metallic pollution is also shown by the MS data for some

left tributaries (e.g. Topolnita R. mouth zone, with k values of

4777.6

Siu., in the MSM phase 1996, and of 2979.5

S i u . ,

in the MSM phase 1997).

As regards the sediment cores, interesting data were obtained for

two cores (longer than 2 m) sampled in the Lake Iron Gates I (at km

969.5 and km 947.2 respectively). They showed similar MS pat-

terns, with a well defined anomalous zone, the result being under

consideration for the following MSM phases and interpretation.

3.2. Danube Delta

One leg each year was performed in the period 1995-1997, and

the bottom sediments were collected from lakes and channels in

both fluvial and marine delta plains, as well as from the Razelm

(Razim) Sinoe lacustrine-lagoonal complex (RSLC).

The MS values range between (0.9)

471.2

S I u .

(1995 phase of the MSM), (3.2)

497.5

SIu. (1996

MSM phase) and (5.6)

565.1

SIu. (1997 MSM phase)

respectively. In all three phases the most frequent MS values are in

the interval 050

SIu., most of the k values were lower than

200

Siu. (see Rãdan et al. 1996, 1997 for the 1995 and 1996

MSM data).

Fig. 1. Magnetic susceptibility map of bottom sediments from the north-western Black Sea (geoecological monitoring, phase 1995). Note: The MS values must

be multiplied by 10

(Slu).

241

The lowest MS values (<10

SIu.) were obtained for organic

sediments (rich in vegetal detritus) sampled from lakes showing

(sometimes locally) more confined environments, while the highest

MS value were measured especially on siltic muds collected from

lakes directly influenced by the Danube River.

Recent environmental changes induced by the digging of a new

canal of connection with the Danube are clearly revealed by the MS

values reflecting the enhancement of the silting up process in some

lakes in the area.

Reliable MS data have been also obtained on sediments sampled

from channels within the Danube Delta, as well as from three impor-

tant lakes of the RSLC. Higher MS values were recorded in the

MSM stations close to two main channel inlets into the Razelm lake

(e.g. Dunavãþ Chn.; 440.9

SIu. in the 1997 MSM phase).

3.3. Black Sea

Subsamples of bottom sediments (muds, muds with shells, silty

muds, sandy muds, clayey muds) were collected for MS study dur-

ing two GEM legs performed in the north-western Black Sea in 1995

and 1997.

As regards the 1995 phase, the MS values are between 1.2

and 448.1

SIu. the limits being determined on box-corer

samples of muds very rich in shells and of silty muds, respectively.

The MS map (Fig. 1) shows a clear anomalous zone, which coin-

cides with the Danube River influenced area (according to Panin et

al. 1997), particulary with the Danube Delta front and the Danube

prodelta zone; the southward drift of the Danube sediment influx in

the shelf zone is also suggested by MS contours, as well as the

Danube deep sea fan.

A more representative collection of sediment subsamples taken

the 1997 cruise is under study.

4. Concluding remarks

Magnetic susceptibility calibration of the Danube Delta lake sedi-

ments resulted from the MS monitoring, an the capability of this

rock-magnetic parameter to be used as sedimentological indicator

was demonstrated by al. MS data obtained in the Danube River

Danube Delta western Black Sea system.

The ability of MSM to identify in bottom sediments the environ-

mental magnetic signature, including its changes, even produced re-

cently by the antropic impact on the deltaic ecosystem, was also

pointed out.

The capacity of MSM to detect the metallic pollution is supported

expecially by the results recorded along the Danube River, but un-

der local conditions the presence of pollutants in the Danube Delta

was shown as well (e.g. L. Dranov).

The MS data confirm the usefulness of MSM as a tool for the geo-

ecological monitoring introduced since 1992 in the Danube River

Danube Deltawestern Black Sea system.

References (selection)

Oaie Gh., Szobotka St., Secrieru D., Rãdan M., Rãdan S. & Rãdan S.C.,

1994: Analele st. Inst. Delta Dunarii, III/2, 335346 (in Romanian).

Rdan S.C., Rãdan M., Rãdan S. & Mihãilescu N., 1993: Annales Geophysi-

cae, Suppl. I to Vol. 11, C91.

Rãdan M., Rãdan S.C., Rãdan, S., Szobotka St. & Oaie Gh., 1996a:

Analele st. Inst. Selta Dunarii, V, 89104 (in Romanian, English sum-

mary).

Rãdan S.C., Rãdan M., Rãdan S., Oaie Gh., Szobotka St. & Mihãilescu N.,

1996b: Annales Geophysicae, Suppl.I to Vol. 14, C149.

Rãdan S.C., Rãdan M., Rãdan S. & Mihãilescu N., 1996c: Geol. Carpathi-

ca, 47, 3, 190191.

Rãdan S.C., Rãdan M. & Rãdan S., 1996d: Analele st. Inst. Delta Dunarii,

V, 305-317 (in Romanian, English summary).

Rãdan S.C., Rãdan M., Rãdan S. & Ganciu A., 1997: Analete st. Inst. Delta

Dunarii, VI (in Romanian, abridged English version)(in print).

Rãdan S.C., Rãdan M., Rãdan S., Ganciu A. & Mihailescu N., 1998: An-

nales Geophysicae, Suppl. Vol. 16 (in print).

Rãdan M., Rãdan S.C., Rãdan S., Oaie Gh. & Szobotka St., 1998: Annales

Geophysicae, Suppl. Vol. 16 (in print).

Szobotka St., Oaie Gh., Craiu C., Mihaeila E., Rãdan S., Rãdan S.C. & Vi-

foreanu A., 1995: Analele st. Inst. Delta Dunarii., IV/2, 6172 (in Ro-

manian, English abstract).

Panin N., Jipa D., Gomoiu M.T. & Secrieru D., 1997: NATO Advances Re-

search Workshop, Constanta, Romania.

MAGNETIC SUSCEPTIBILITY

OF FOREST SOILS IN POLISH-GERMAN

BORDER AREA

Z. STRZYSZCZ and T. MAGIERA*

Institute of Environmental Engineering PAS, M. Curie-Sklodowskej 34,

41-819 Zabrze, Poland; *sledz@ipis.zabrze.pl

Polish-German border is demarcated along the rivers Nysa

£u¿ycka and Odra. Because of brown coal exploration on both

sides of the Nysa Luzycka River, five large power plants of more

than 1000 MW are located close to the border and 16 others are lo-

cated at distances less then 100 km from the border (Fig. 1). Be-

cause of westerly or south-westerly winds predominating in this

area most of dust pollution emitted from these sources have been

accumulated in soils on the Polish side of the border and magnetic

particles occurring in fly ashes cause an increase in magnetic sus-

ceptibility of topsoil, especially in forest area. Measurements of

topsoil magnetic susceptibility seem to be very rapid and easy tool

for assessing levels of industrial pollution in soils especially on

forest areas where an accumulation of dust pollution was continu-

ous through many years. Six areas of study were selected on the

Polish side of the border: Sieniawka, Zgorzelec Zary, Gubin, Kos-

trzyn and Gryfino (Fig. 1). Specific low-field magnetic suscepti-

bility (

) has been measured on samples taken from 35 soil pits

excavated in forest in each studied area.

Going along the Polish-German border from the south to the

north, magnetic susceptibility of forest topsoil decreases. In the

southernmost Sieniawka area,

values in topsoil varied between

400 and 550

and were the highest from the 6 study ar-

eas. The maxima of

within soil profiles were noticed in organic

horizon (O

or O

subhorizons) and enhancement of

values with

respect to lower part of profile as 26 times. The relatively high

values and strong enhancement as probably due to vicinity of large

Turów Power Plant located on the study area and 2 smaller German

power plants (Hirschfelde and Neugersdorf) located close to the

border. Moreover an influx of immission from north-western part of

Czech Republic, were also 7 large power plants are located, can not

be excluded as well. In the Zgorzelec area, 5 soil pits were excavat-

ed in transect from Sulików (ca. 20 km E from the border) towards

the border. The

values varied between 220 and 520

k g

In soil profiles from the east part of this area (close to Sulików) the

highest

values were in lower part of the profile (C horizon) and

there was no magnetic enhancement in topsoil. It was the effect of

weathering of magmatic and metamorphic rocks from geological

bedrock. In soil profiles located closer to the border in the Nysa

£u¿ycka Valley, magnetic enhancement of topsoils as about 6 times.

This area is stronghly influenced by a Hagenwerder Power Plant lo-

cated in Germany close to the border. Also Turów Power Plant, lo-

cated 50 km south, may have a great influence on this area as well

as the Hirschfelde Power Plant and some smaller plants located in

Görlitz. The

values of topsoil within the third study area, located

between ¯ary on the east side and Leknica on the west, are more sta-

242

ble, between 300 and 350

. The considerably lower

susceptibility can be a result of soil profile location. Most of the

profiles were located 3050 km from the border (due to the close

military area). On the Polish side of the border in this area, there are

no power plants or other greater sources of pollution. However,

magnetic enhancement of topsoil in noticeable (

values about 2

times higher then in lower part). This area is probably mostly influ-

enced by the German Power Plants Boxberg, Schwarze Pumpe and

Hagerswerde. In the Gubin are the

values varied between 300 and

520

and magnetic enhancement with respect to a low-

er part of profile was about 3 times. Relatively high

values in this

ares are due to emissions from a large Jänschwalde Power Plant lo-

cated 10 km west from the sampled area. This area may also be in-

fluenced by a smaller, but closer Power Plant in Guben (German

part of Gubin). Magnetic susceptibility in the Kostrzyn area is al-

ready considerably lower then in the southern part of border area

(220250

). Also magnetic enhancement in topsoil is

lower, but still noticeable. In this area, there are us larger sources of

pollution within a 50 km radius except for a small power plant in

Kostrzyn. The closest dust emitters are in Gorzów and Frankfurt.

Between the Gubin and Kostrzyn areas not far from the border, a

large steel plant in Eisenhüttenstadt is located which can also affect

the study area. However, an earlier study suggested rather local in-

fluence of metallurgical emissions on soil magnetic susceptibility

which is limited to the area of several kilometers from the source

due to diameters of magnetic particles (Strzyszcz 1989a; Strzyszcz

et al. 1994). In the Gryfino area the soil susceptibility varied from

180 to 200

and were the lowest in all of the study areas.

However, about 30 km north, the a Dolna Odra power plant is

located and somewhat further in the same direction industrial and

power plants of Szczecin are placed, but the influence of their

emission in this area were not so clear. South-westerly winds in

this area predominate and this is probably the main factor limiting

dust immission from the Dolna Odra power plant and Szczecin.

The largest source of dust emission in this area and also potential

source of magnetic particles is refinery in Schwedt (Germany),

about 60 km SW away from here. At present, literature data about

the emission of magnetic particles by refinery are not sufficient.

The

values exceeding 500

are characteristic for

areas with a large industrial immission, for example Upper Sile-

sian (Strzyszcz 1989a, 1993; Strzyszcz et al. 1996). So high

val-

ues were measured in the Sieniawka and Zgorzelec areas and also

in some profiles in the Gubin area, where the influence of the Jän-

schwalde Power Plant is still noticeable. Further to the noerth, X

values decrease to 200

. These values are characteris-

tic for areas with moderate and low level of industrial immissions

(Strzyszcz et al. 1994).

References

Strzyszcz Z., 1989a: Anwesenheit des ferromagnetischen Eisen in ober-

schlesischen Waldböden und deren Ursachen. Mitt. Deut. Boden. Ge-

sell., 59, 11971202.

Strzyszcz Z., 1989b: Ferromagnetic properties of forest soils being under

influence of industrial pollution. Air pollution and forest decline. In:

Proc. 14th Int. Meeting for Specialist in Air Pollution Effects on For-

est Ecosystems. IUFRO, Interlaken, 201207.

Strzyszcz Z., 1993: Magnetic susceptibity of soils in the area influenced by

industrial emissions. In: Monte Verita (Ed.): Soil Monitoring.

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Fig. 1. Power plants in East Germany: 1 Truttendorf; 2 Lubenau; 3

Vetschau; 4 Boxberg; 5 Jenschwalde; 6 Hirschwelde; 7 Hagenwer-

den; 8 Schwarze Pumpe; 9 Vackerode; 10 Lippendorf; 11 Thier-

bach; 12 Epenheim; 13 Lelua; 14 Regis; 15 Halle; 16 Schwedt

Refinery. Power plants is north-western Czech Republik: 1 Tisowa; 2

Prunerov; 3 Tusimice; 4 Pocerady; 5 Pomorany; 6 Litvinov Refin-

ery; 7 Melnik; 8 Chvaletice; 9 Detmarovice; 10 Novaky; 11

Vojany. Main Polish sources of emission: 1 Turów Power Plant, 2 Lubin

and Glogów copper plants and mines; 3 Dolna Odra Power Plant. Areas of

study: A Sieniawka; B Zgorzelec; C Zary; D Gubin; E Kos-

trzyn; F Gryfino.