GEOLOGICA CARPATHICA, 49, 1, BRATISLAVA, FEBRUARY 1998
514
PRELIMINARY RESULTS FROM ROCK MAGNETIC ANALYSES
OF QUATERNARY AND TERTIARY BASALTS
FROM THE GULF COAST OF MEXICO
JOHN-PAUL J. POLLARD
1
, GRAHAM J. SHERWOOD
1
and HARALD BÖHNEL
2
1
School of Biological and Earth Sciences, Liverpool John Moores University, Byrom Street, Liverpool L3 3AF, UK
2
Instituto de Geofisica, UNAM, Cd. Universitaria, 04510 Mexico-City, Mexico
(Manuscript received March 18, 1997; accepted in revised form December 11, 1997)
Abstract: As the foundation for a paleomagnetic study of the Gulf Coast of Mexico, rock magnetic measurements
were carried out on basaltic samples from 40 sites in the Gulf Coast region. Strong-field thermomagnetic and hyster-
esis techniques as well as room and low temperature susceptibility analyses were employed. These measurements
show that the samples contain titanomagnetites with domain states from single- to multi-domain, with few
superparamagnetic grains. Most samples have undergone at least partial deuteric oxidation though a significant quan-
tity have not. The titanomagnetites have a range of compositions from Ti-poor to Ti-rich, where the latter appear to
have endured varying degrees of maghematization.
Key words: Mexico, Gulf Coast, basalts, rock magnetism.
probably associated with the occurrence of northsouth rift-
ing along the border of the coastal plain and Altiplano regions
93 Ma ago (Cantagrel & Robin 1979; Robin 1982).
Introduction and geological setting
As part of a project to determine paleomagnetically the
long-term tectonic characteristics of the Gulf Coast of Mex-
ico, samples have been taken from various localities within
the Gulf Coast region for rock magnetic investigation. This
investigation is needed to give a detailed knowledge of the
magnetic mineralogy and domain states of the magnetic
minerals as it is essential that the rock is carrying a primary
remanent magnetization for tectonic reconstruction.
The study area (see Fig. 1 and Tables 1a and 1b) includes
the eastern end of the Trans-Mexican Volcanic Belt (TMVB)
and a large portion of the Eastern Alkaline Province (EAP) as
described by Robin (1976, 1982) and Thorpe (1977). The vol-
canism of the TMVB is related to subduction of the Rivera
and Cocos plates along the Middle America Trench (Nixon
1982; Burbach et al. 1984; Nixon et al. 1987; Böhnel 1997).
The TMVB itself is composed of Tertiary volcanic and intru-
sive rocks of andesitic and basaltic composition. The part of
the TMVB in the study area is made up primarily of andesites
and dacites with ages less than 15 Ma (Cantagrel & Robin
1979). The EAP is a discontinuous belt of associated alkaline
and hyperalkaline magmatic suites that lie between the Trans-
Pecos province, southern Texas, and the Tuxtla Massif,
southern Veracruz. EAP volcanism extended southward dur-
ing late Tertiary times. The alkaline lavas of Texas have ages
between 43 and 16 Ma (Parker & McDowell 1973; Barker
1977). In Mexico alkaline magmatism began in Oligocene
times in the north, in the Miocene in the central zone, and in
PlioceneQuaternary times in the south (Robin & Tournon
1978; Cantagrel & Robin 1979). The EAP suites are of
PlioceneQuaternary age within the study area, and are com-
posed of a range of alkaline rocks including nephelinites,
basanites and alkali basalts (Thorpe 1977; Robin & Tournon
1978). The cause of the EAP volcanism is uncertain but is
Fig. 1. Map showing the general geology of the study area and
sample site locations (after Cantagrel & Robin 1979). EAPeast-
ern alkaline province; TMVBtrans-Mexican volcanic belt;
MATmiddle america trench (inset map).
6 POLLARD, SHERWOOD and BÖHNEL
Table 1a: Site location with rock type and approximate age for EAP sites. Sites MOL A and MOL B are from the same location but are
slightly different rock types.
EAP SITES
Site
Location
General rock type and approx. age.
TEC
19°75.94N, 96°55.57W
Fine grained basalt with occasional small (< 2 mm diam.) vesicles. Quaternary.
DED
19°80.84N, 96°53.67W
Fine grained basalt containing very small (< l mm diam.) vesicles. Quaternary.
SAN
19°86.69N, 96°50.69W
Fine grained basalt with some yellow alteration. Rare vesicles. Quaternary.
POZ
20°47.35N, 97°63.84W
Very fine grained basalt with rare vesicles. Late Tertiary.
MIC
22°77.03N, 98°57.62W
Very fine grained basalt containing no vesicles. Late Tertiary.
BAR
22°95.24N, 97°85.76W
Fine grained basalt. No vesicles. Signs of alteration in some samples. Late Tertiary.
STV
22°94.01N, 97°95.55W
Basalt containing mostly fine grains but with some larger (>2 mm diam.) crystals.
Late Tertiary.
ALE
22°93.90N, 97°95.58W
As STV
NTI
22°93.57N, 97°97.55W
As STV
ALD
22°99.19N, 98°05.42W
Very fine grained basalt. No vesicles. Late Tertiary.
ALT
21°02.26N, 98°61.85W
Basalt with many yellow alterations. Fine grained with occasional large elongate
vesicles. Late Tertiary.
ARD
20°97.00N, 98°65.81W
Very line grained basalt containing no vesicles. Late Tertiary.
MOL A
20°79.17N, 98°72.32W
Basalt with many white alterations. Mostly fine grained with some larger crystals.
Tertiary.
MOL B
(Location as MOL A)
Yellowish coloured igneous rock with large crystals in a finematrix. Very altered.
Late Tertiary.
FLO
20°59.13N, 98°62.19W
Light grey flow textured igneous rock with occasional small vesicles.
Very fine grain size. Late Tertiary.
HUA
20°55.76N, 98°62.22W
Fine grained basalt containing some small vesicles. Late Tertiary.
TMVB SITES
Site
Location
General rock type and approx. age.
MET
20°43.84N, 98°67.45W
Basalt with orange coloured alterations. Fine grain size but with some larger
(>1 mm diam.) crystals. Late Tertiary.
ZQU
20°40.98N, 98°68.56W
Fine grained basalt with many small vesicles, and occasional large
(>2 mm diam.) ones. Tertiary.
ATO
20°38.69N, 98°72.04W
Fine grained basalt with vesicles of 12 mm diam. Late Tertiary.
DYK
20°16.76N, 98°65.06W
Fine grained basalt with vesicles of 12 mm diam. Late Tertiary.
FUD
20°15.58N, 98°65.28W
As DYK but more altered appearance. Late Tertiary.
PAC
20°13.36N, 98°70.09W
As FUD.
TEO
19°40.11N, 96°97.45W
Very fine grained basalt containing some small (< l.5 mm diam.) vesicles. Quaternary.
CEL
19°40.08N, 96°97.23W
Fine grained basalt with elongate vesicles. Quaternary.
TUZ
19°38.98N, 96°87.11W
Fine grained basalt with a few larger (approx. l mm diam.) crystals.
Some rare vesicles. Quaternary.
ORI
18°94.17N, 97°41.91W
Fine grained basalt with some larger (approx. 1 mm diam.).
Abundant small elongate vesicles. Quaternary.
ZAB
18°95.25N, 97°42.02W
Very fine grained basalt containing some very small vesicles.
Some pale alterations evident. Quaternary.
PIZ
19°54.06N, 98°12.33W
Fine grained basalt containing many small (<1 mm diam.) vesicles.
Slightly weathered. Late Tertiary.
TLA
19°63.54N, 98°l 1.69W
Fines grained basalt containing small vesicles. A few larger elongate vesicles
exist in some samples. Late Tertiary.
SAM
19°63.88N, 98°10.79W
Fine grained basalt with some slightly larger crystals. Some small elongate vesicles exist
within the basalt. Late Tertiary.
GAS
19°65.69N, 98°09.31W
Fine grained basalt with rare small vesicles. Late Tertiary.
RED
19°77.31N, 98°50.65W
Fine grained basalt matrix surrounding some larger (>lmm diam.) pale crystals.
A few very small vesicles. Late Tertiary.
PAN
19°77.03N, 98°50.39W
As RED.
MID
19°77.20N, 98°50.48W
As RED.
MAR
19°78.34N, 98°51.74W
As RED plus some samples have abundant large (>2 mm across) vesicles whilst others
have none. Late Tertiary.
POC
19°77.49N, 98°51.68W
As MAR.
SAH
19°77.29N, 98°57.25W
Fine grained basalt with many vesicles between 1 and 5 mm+ across. Late Tertiary.
GUN
19°77.28N, 98°57.34W
As SAH.
BUG
19°83.54N, 98°57.04W
Very fine grained basalt containing no vesicles. Late Tertiary.
TAC
19°83.39N, 98°45.60W
Very fine grained basalt which contains many very small (< l mm diam.) vesicles. Late
Tertiary.
RIO
19°80.20N, 98°50.52W
Fine grained basalt with a locally inconsistent distribution of large and smaller vesicles.
Obvious white and red coloured alterations. Late Tertiary.
Table 1b: Site location with rock type and approximate age for TMVB sites.
PRELIMINARY RESULTS FROM ROCK MAGNETIC ANALYSES OF BASALTS 7
A total of 40 sites were selected within both the TMVB
and EAP from which samples were taken of predominantly
basaltic late Miocene to Quaternary rocks.
Sampling and laboratory methods
Sampling sites (see Fig. 1) were selected according to
rock age, freshness and accessibility of the exposure. A
petrol-driven water-cooled rock drill was used to take be-
tween six and ten 2.5 cm cores from each site. Each of these
were then orientated in situ using a clinometer, magnetic
compass and, where possible, a sun compass. The cores
were prepared for laboratory analysis using a water-cooled
rock saw to cut them into 23 mm long samples.
Various rock magnetic laboratory techniques employed for
this study include hysteresis parameters, strong-field thermo-
magnetic analysis, plus low- and room-temperature suscepti-
bility measurements.
Hysteresis parameters were obtained using a Molspin vi-
brating sample magnetometer (VSM) with a maximum field
of 1 T. The parameters determined were: saturation magne-
tisation, M
S
; saturation remanence, M
RS
; coercive force,
H
C
; and the ratio M
RS
/M
S
. A computer controlled horizontal
Curie balance was employed to measure strong-field mag-
netization from room temperature to 700
o
C in a field of
0.35 T. From the thermomagnetic curve produced, the Curie
temperature was estimated using the method of Grommé et
al. (1969), and the change in magnetization at 100
o
C (RM)
due to heating was determined. Low temperature suscepti-
bility analyses were carried out by measuring the variation
of low-field susceptibility from liquid nitrogen temperature
(78 K) to room temperature(293 K). These measurements
were done using a computer-controlled Bartington MS2
susceptibility meter with a water-jacketed probe. The rela-
tive susceptibility values (RS) were calculated from the
temperature versus susceptibility curves. Room temperature
susceptibility measurements were carried out using a Bar-
tington MS2 susceptibility meter. A dual frequency Barting-
ton MS2B probe allowed the measurement of low (0.46 kHz)
and high (4.6 kHz) frequency susceptibility. Mass suscepti-
bility was calculated for each sample at each frequency. The
use of the dual frequency meter also enabled the calculation
of frequency dependant susceptibility (
χ
fd%).
Results
Hysteresis parameters
The data gained from the VSM (Table 2) show that the val-
ues of M
S
are spread between 0.0042.04 Am
2
kg
-1
, though
most of the samples are in the range 0.21.21 Am
2
kg
-1
. Such
values show there is a significant concentration of magnetic
minerals within most of the samples.
Values for the ratio M
RS
/M
S
permit the determination of
whether a sample contains multi-domain (MD M
RS
/M
S
<0.05), single-domain (SD M
RS
/M
S
ca. 0.5), or a mixture
of both types of material (0.05<M
RS
/M
S
<0.5) as suggested by
OReilly (1984). The data gained show a broad range of val-
ues between 0.060.46 which indicates that the majority of
the samples have a mixture of MD and SD grains, though
there are samples that are almost completely MD or SD. Co-
ercive force (H
C
) values are distributed evenly between 3.38
and 27.64 mT. Some samples exhibit slightly wasp-waisted
loops (e.g. SAH 8, Fig. 2) probably due to two different popu-
lations of grains, one with hard and one with soft coercivity.
The hysteresis loops of other samples such as POZ 7 show
magnetic behaviour dominated by paramagnetic material, as
identified by stretched loops (see Fig. 2).
Strong-field thermomagnetic behaviour
Samples from each site had their strong-field thermomag-
netic behaviour determined and the results are listed in Ta-
ble 3. The types of thermomagnetic behaviour exhibited by
the samples have been classified into different groups. The
classifications chosen have been adapted from the methods
used by Mankinen et al. (1985) and Sherwood (1988).
Type 1 curves (Fig. 3) show a single ferrimagnetic phase
with a low to intermediate (<500
o
C) Curie temperature.
There is a marked difference between heating and cooling
curves with an increase in Curie temperature and magneti-
zation in the cooling phase, and therefore the samples have
RM values of >1. These type 1 curves are divided into two
subgroups: in type1a the Curie temperature on the cooling
curve has irreversibly increased to a high temperature phase
(>500
o
C), but there is no disproportionation peak upon
heating. Note that RM values for samples of site POZ are
Fig. 2. Hysteresis curves of two samples: SAH 8 shows a wasp
waisted loop whilst the curve of POZ 7 is dominated by paramag-
netic material.
SAH 8
POZ 7
8 POLLARD, SHERWOOD and BÖHNEL
anomalously high and are subsequently labelled type 1a*.
Type 1b curves are almost identical to type 1a curves except
that a disproportion peak is present on the heating curve.
These type 1 curves are interpreted as Ti-rich titanomagne-
tites which cooled so quickly that no high temperature deu-
teric oxidation took place, but have since been subjected to
low temperature oxidation (maghemitization). The extent of
maghemitization is greater in type 1b samples. The fact that
LFMS
HFMS
% FDS
RS
Ms
Mrs [=SIRM] Mrs/Ms
Hc
Site
10
-8
(m
3
kg
-1
) 10
-8
(m
3
kg
-1
)
(?fd%)
(?
78
/?
298
)
(Am
2
/kg)
(Am
2
/kg)
(Am
2
/kg)
(mT)
EAP SITES
TEC
1327.12
1312.09
0.50
1.08
1.04
0.29
0.28
27.19
DED
414.34
414.36
-0.01
0.51
0.43
0.06
0.14
8.01
SAN
790.14
781.70
1.07
0.42
0.58
0.09
0.16
6.22
POZ
279.99
283.44
-1.10
0.67
0.11
0.03
0.24
9.69
MIC
2141.99
2138.70
0.16
0.55
2.04
0.41
0.20
17.72
BAR
900.85
902.37
-0.16
0.25
0.76
0.05
0.07
3.90
STV
533.71
537.98
-0.62
0.20
0.46
0.03
0.06
3.38
ALE
764.24
764.81
-0.24
0.52
0.42
0.03
0.07
3.87
NTI
618.11
618.75
-0.16
0.74
0.60
0.08
0.13
14.54
ALD
564.17
564.54
-0.06
0.53
0.50
0.07
0.14
16.17
ALT
382.51
380.30
0.39
1.41
0.40
0.08
0.22
21.80
ARD
1314.16
1304.82
0.85
0.46
0.68
0.11
0.17
5.82
MOL A
415.36
416.54
-0.29
0.95
0.65
0.15
0.23
16.62
MOL B
7.50
7.34
1.85
0.64
0.007
0.0011
0.20
18.10
FLO
130.57
130.52
0.08
0.80
0.26
0.08
0.29
21.95
HUA
93.33
92.72
0.66
1.13
0.20
0.09
0.46
27.64
TMVB SITES
MET
104.76
102.23
2.86
1.25
0.09
0.02
0.22
16.50
ZQU
117.31
115.75
1.28
1.45
0.17
0.05
0.27
14.27
ATO
186.55
186.13
0.23
2.41
0.34
0.09
0.26
24.76
DYK
43.46
43.15
0.90
0.63
0.04
0.01
0.26
24.88
FUD
11.97
11.89
0.72
1.05
0.004
0.0008
0.20
10.30
PAC
26.23
25.84
1.75
0.01
0.0027
0.20
17.00
TEO
669.40
669.36
0.00
1.28
0.85
0.07
0.08
5.95
CEL
427.56
426.92
0.15
1.15
0.90
0.15
0.17
12.74
TUZ
292.96
290.89
0.71
1.12
0.50
0.06
0.11
9.07
ORI
606.98
606.43
0.10
1.02
1.20
0.23
0.19
17.30
ZAB
618.93
619.02
0.01
1.14
1.12
0.17
0.15
11.66
PIZ
607.32
602.72
0.96
1.12
0.79
0.20
0.25
20.82
TLA
327.82
327.42
0.12
0.55
0.73
0.18
0.24
18.30
SAM
214.51
212.17
1.20
0.60
0.37
0.12
0.32
13.35
GAS
355.86
355.41
0.11
0.74
0.63
0.09
0.14
9.94
RED
474.94
474.32
0.15
1.19
1.06
0.17
0.16
13.76
PAN
382.04
381.05
0.32
0.50
0.82
0.14
0.17
12.17
MID
559.48
559.57
-0.02
0.58
1.21
0.23
0.19
14.82
MAR
261.50
262.30
-0.28
0.35
0.59
0.12
0.20
7.99
POC
290.60
289.38
0.39
0.44
0.64
0.16
0.25
11.18
SAH
238.10
237.19
0.40
0.47
0.52
0.18
0.34
18.29
GUN
246.70
245.48
0.65
0.73
0.78
0.26
0.33
25.05
BUG
212.63
213.53
-0.42
1.55
0.38
0.08
0.20
17.67
TAC
411.35
410.40
0.25
1.13
0.88
0.11
0.13
14.91
RIO
509.69
508.65
0.22
1.17
1.03
0.12
0.12
11.47
Table 2: Hysteresis, susceptibility and low temperature susceptibility properties of sites sampled. LFMS low-field mass susceptibility;
HFMS high-field mass susceptibility; % FDS percentage frequency dependent susceptibility; RS relative susceptibility; Ms satu-
ration magnetization; Mrs saturation remanent magnetization; Hc coercive force. Note, all susceptibility figures are site mean values.
these measurements were not performed in a vacuum or a
nitrogen atmosphere does make it possible that these sam-
ples were oxidized during the experiment. However, it is as-
sumed that this is unlikely due to the small number of sam-
ples that exhibit this behaviour.
Type 2 curves (Fig. 4) show a single ferrimagnetic phase
with a high Curie temperature, and a decrease in magnetiza-
tion after heating. This type has been subdivided according to
PRELIMINARY RESULTS FROM ROCK MAGNETIC ANALYSES OF BASALTS 9
rich. Consequently, it is inferred that the Ti-poor phase is
caused by the high temperature deuteric oxidation of a Ti-rich
titanomagnetite to a Ti-poor titanomagnetite containing il-
menite lamellae.
Type 3 curves (Fig. 5) are similar in most respects to type
2 curves i.e. they exhibit a single ferrimagnetic phase with a
high Curie point. The only difference between these and
type 2 curves is that the cooling curve crosses the heating
curve and therefore the magnetization after cooling is slight-
ly higher than that noted upon heating. Despite this slight
difference, the magnetic phase for type 3 curves is assumed
to originate by the same process as that of type 2 curves.
Fig. 4. Examples of all type 2 Curie curves.
Fig. 3. Examples of all type 1 Curie curves.
the degree of irreversibility where type 2c is more irreversible
than type 2b which is more irreversible than type 2a. Type 2a
curves have similar shaped heating and cooling curves where
RM values are between 0.9 and 1 (a <10 % decrease in mag-
netization upon cooling at 100
o
C). The heating and cooling
curves of the type 2b plots are similarly shaped but have RM
values of <0.9. Type 2c curves also have RM values of <0.9
but exhibit a marked difference in curve shape between heat-
ing and cooling. The production of this type 2 magnetic phase
is inferred to have been caused by the presence of a Ti-poor
titanomagnetite. This Ti-poor phase is unlikely to have been
of primary origin as primary titanomagnetites tend to be Ti-
PAN 5 (Type 1a)
0
200
400
600
800
Temperature
o
C
Magnetization
Heating
Cooling
POZ 3 (Type 1a*)
0
200
400
600
800
Temperature
o
C
Magnetization
Heating
Cooling
STV 5 (Type 1b)
0
200
400
600
800
Temperature
o
C
Magnetization
Heating
Cooling
MIC 7 (Type 2c)
0
200
400
600
800
Temperature
o
C
M
agnetizatio
n
Heating
Cooling
TEC 7 (Type 2b)
0
200
400
600
800
Temperature
o
C
M
agnetis
atio
n
Heating
Cooling
TEO 7 (Type 2a)
0
200
400
600
800
Temperature
o
C
M
agnetizatio
n
Heating
Cooling
10 POLLARD, SHERWOOD and BÖHNEL
Table 3: Strong-field thermomagnetic properties and low temperature susceptibility groupings of selected samples. Tc1 and Tc2 Curie
temperatures; Curie type classification as described in text; RM the ratio at 100
o
C of magnetization during cooling to magnetiza-
tion during heating; LT
χ
Gp. low temperature susceptibility group.
Tc 1
Tc 2 Curie
LT?
Tc 1
Tc 2
Curie
LT?
Samples
(°C)
(°C)
type
RM
Gp.
Samples
(°C)
(°C)
type
RM
Gp.
EAP SITES
TEC3
580
2b
0.78
3
ALE3
290
1a
1.53
1
TEC7
570
2b
0.66
3
ALE4
590
2a
0.90
3/1
DED2
335
540
4b
1.21
1
NTI2
600
2b
0.86
3/1
DED6
315
545
4b
1.25
1
NTI6
335
1b
1.34
1/3
SAN3
185
500
4b
1.93
1
ALD7
485
3
1.09
3/1
SAN5
190
575
4a
1.10
1
ALD1
530
2a
1.01
1
POZ3
310
1a*
7.27
4
ALT5
200
585
4b
1.21
1
POZ8
315
1a*
5.00
4
ALT4
570
2b
0.67
2
MIC1
560
2c
0.84
1
ARD7
180
1a
1.79
1
MIC7
560
2c
0.79
1
ARD2
320
535
4a
1.02
2
BAR2
310
570
4b
1.26
1
MOL5
BAR9
375
1a
1.76
1
FLO3
580
2b
0.68
1
STV5
325
1b
1.56
1
FLO6
570
2b
0.65
STV6
325
1b
1.52
1
HUA2
330
595
4b
1.73
HUA6
340
575
4b
1.66
TMVB SITES
MET3
590
2b
0.84
SAM5
260
1a
1.89
1
MET6
550
2b
0.86
SAM2
210
530
4b
1.22
1
ZQU1
625
2b
0.82
GAS4
545
2a
0.94
ZQU4
580
2a
0.97
2
GAS6
570
2a
0.93
1
ATO3
560
2b
0.86
2
RED3
530
2a
1.00
3
ATO7
560
2b
0.78
2
RED8
580
2b
0.85
3
DYK4
585
2a
0.94
PAN1
340
550
1a
1.28
1
DYK8
PAN5
305
1a
1.52
1
FUD3
MID5
325
550
4b
1.21
1
PAC2
MID3
330
560
4a
1.02
1
TEO2
560
2b
0.89
3
MAR4
235
1a
1.70
1
TEO7
560
2a
0.90
3
MAR8
220
1a
1.75
1
CEL3
540
2a
0.92
2
POC6
230
1a
1.83
1
CEL8
540
4a
1.02
2
POC5
235
1a
1.72
1
TUZ5
570
2b
0.86
2
SAH9
350
1b
1.46
1
TUZ2
560
2b
0.85
2
SAH4
350
545
4b
1.26
1
ORI6
560
2a
0.94
2
GUN5
590
2b
0.81
3
ORI9
340
545
4a
1.00
3
GUN1
535
2a
1.09
1
ZAB2
565
2b
0.86
3
BUG1
565
2b
0.81
2
ZAB4
560
2b
0.86
3
BUG7
580
2b
0.78
3
PIZ4
575
2a
0.96
3
TAC2
590
2b
0.86
3
PIZ2
535
2b
0.86
3
TAC4
590
2b
0.88
3
TLA7
275
590
4b
1.13
3/1
RIO2
580
2b
0.89
3
TLA4
590
2a
0.97
RIO5
590
2b
0.78
3
Type 4 curves (Fig. 5) show two distinct ferrimagnetic
phases. One is a low temperature phase and usually occurs
at <350
o
C. The second phase is higher with Curie tempera-
tures >500
o
C. Only the high temperature phase exists on
the cooling curve. Most of the type 4 curves also record a
higher magnetization after cooling than at the start of heat-
ing. The low and high temperature ferrimagnetic phases are
interpreted to be caused by Ti-rich and Ti-poor populations
of titanomagnetites respectively. The occurrence of the two
populations together is inferred to be as a result of partial
deuteric oxidation, where some of the Ti-rich titanomagne-
tite is oxidized to the Ti-poor variety with ilmenite lamellae.
As only the high temperature phase is found on the cooling
curve this suggests that the low temperature phase is a cat-
ion-deficient titanomagnetite, probably as a result of low
temperature oxidation (Sherwood 1988).
Of the samples tested on the Curie balance the majority
(over 50 %) exhibited type 2 and 3 curves, whilst the sam-
PRELIMINARY RESULTS FROM ROCK MAGNETIC ANALYSES OF BASALTS 11
ples with a low Curie temperature make up approximately
22 % of those tested. However, most of the type 4 samples
appear to be dominated by the low temperature phase. This
indicates that there is approximately an equal number of
samples with a primarily Ti-poor phase to those which are
dominated by a Ti-rich phase.
Low temperature susceptibility
Information gained from low temperature susceptibility
experiments provides some information about the grain size
and composition of magnetic minerals. Since this technique
was first developed, a generally accepted method of classi-
fication and interpretation has evolved (see Radhakrishna-
murty et al. 1977; Radhakrishnamurty 1985, 1993; Senan-
ayake & McElhinny 1981; Shaw et al. 1991; Sherwood
1988, 1992; and Sherwood & Basu Mallik 1996), which has
been adapted for this study. Classification of a sample is
done by observing the shape of the curve gained from the
experiment and placing it into one of three main groups:
group 1 curves (Fig. 6) exhibit an increase in susceptibility
from 78 K to room temperature (293 K) and so have RS val-
ues <1. Group 2 curves (Fig. 6) show a decrease in suscepti-
bility from 78 K to 293 K and therefore RS is >1. Group 3
curves (Fig. 6) exhibit a peak at around 125 K whilst the
susceptibility values at 78 and 293 K are similar having RS
values ca. 1. Samples which exhibit curves with compo-
nents common to two of the three groups are labelled 3/1 or
1/3 depending on the group from which the dominant com-
ponent is derived (the first number represents the dominant
component group). Unusual samples which show a marked
peak at 250 K (group 4) from one site within this study are
discussed later.
The mean values of RS (
χ
78
/
χ
293
) for susceptibility groups
1, 2 and 3 are 0.43, 1.54 and 1.14 respectively. These values
are similar to those found by Senanayake & McElhinny
(1981) and by Sherwood (1988).
Low temperature susceptibility behaviour can not be attrib-
uted to any chemical or physical state of magnetic minerals
without further information (Radhakrishnamurty 1993), ex-
cept that the behaviour of group 3 samples is almost certainly
caused by MD magnetite. From the information shown in Ta-
ble 2 it can be seen that the vast majority of samples in group
1 have a Curie temperature indicative of Ti-rich titanomagne-
tites, and that all the samples that have been classed as group
3 show Curie temperatures which indicate Ti-poor magnetic
minerals. These two observations agree with the hypotheses
that Ti-rich titanomagnetites will have group 1 behaviour and
that MD magnetite will have group 3 behaviour (Radhakrish-
namurty 1985, 1993; Senanayake & McElhinny 1981). Like
group 3 samples, the samples of group 2 all have Curie tem-
peratures in excess of 480
o
C, and this is consistent with the
findings of other authors. Unlike group 3 samples however,
there are no generally accepted conclusions that can be
reached about the behaviour of group 2 samples. Radha-
krishnamurty (1977, 1985) ascribes group 2 behaviour to the
presence of cation-deficient magnetite, but as in Sherwood
(1988), all except one of the samples in this study which have
titanomaghemite inversion peaks (Curie type 1b), have group 1
behaviour. Senanayake & McElhinny (1981) explain group 2
behaviour as a result of the presence of titanomagnetite grains
with exsolved ilmenite lamellae. Sherwood & Basu Mallik
(1996) suggested that the presence of primary ilmenite is likely
to be the principle cause of this type of behaviour. Reflected
light microscopy will be able to detect the presence of ilmenite
in our samples. The results presented here do not support any
definite conclusions concerning this behaviour, although hys-
teresis plots do suggest the presence of paramagnetic material
(possibly ilmenite) in most of the group 2 samples.
Repeatable plots were obtained from the POZ samples
(Fig. 6) which have a marked peak at ca. 250 K. Additional-
Fig. 5. Examples of Curie curve types 3 and 4.
ALD 7 (Type 3)
0
200
400
600
800
Temperature
o
C
Magnetizatio
n
Heating
Cooling
SAN 5 (Type 4a)
0
200
400
600
800
Temperature
o
C
Magnetizatio
n
Heating
Cooling
BAR 2 (Type 4b)
0
200
400
600
800
Temperature
o
C
Magnetizatio
n
Heating
Cooling
12 POLLARD, SHERWOOD and BÖHNEL
ly, thermomagnetic analyses of these samples produced
curves with a single low Curie temperature (Ti-rich) phase
spread over a range of ca. 100
o
C (see Fig. 3). Shaw et al.
(1991) and Sherwood (1992) describe similar peaks in their
susceptibility analyses, and have attributed them to the un-
blocking of a low blocking temperature SD titanomagnetite.
Room temperature susceptibility
The values for low- and high-field mass-specific suscepti-
bility (see Table 2) are almost exactly the same for each in-
dividual sample site: a scale of the difference between the
two values is given by percentage frequency dependent sus-
ceptibility (
χ
fd%). From the
χ
fd% value one can tell wheth-
er the sample has a significant quantity of ferri- and ferro-
magnetic grains lying close to the superparamagnetic (SP)/
single domain (SD) grain size boundary (Thompson & Old-
field 1986). The values of
χ
fd% obtained for all but 6 of the
samples is <1%, i.e. <1% of the overall low frequency sus-
ceptibility is contributed to by SP grains. The other samples
have
χ
fd% values <3 which is still only a minor overall con-
tribution from SP grains.
Discussion
The rock magnetic data from both the Late Tertiary and
Quaternary sites (Table 1) were compared to note any chro-
nological variation in characteristics. The only obvious dif-
ference noted is that all the Quaternary samples tested have
a high temperature Curie phase, whereas the Tertiary sites
have a significant proportion of samples that yielded a low
temperature phase. Therefore, all the Quaternary samples
have Ti-poor titanomagnetite, whilst the magnetic minerals
of the Late Tertiary samples are either Ti-rich or Ti-poor ti-
tanomagnetite. There appear to be no other obvious chrono-
logical patterns in the remaining rock magnetic data.
A comparison of our data with previous findings from the
same region show similar rock magnetic properties (note
that results from sites south of latitude 20
o
N can only be di-
Fig. 6. Typical low temperature susceptibility curves for each group.
A R D 5
( g r o u p 1 )
0
0 .1
0 .2
0 .3
0 .4
0 .5
0 .6
0 .7
0 .8
0 .9
- 2 0 0
- 1 0 0
0
T e m p e ra tu r e
o
C
χ/χ
0
A L T 4
( g r o u p 2 )
0
0 .2
0 .4
0 .6
0 .8
1
1 .2
1 .4
1 .6
1 .8
2
- 2 0 0
- 1 0 0
0
T e m p e ra tu re
o
C
χ/χ
0
T E C 5
( g r o u p 3 )
0
0 .2
0 .4
0 .6
0 .8
1
1 .2
1 .4
1 .6
1 .8
- 2 0 0
- 1 0 0
0
T e m p e ra tu r e
o
C
χ/χ
0
P O Z 7
( g r o u p 4 )
0
0 .5
1
1 .5
2
2 .5
3
3 .5
4
4 .5
- 2 0 0
- 1 0 0
0
T e m p e ra tu re
o
C
χ/χ
0
PRELIMINARY RESULTS FROM ROCK MAGNETIC ANALYSES OF BASALTS 13
rectly compared as there is no comparable data north of this
latitude in the Gulf Coast region). Strong field thermomag-
netic analysis by Böhnel (1985, 1997) found samples with
Curie temperatures mostly in the range 520
o
C to 580
o
C
which generally agree with the findings of this study. The
only slight difference is that this study revealed a greater
number of samples dominated by a low temperature Curie
phase. Gonzalez et al. (1997) studied the central and west-
ern TMVB and show a similar distribution of high and low
temperature Curie phases.
Hysteresis parameters obtained by Böhnel (1985) give
values for M
RS
/M
S
that are very similar to the values gained
in this study, indicating that the vast majority of volcanic
rocks from this area have a mixture of SD and MD grains.
Only a small minority has either predominantly one or the
other domain states.
A comparison between the results of EAP and TMVB
samples (Tables 2 and 3) reveal no significant differences in
rock magnetic characteristics. Differences do exist however,
between samples, and these are assumed to be due to factors
such as compositional variation, exposure to weathering etc.
Summary and further work
These results indicate that for the majority of the sites
studied there are significant amounts of stable magnetic
minerals within the rocks. Therefore, they should be suit-
able for a paleomagnetic study in which the primary objec-
tive will be to obtain site-mean paleomagnetic directions
and paleopoles. These values will be used to try to recon-
struct the tectonic history of the Gulf Coast region from the
Late Tertiary to present day, which is uncertain at this time.
A further field season has been completed and these results
are being processed at present. The combination of the new
data with that presented here will give a more even spatial
distribution of site locations. However, these results at least
prove that further study is worthwhile.
Acknowledgements: This research was funded by Liverpool
John Moores University and Universidad Nacional Autono-
ma de Mexico. Thanks are due to the staff at the Geomag-
netism Laboratory, University of Liverpool where some of
the measurements were carried out. We are also grateful to
the Gonzalez-Huesca family, Ramon Sandoval and the Insti-
tuto de Geofisica for their help with the fieldwork.
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