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, FEBRUARY 2014, 65, 1, 83—95 doi: 10.2478/geoca-2013-0006
Introduction
Chromian spinel [hereafter Cr-spinel, (Mg,Fe
2+
)(Cr,Al,Fe
3+
)
2
O
4
]
is a common accessory mineral in ultramafic and mafic
rocks. Its composition has often been used as a sensitive in-
dicator in order to determine the degree of partial melting in
the mantle source region and/or the composition of the pro-
duced mafic melt (e.g. Dick & Bullen 1984; Arai 1992; Zhou
et al. 1998; Proenza et al. 1999; Barnes & Roeder 2001;
Hellebrand et al. 2001; Kamenetsky et al. 2001; Zhou et al.
2005; González-Jiménez et al. 2011). In addition, variations
concerning Cr-spinel composition in peridotites are also
known to reflect dissimilarities in the processes involved in
the evolution of upper mantle rocks, such as partial melting
and mantle metasomatism (e.g. Kubo 2002; Arif & Jan 2006),
or even discrepancies in the geotectonic setting in which they
were formed (e.g. Ishii et al. 1992; Ahmed et al. 2012).
Compared to other high-T igneous phases Cr-spinel is
thought to be resistant to post-magmatic processes such as al-
teration and regional metamorphism (e.g. Burkhard 1993;
Barnes 2000; Mellini et al. 2005). Therefore, it is especially
useful in evaluating the tectonic provenance of strongly
hydrated mantle peridotites (serpentinites), since it commonly
represents the only preserved primary phase (e.g. Saumur &
Hattori 2013). However, there is now plenty of convincing
Composition and alteration of Cr-spinels from Milia and
Pefki serpentinized mantle peridotites (Pindos Ophiolite
Complex, Greece)
ARGYRIOS KAPSIOTIS
Department of Geology, Section of Earth Materials, Panepistimiopolis of Rion, University of Patras, 265 04 Patras, Greece;
Present address: Department of Earth Sciences, Sun Yat-sen University, 510275 Guangzhou, P.R. China; kapsiotisa@yahoo.gr
(Manuscript received May 9, 2013; accepted in revised form October 16, 2013)
Abstract: The Pindos Ophiolite rocks include variably serpentinized peridotites derived from a harzburgitic and subor-
dinately dunitic mantle. In the serpentinized matrix of these rocks pseudomorphic (mesh, bastite) and non-pseudomor-
phic (interpenetrating, type-2 hourglass) textures were recognized. Chromian spinel (Cr-spinel) is anhedral to subhedral
and often replaced by a porous opaque phase. Chemistry data show that Cr-spinel cores retain their original composi-
tion, having Cr#[Cr/(Cr + Al)] that ranges between 0.45 and 0.73, and Mg#[Mg/(Mg + Fe
2+
)] that varies between 0.52
and 0.65, accompanied by low content in TiO
2
( < 0.11 wt. %). The relatively wide variation of their Cr# values reflects
that the studied peridotites were produced by variable degrees of melting. It is likely that the Pindos peridotites repre-
sent mantle residues originally formed in a mid-ocean ridge (MOR) environment, which were subsequently entrapped
as part of a mantle wedge above a supra-subduction zone (SSZ) regime. Cr-spinel adjacent to clinochlore systematically
displays limited compositional and textural zoning along grain boundaries and fractures. However, the degree of peri-
dotite serpentinization does not correlate with the abundance of zoning effects in accessory Cr-spinel. Thus, Cr-spinel
zoning is thought to represent a secondary feature obtained during the metamorphic evolution of the host peridotites.
Core to rim compositional trends are expressed by MgO and Al
2
O
3
impoverishment, mainly compensated by Cr
2
O
3
and
FeO increases. Such chemical trends are produced as a result of Cr-spinel re-equilibration with the surrounding serpen-
tine, and their subsequent replacement by ferrian (Fe
3+
-rich) chromite and clinochlore, respectively, during a brief, fluid
assisted, greenschist facies metamorphism episode (T > 300 °C). The limited occurrence of ferrian chromite with high
Fe
3+
# values suggests that elevated oxidizing conditions were prevalent only on a local scale during Cr-spinel alteration.
Key words: Pindos, metamorphism, peridotites, Cr-spinel, ferrian chromite.
evidence that Cr-spinel may undergo significant chemical
modifications related to sub-solidus equilibration during the
post-magmatic stage. More specifically, textural observations
and mineral chemistry data indicate that hydrothermal alter-
ation and metamorphism can significantly modify primary
Cr-spinel composition (e.g. Bliss & MacLean 1975; Evans &
Frost 1975; Wylie et al. 1987; Kimball 1990; Suita & Streider
1996; Barnes 2000; Mellini et al. 2005; Farahat 2008; Merlini
et al. 2009; Rollinson et al. 2012; Sansone et al. 2012).
High Cr/Al, low Mg/Fe
2+
and considerable to high Fe
3+
contents describe the most common alteration trend of
Cr-spinel. The composition of this alteration product can be
expressed as (Fe
2+
,Fe
3+
,Mg)(Cr,Fe
3+
,Fe
2+
,Al)
2
O
4
and is
known in the literature as ferrian chromite (formerly called
‘ferritchromite’ or ‘ferritchromit’, Spangenberg 1943; and
cited in, among others, Evans & Frost 1975; González-
Jiménez et al. 2009; Mukherjee et al. 2010; Gervilla et al.
2012; Derbyshire et al. 2013). Various investigations sug-
gest that ferrian chromite forms during low-T hydrothermal
alteration (e.g. Burkhard 1993; Wylie et al. 1987; Mukherjee
et al. 2010). However, the majority of studies indicate that
ferrian chromite is produced as a consequence of metamor-
phism (e.g. Bliss & MacLean 1975; Barnes 2000; Proenza et
al. 2004; Mellini et al. 2005; Merlini et al. 2009; Grieco &
Merlini 2012).
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The present paper discusses the compositional variability
of accessory Cr-spinels from a set of mantle peridotite bod-
ies, located in the areas of Milia and Pefki, Pindos Ophiolite
Complex, Greece. In this study, Cr-spinel compositions are
used as a petrogenetic tool to conclude the genesis and geo-
tectonic origin of the studied mantle peridotites. Moreover,
the current study attempts to provide insights into the influ-
ence of serpentinization and metamorphism on Cr-spinels,
with the aim of explaining their alteration patterns on the ba-
sis of textural and mineral chemistry data.
Geological framework
The External Hellenides are part of the Alpine orogenic
belt, representing a typical fold-thrust belt. They are mainly
composed of Mesozoic and Cenozoic sedimentary rocks de-
posited in a series of platforms (pre-Apulian and Gavrovo
Zones) and basins (Ionian and Pindos Zone). According to
Jones & Robertson (1991) the Pindos Zone is made up of a
series of Mesozoic and Tertiary tectono-stratigraphic units
including: 1) the Pindos Ophiolites (Jurassic), 2) the shallow-
water Orliakas limestones (Late Cretaceous), 3) the Avdella
Mélange (Late Triassic—Late Jurassic), 4) the Dio Dendra
Group deep-water sediments (Late Jurassic—Late Cretaceous)
and 5) the Pindos flysch (Late Cretaceous—Tertiary).
The ophiolites of northwestern continental Greece are con-
sidered to be oceanic remnants after the progressive closure
of the Neotethyan Ocean. Among these the Pindos Ophiolite
Complex (Fig. 1) has been extensively studied with regard to
its structural features and tectonic position (e.g. Ross & Zim-
merman 1996; Rassios & Smith 2000; Rassios & Moores
2006; Ghikas et al. 2009; Rassios & Dilek 2009).
The Pindos Ophiolite Complex is located in northwestern
Greece and corresponds to a piece of Middle to Upper Jurassic
oceanic crust (Rassios & Smith 2000). It is tectonically em-
placed over the autochthonous Maastrichtian-Eocene Pindos
flysch. It can be subdivided into four principal tectonic units:
the Dramala Ultramafic Complex, the Loumnitsa Unit and the
Aspropotamos Complex, all structurally overlying a sub-
ophiolitic chaotic lithological formation known as the Avdella
Mélange (Jones & Robertson 1991). The Dramala Complex
represents oceanic mantle and part of its crustal sequence and
comprises variably depleted spinel harzburgite—dunite masses
(>1000 km
2
), which may host small chromitite pods, pyroxen-
ite and ultramafic cumulates (Jones & Robertson 1991). Lo-
Fig. 1. Simplified geological map of the Pindos Ophiolite Complex showing the location of the areas of Milia and Pefki (modified after
Jones & Robertson 1991) and inset map illustrating the location of the Pindos Ophiolites in the Greek peninsula.
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COMPOSITION AND ALTERATION OF Cr-SPINELS FROM SERPENTINIZED PERIDOTITES (GREECE)
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cally, harzburgite and chert breccias are cemented by ophical-
cite, indicating that the Dramala ultramafic rocks were once
exposed on the sea floor (Jones et al. 1991). The intrusive and
extrusive crustal rocks of the Aspropotamos Complex cover a
wide range of geochemical affinities, varying from N-MORB
through MORB/IAT to IAT and boninites cross-cutting all the
previous types of volcanics (Kostopoulos 1989). The Loum-
nitsa Unit represents the metamorphic sole of both Dramala
and Aspropotamos complexes, consisting of low amphibolite-
and greenschist-facies metaigneous and metasedimentary
rocks that have yielded amphibole Ar-Ar ages of 169 ± 5 and
165 ± 3 Ma (Whitechurch & Parrot 1978; Spray & Roddick
1980). The Avdella Mélange represents a subduction-accre-
tion formation and includes sediments, volcanic and plutonic
rocks, as well as metamorphic rocks of the Loumnitsa Unit.
Sampling and field observations
A total of 17 serpentinized peridotite samples were col-
lected from the Pindos Ophiolite Complex. The peridotite
samples were taken from the Milia and Pefki districts, each
of them located in the southeastern part of the Dramala Mas-
sif. Care was taken to sample variably serpentinized rocks
from different mantle lithologies.
The studied areas are extremely mountainous (with sum-
mits between 1500 and 2150 m above sea level). Extensive
outcrops of peridotites are well exposed both in Milia and
Pefki. The peridotites are coarse-grained and strongly de-
formed. Deformed massive and variably serpentinized
harzburgite is the dominant rock type in the mantle section
of both investigated regions, commonly cross-cut by pyrox-
enite dykes and gabbroic veins, especially in the area of Pefki.
Milia harzburgites may contain blocky-shaped pyroxene,
which probably represents a preserved high-T ‘asthenospheric’
(or ‘mantle’) fabric (Rassios & Dilek 2009). On the other
hand, dunite is subordinate to harzburgite in both areas and
relatively more uncommon in the area of Pefki. Dunite oc-
curs in the form of small pods or bodies ranging from a few m
to a few tens of m in size. The largest dunite bodies com-
monly host podiform chromitites. The contact between
harzburgite and dunite is transitional and frequently sheared
and serpentinized. Moreover, dunite is commonly more ser-
pentinized and mylonitized compared to harzburgite.
Serpentinization is almost pervasive through the whole
mantle section in Milia, whereas in Pefki serpentinization ef-
fects are only local within the ultramafic section. Serpenti-
nized rocks are frequently strongly deformed, displaying
foliation along sizeable shear zones (a few m in thickness),
which further indicates that these rocks were affected by re-
gional metamorphism. They are commonly covered by a thick
reddish to dark brown crust made up of a complex mixture of
iron oxide with clay minerals, owing to local weathering.
Petrography
The peridotite samples are variably serpentinized and gener-
ally composed of harzburgite and subordinate dunite. The de-
gree of serpentinization is up to 40 vol. % in harzburgite and
does not exceed 50 vol. % in dunite. In addition, completely
serpentinized rocks originating from both peridotite types
were also found. In serpentinized harzburgite, the primary min-
eral phases include olivine and orthopyroxene porphyroclasts
accompanied by minor relicts of clinopyroxene and accessory
Cr-spinel, whereas secondary phases include serpentine, chlo-
rite and magnetite accompanied by minor tremolite and talc.
Serpentinized dunite contains relicts of olivine, Cr-spinel and
serpentine accompanied by subordinate chlorite and tremolite.
In the serpentinized peridotite matrix four different types
of texture were recognized, in decreasing order of abun-
dance: I) mesh texture defined by serpentine and magnetite
that replace olivine, II) bastite texture represented by serpen-
tine pseudomorphs after orthopyroxene, III) interpenetrating
texture and IV) type-2 hourglass texture.
The mesh cores consist of serpentine, magnetite, chlorite
(Fig. 2a) or olivine relicts, since replacement of olivine by
serpentine and magnetite proceeds towards the olivine por-
phyroclast core. In some cases, outlines of large olivine crys-
tals ( > 1 mm) may be preserved in the groundmass of
mesh-textured serpentinites after dunite, which indicates that
dunites are mostly coarse-grained similarly to their harzbur-
gitic equivalents. In the mesh rims, serpentine fibres are
commonly perpendicularly oriented to the mesh cells. Bas-
tite pseudomorphs are made up of serpentine (Fig. 2b), trem-
olite, chlorite or talc. They have elongated ovoidal shape and
commonly exhibit Cr-spinel exsolution lamellae. Deformation
characteristics such as kinking and undulatory extinction may
still be recognizable at some bastite grains. Interpenetrating
texture consists of elongated, intersecting blades of serpentine,
whereas type-2 hourglass texture exhibits wavy extinction due
to recrystallization (Fig. 2c; O’Hanley & Wicks 1995). In
some samples these non-pseudomorphic textures are superim-
posed on the pseudomorphic ones (mesh and bastite).
Except for serpentine the other secondary silicate phases
replace mantle exsolved orthopyroxene and clinopyroxene
crystals. Moreover, chlorite was also found to form either in-
tergrowths with serpentine or aureoles surrounding Cr-spinel
grains. In the last case, chlorite is subhedral and bladed in
shape and it was found to overprint mesh serpentine. Apart
from replacing pyroxenes, tremolite also occurs as randomly
oriented prismatic crystals scattered in the serpentinized
groundmass. Magnetite commonly forms irregular networks
(Fig. 2d) or occurs as fine dusty grains scattered in the ser-
pentinized matrix. In some samples magnetite is overgrown
on Cr-spinel grains (Fig. 2e), which represents textural evi-
dence supporting its formation during serpentinization (syn-
serpentinization magnetite).
Spinel textures
Cr-spinel constitutes less than 2—3 vol. % of the studied
rocks. It occurs as isolated and fractured crystals in the peri-
dotite groundmass. Grain sizes vary within samples, but
most are between 0.1 and 1.5 mm. They appear to form red
to dark brown coloured anhedral to subhedral crystals. More-
over, optical microscopy revealed that several Cr-spinel
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Fig. 2. a – Mesh texture (crossed nicols/XPL); b – Bastite texture marked by the white dashed line (XPL); c – Type-2 hourglass texture
marked by the white dashed line (XPL); d – Magnetite network in serpentinized peridotite (back-scattered electron image/BSE); e – Syn-
serpentinization magnetite overgrown on unaltered anhedral Cr-spinel (BSE); f – Cr-spinels exhibiting optical zoning (plane polarized
nicols/PPL); g – Complete ferrian chromite rim in a Cr-spinel grain armoured by clinochlore aureole (BSE, micrograph taken from Kapsiotis
et al. 2007). h – Partly altered Cr-spinel grain. Abbreviations: Ser – serpentine, Chl – clinochlore, Spn – Cr-spinel, Mgt – magnetite,
Bst – bastite, Ol – olivine, Fe-Chr – ferrian chromite, Tlc – talc.
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COMPOSITION AND ALTERATION OF Cr-SPINELS FROM SERPENTINIZED PERIDOTITES (GREECE)
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grains were optically inhomogeneous (Fig. 2f). Most of them
show irregular zoning due to replacement by an opaque
phase along cracks and grain boundaries. In petrographic
terms only Cr-spinels in contact with chlorite display zoning.
On the other hand, Cr-spinel grains solely in contact with
serpentine are homogeneous.
Combined electron microscopy and mineral chemistry ana-
lytical work revealed that the opacity of the studied Cr-spinels
is mainly correlated with an increase in their Fe
2+
content.
Their compositional zoning is apparent in back-scattered elec-
tron images (BSE), in which the edges of the zoned Cr-spinel
grains appear to be bright and separated from the inner part of
the grains by sharp contacts. Zoning, although commonly lim-
ited, advances from grain boundaries and fractures towards the
inner part of the grain, which gives the crystal edges the ap-
pearance of being ragged and poorly defined (Fig. 2g). On the
other hand, a few grains exhibiting patchy zoning were also
found (Fig. 2h). Generally, the opaque zones have a porous
structure displaying sieve texture. Analytical work revealed
that the most sizeable pores are filled with chlorite. Intrigu-
ingly each zoned Cr-spinel is surrounded by chlorite aureoles
(Fig. 2g). These aureoles are up to 70 µm thick and display
grey to purple colours under cross-polarizers.
It is worthy mentioning that the extent of Cr-spinel zoning
is not directly correlated with the degree of serpentinization
of the host rocks. Even though harzburgite is generally less
altered compared to dunite, harzburgitic Cr-spinels display
zoning more frequently. Moreover, zoning extent and thick-
ness are greater in Cr-spinels hosted in harzburgite. However,
there are samples in which some accessory Cr-spinels may
display zoning and some others not. Additionally, no zoning
effect was observed in accessory Cr-spinel from 3 serpenti-
nized dunite samples.
Analytical techniques
Cr-spinels were investigated in situ and imaged using a Su-
per JEOL JSM—6300 scanning electron microscope (SEM) at
the University of Patras, Greece. The quantitative analyses of
Cr-spinel core—rim pairs, serpentine and chlorite (clinochlore)
were performed using a Super JEOL JSM-6300 microprobe
operated in wavelength-dispersive spectrometry (WDS) mode.
Its operating conditions were 15 kV accelerating voltage and
20 nA beam current, with 4 µm beam diameter. The ZAF cor-
rection software was put into use (Reimer 1998). Calibrations
were done using natural and synthetic reference materials. The
proportion of Fe
3+
in Cr-spinel was calculated assuming ideal
spinel stoichiometry (AB
2
O
4
). 27 Cr-spinel and ferrian
chromite pair analyses (19 from harzburgite and 8 from dun-
ite) were carried out on 14 serpentinized mantle rock samples.
Selected pair analyses of Cr-spinel and ferrian chromite from
the studied serpentinized peridotites are listed in Table 1,
whereas representative analyses of serpentine and clinochlore
are listed in Table 2. Selected spinel-group minerals and cli-
nochlore analytical data are taken from Kapsiotis et al. (2007).
Qualitative X-ray powder diffraction (XRD) analyses were
performed using a Philips PW 1410 powder diffractometer
to better characterize serpentine minerals. Powdered samples
were scanned from 3 to 60° 2
θ, with a step size of 0.02° 2θ
and a count time of 4 sec per step.
Mineral chemistry (and XRD data)
Cr-spinel
Analytical traverses across optically zoned Cr-spinel grains
revealed detectable chemical zoning. In serpentinized rocks
from both areas Cr-spinel cores have quite similar composi-
tions. In particular, Cr-spinel cores have Cr#[Cr/(Cr + Al)]
that ranges between 0.45 and 0.73, and Mg#[Mg/(Mg + Fe
2+
)]
that varies between 0.52 and 0.65 (Fig. 3a), thus they can
Fig. 3. Compositional variations of Cr-spinel cores and ferrian
chromite from the studied serpentinized peridotites in terms of:
a – Cr#[Cr/(Cr+Al)] versus Mg#[Mg/(Mg+Fe
+2
)]. Data for spinel in
modern abyssal peridotites are from Dick & Bullen (1984) and Juteau
et al. (1990). Field for spinel in equilibrium with boninites and
N-MORB’s is taken from Dick & Bullen (1984). Data for spinel in
fore-arc peridotites are from Ishii et al. (1992) and Ohara & Ishii
(1998). b – Classification of the composition of Cr-spinel and ferrian
chromite from the studied peridotites in terms of Cr#[Cr/(Cr + Al)]
versus Mg#[Mg/(Mg + Fe
+2
)]. Cr-spinel composition is also contoured
at a nominal temperature of 1200 °C for olivine compositions from
Fo
90
to Fo
96
(quantitatively computed by Dick & Bullen 1984).
Symbols: black squares – harzburgitic Cr-spinel cores, open cir-
cles – dunitic Cr-spinel cores, grey squares – ferrian chromite.
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Table 1:
Representative
electron-microp
robe
analyses
of
Cr-spinel
and
ferrian
chromite
pairs
from
the
studied
serpentinized
peridotite
s
(wt. %
–
not
detected,
Spn
–
Cr-spinel,
Fe-chr
–
fer
rian
chromite).
Continued
on
the
next
page.
be classified as spinel to magne-
siochromite solid solution mem-
bers (Fig. 3b). Moreover, Fe
3+
/
(Fe
3+
+ Fe
2+
) is up to 0.30. Cr
2
O
3
ranges
between
37.82
and
55.56 wt. %, Al
2
O
3
between 14.12
and 31.27 wt. %, MgO between
10.78 and 14.13 wt. %, and FeO
t
between 15.87 and 26.06 wt. %. In
terms of Cr# and Mg# values Cr-
spinel core analyses plot mostly in
the field of spinel from fore-arc
peridotites (Fig. 3a; Ishii et al.
1992; Ohara & Ishii 1998), whereas
a few analyses plot on the bound-
ary between the fields of spinel
form fore-arc peridotites and abys-
sal peridotites (Fig. 3a; Dick &
Bullen 1984; Juteau et al. 1990).
Furthermore, Cr# is lower in Cr-
spinel from harzburgite (0.45—0.63)
compared to dunite (0.67—0.73;
Fig. 3a).
On the other hand, the composi-
tion of opaque regions varies
within the following ranges:
Cr# = 0.75—0.90, Mg# = 0.12—0.40,
which is indicative of chromite
chemistry (Fig. 3b), while Fe
3+
/
(Fe
3+
+ Fe
2+
) < 0.55. Cr
2
O
3
ranges
between 34.10 and 63.82 wt. %,
Al
2
O
3
between
4.02
and
11.07 wt. %, MgO between 2.27
and 8.05 wt. %, and FeO
t
between
26.21 and 56.04 wt. %.
The TiO
2
content of Cr-spinel
cores is generally very low
(<0.11 wt. %), whereas it can be up
to 0.48 wt. % in the porous opaque
regions. MnO and SiO
2
contents are
also lower in cores (<0.46 wt. %
and <0.32 wt. %, respectively) com-
pared to the rims (<1.88 wt. % and
< 2.39 wt. %). Moreover, Fe
3+
#[Fe
3+
/
(Fe
3+
+Cr+Al)] is low (<0.12), com-
monly showing a slight increase
from core to rim. However, in two
rims it was found to be relatively
elevated (0.38 and 0.40).
Regarding certain mineral chem-
istry features (elevated FeO
t
and re-
duced Al
2
O
3
and MgO contents)
combined with their physical prop-
erties (high reflectivity and low
hardness), it can be claimed that
the opaque regions correspond to a
FeO
t
- and Cr
2
O
3
-rich, Al
2
O
3
-poor
spinel phase generally known as
ferrian chromite.
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Table 1: Continued.
Table 2: Representative electron-microprobe analyses of serpentine and clinochlore from the studied peridotites (wt. % – not detected;
analyses PM11/1 and P22/1 are taken from Kapsiotis et al. 2007).
Area Milia
Pefki Milia
Pefki
Sample PM
2A
PM
2A
PM
5
P
15
P
16
P
21
P
22
PM
2A
PM
11
P
21
P
21
P
22
Mineral Serpentine
Clinochlore
Analysis
1 2 8 6 2 2 1 1 1 4 5 1
SiO
2
44.55
44.04
39.69
41.79
40.64
40.18
40.9
30.04
34.85
34.24
35.37
37.15
TiO
2
0.18
–
0.17
–
–
–
0.05 –
–
–
–
–
Al
2
O
3
1.20
0.54
0.54
0.04
0.17
0.86
0.32
21.07
13.06
18.85
16.75
9.64
Cr
2
O
3
0.40 0.17 –
–
–
0.54 – 0.29 3.33 2.45 2.02 2.37
FeO
t
2.20
1.32
5.39
7.29
7.74
7.51
6.57
5.38
1.42
3.05
3.16
3.47
MgO
41.00
36.83
35.76
39.36
37.01
34.75
35.32
28.04
33.14
33.34
34.63
34.52
NiO
– 0.17 – 0.59
0.23 –
0.21 0.06 0.22 0.16 – 0.42
MnO
0.34
–
0.42
0.08
0.03
–
– 0.01 – 0.10 –
–
CaO
–
0.15
0.08
–
–
–
–
–
0.05 –
–
–
Na
2
O
–
0.44 –
–
–
–
–
–
–
–
–
–
K
2
O
–
0.69 –
–
–
–
–
–
–
–
–
–
Total
89.87
84.35
82.05
89.15
85.82
83.84
83.37
84.89
86.07
92.19
91.93
87.57
Atoms pfu based on O (+ OH) = 9
Atoms pfu based on O = 28
Si
2.001 2.100 1.978 1.936 1.952 1.970 2.007 5.840 6.613 6.099 6.307 6.988
Al
IV
–
–
0.022
0.002
0.010
0.030
–
2.160
1.387
1.901
1.693
1.012
Al
VI
0.063 0.030 0.010 –
–
0.020 0.018 2.664 1.531 2.053 1.824 1.123
Ti
0.006
–
0.006
–
–
– 0.002 –
–
–
–
–
Cr
0.014 0.006 –
–
–
0.021 –
0.045 0.499 0.345 0.284 0.352
Fe
+3
0.070 0.050 0.20 0.250 0.280 0.280 0.240 –
–
–
–
–
Fe
+2
–
–
–
–
–
–
–
0.875 0.225 0.454 0.471 0.546
Ni
–
0.010 – 0.020
0.010 –
0.010 0.010 –
0.020 –
0.060
Mn
0.013 –
0.018 0.003 0.001 –
– 0.002 –
0.015 –
–
Mg
2.745 2.618 2.657 2.718 2.651 2.539 2.583 8.126 9.293 8.854 9.206 9.680
Ca
–
0.008 0.004 –
–
–
–
–
–
–
–
–
Na
–
0.041 –
–
–
–
–
–
–
–
–
–
K
–
0.042 –
–
–
–
–
–
–
–
–
–
Cation sum
4.912
4.905
4.895
4.929
4.904
4.860
4.860 19.722 19.746 19.741 19.785 19.761
Rock Dunite
Area Milia
Pefki
Sample M
2B
M
2B
M
2B
M
2B
PM
12
PM
12
PM
12
PM
12
P
11
P
11
P
11
P
11
Mineral
Spn Fe-chr Spn Fe-chr Spn Fe-chr Spn Fe-chr Spn Fe-chr Spn Fe-chr
Analysis
1core 1rim 3core 3rim 1core 1rim 2core 2rim 1core 1rim 3core 3rim
SiO
2
–
0.36
–
0.24
–
1.57 0.16 0.96
–
0.94
–
0.13
TiO
2
–
–
–
–
–
0.22
–
0.27 0.06 0.24
–
0.10
Al
2
O
3
16.55 7.03 16.71 7.24 14.95 4.95 14.12 5.11 16.49 5.05 17.56 5.23
Cr
2
O
3
49.94 58.08 50.45 57.98 55.55 58.98 55.56 59.01 54.57 59.65 52.75 59.01
Fe
2
O
3
6.04 4.74 4.24 4.43 1.38 2.79 0.90 3.74 0.07 2.73 2.06 4.63
FeO
14.10 24.99 15.41 25.08 17.93 26.30 17.73 26.38 16.93 26.88 16.06 25.46
MnO
0.23 0.23 0.12 0.28
–
1.13 0.05 0.85
–
0.91
–
0.79
MgO
13.27 5.71 12.34 5.45 10.93 5.23 10.78 4.89 11.57 4.41 12.44 4.43
ZnO
–
–
–
–
–
–
–
–
–
–
–
–
Total
100.12 101.13 99.26 100.69 100.74 101.17 99.30 101.21 99.69 100.80 100.88 99.77
Atoms pfu based on O = 4
Si
– 0.012
– 0.008
– 0.054
0.005
0.033
– 0.033
–
0.005
Ti
–
–
–
–
–
0.006 –
0.007 0.001 0.006 –
0.003
Al
0.614
0.283
0.628 0.293 0.563 0.201 0.541 0.208 0.620 0.207 0.647 0.217
Cr
1.243
1.570
1.271 1.576 1.404 1.607 1.427 1.614 1.376 1.643 1.304 1.645
Fe
+3
0.143
0.122
0.102 0.115 0.033 0.072 0.022 0.097 0.002 0.071 0.049 0.123
Fe
+2
0.371
0.715
0.411 0.721 0.479 0.758 0.482 0.763 0.451 0.783 0.420 0.751
Mn
0.006
0.007
0.003 0.008 –
0.033 0.001 0.025 –
0.027 –
0.024
Mg
0.623
0.291
0.586 0.279 0.521 0.269 0.522 0.252 0.550 0.229 0.580 0.233
Zn
– –
–
–
–
–
–
–
–
–
–
–
Cation sum
3.000
3.000
3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000
Cr#
0.67 0.85 0.67 0.84 0.71 0.89 0.73 0.89 0.69 0.89 0.67 0.88
Mg#
0.63 0.29 0.59 0.28 0.52 0.26 0.52 0.25 0.55 0.23 0.58 0.24
Fe
3+
#
0.072
0.062
0.051 0.058 0.017 0.038 0.011 0.051 0.001 0.037 0.025 0.062
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Serpentine
As it was identified by XRD data, the serpentine ground-
mass of the studied hydrated peridotites consists of antigorite
and chrysotile (Fig. 4). Antigorite was found to be the domi-
nant serpentine polymorph in interpenetrating and type-2
hourglass texture. Most of serpentine grains contain relatively
elevated concentrations of Al
2
O
3
(up to 2.37 wt. %), consis-
tently with antigorite composition. SiO
2
ranges between 41.94
and 46.67 wt. %, MgO between 35.71 and 42.04 wt. %,
whereas Cr
2
O
3
is up to 0.88 wt. %. Chrysotile grains contain
lower Al
2
O
3
(up to 0.46 wt. %), whereas Cr
2
O
3
content is be-
low the detection limit of the electron microprobe.
Chlorite
Chlorites are characterized by relatively high Cr
2
O
3
(up to
3.33 wt. %). Their SiO
2
content varies between 30.04 and
37.15 wt. %, whereas Al
2
O
3
concentration may be up to
21.07 wt. % and MgO content ranges between 28.04 and
34.63 wt. %. Concentrations of TiO
2
, MnO and NiO are low
and sometimes even below detection limits. The Si contents
of chlorite classify them as clinochlore after the classifica-
tion of Bailey (1980). Their chemical characteristics are sim-
ilar to those of chlorites from other hydrated ophiolitic
peridotites (e.g. Jan & Windley 1990).
Discussion
Primary Cr-spinel compositions
The compositions of Cr-spinel cores are generally similar
within individual samples. They are characterized by moder-
ate to elevated Cr#(0.45—0.73), high Mg#(0.52—0.65), low
Fe
3+
# resembling in composition Cr-spinels in podiform
chromitites from the Rayat area, northeastern Iraq (Arai et al.
2006). In addition, their TiO
2
and MnO contents are low
( < 0.11 wt. % and < 0.46 wt. %, respectively), which is rather
usual for unaltered Cr-spinel in ultramafic rocks (e.g. Barnes
2000; Singh & Singh 2013). Cr-spinel cores should form at
high T’s in equilibrium with olivine containing ~ Fo
93
, since
their composition runs between the Fo
90
and Fo
96
contours
(Fig. 3b). Moreover, Cr-spinel core compositions plot out-
side of the fields of metamorphic spinel compositions
(Fig. 5), which further implies that they have not been affected
by post-magmatic processes and retain their primary compo-
sition. Therefore, they can be used as indicators to unravel
the petrogenesis of peridotites.
The composition of accessory Cr-spinel in peridotites is
regarded as a useful tool for revealing melting processes in
the mantle (e.g. Okamura et al. 2006; Uysal et al. 2007). It is
known that Cr# of spinel is sensitive to melting processes
and that systematically increases with the degree of peridotite
depletion (e.g. Zhou et al. 2005; Uysal et al. 2012). Based on
that criterion it is deduced that the studied peridotites were
produced by variable degrees of mantle melting and that
dunite represents a mantle residue resulting from higher
melting degrees compared to harzburgite.
Except for partial melting, melt-peridotite interaction pro-
cess may account for the high Cr# values in spinel (e.g. Uysal
et al. 2012). In contrast to metasomatically added Cr-spinel
grains the studied ones are commonly anhedral to subhedral
in shape and depleted in TiO
2
, which are suggestive of their
residual origin. Additionally, Cr-spinels analysed in the
present study bear no compositional similarities with spinel
in equilibrium with boninite melts. On the other hand, a few
analyses plot on the boundary of the field representing the
composition of spinel in equilibrium with normal mid ocean
ridge basalts (N-MORBs, Fig. 3a). However, a MORB melt
would crystallize spinels having low Cr# ( < 0.60) and elevated
TiO
2
. Therefore, any magmatic/metasomatic origin of the
examined Cr-spinels should be precluded.
It has been established that Cr-spinel composition can re-
flect formation of mantle rocks in different tectonic regimes
(e.g. Zhou et al. 2005; Ahmed et al. 2012). According to a
number of studies Cr-spinels with Cr#<0.60 are commonly
found in abyssal peridotites related to the lithospheric mantle
emplaced near the ocean ridge, whereas those having higher
values are found in peridotites produced in a supra-subduction
zone (SSZ) environment (e.g. Dick & Bullen 1984; Juteau et
al. 1990; Ishii et al. 1992; Ohara & Ishii 1998). In such set-
tings, fluid assisted partial melting leads to higher peridotite
melting degrees, thereby elevating Cr# in Cr-spinel (e.g.
González-Jiménez et al. 2011; Derbyshire et al. 2013). In
terms of Cr# vs Mg# values the majority of Cr-spinel analyses
plot in the field of spinel from fore-arc peridotites (Fig. 3a).
On the other hand, a few harzburgitic Cr-spinel analyses plot
near the intersection of spinel fields form fore-arc peridotites
and modern abyssal peridotites (Fig. 3a), thus indicating that
they represent hybrid mineral compositions.
The compositional features of Cr-spinels from the Pindos
peridotites indicate that these rocks were probably affected
by a two-stage melting process, including: 1) a partial melt-
ing episode in a mid ocean ridge (MOR) setting, recorded in
residual harzburgites with low Cr# (<0.60) Cr-spinels, 2) fol-
lowed by subsequent entrapment and melting of harzburgites
in the mantle wedge above an intra-oceanic SSZ. The latter
melting episode is documented by the existence of high Cr#
Fig. 4. X-ray diffraction (XRD) diagram showing the presence of
antigorite and chrysotile polymorphs in the studied serpentinized
peridotites. Abbreviations: Ant – antigorite, Chr – chrysotile.
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( > 0.60) Cr-spinels in both peridotite types and was most
likely responsible for the formation of dunites.
Origin of zoning in Cr-spinel: implications for the post-
magmatic evolution of peridotites
Zoning in Cr-spinel might originate as a result of various
processes. For instance in cumulate rocks zoning in Cr-spinel
may be the product of reactions with the intercumulus liquid
(e.g. Saumur & Hattori 2013). However, the studied serpen-
tinized ultramafic rocks represent hydrated residual mantle
peridotites, having petrographic and mineral chemistry fea-
tures inconsistent with a cumulate origin. On the other hand,
the examined peridotites could have reacted with mafic
melts produced by fluid assisted partial melting of the mantle
wedge above a subducted slab. Melt-peridotite interaction
may produce zoning patterns in accessory Cr-spinel (e.g.
Mondal & Zhou 2010). However, there is no textural evidence
to support such a possibility and as was discussed above, the
studied Cr-spinels are supposed to have a residual origin.
Textural evidence indicates that the zoning pattern in
Cr-spinel is not a primary feature. In fact, it is apparently re-
lated to the post-magmatic processes by which the peridotites
were affected since zoned Cr-spinel occurs exclusively in al-
tered rock samples. However, it is not clear which process,
serpentinization or regional metamorphism, is responsible
for the alteration of Cr-spinel. The investigated peridotites
are not uniformly serpentinized. Moreover, zoning is not so
common in accessory Cr-spinel from serpentinized dunite. In
addition, Cr-spinels bordered by serpentine do not exhibit
any alteration marks. Furthermore, the common finding of
clinochlore aureoles surrounding zoned Cr-spinel and over-
printing mesh serpentine provides evidence that the crystalli-
zation of clinochlore post-dated serpentinization. Equally,
clinochlore in the examined hydrated peridotites contains
higher Al
2
O
3
, Cr
2
O
3
and Mg/Fe compared to those in olivine
or pyroxene, thus it could not have formed by isochemical
replacement of these silicate phases during serpentinization.
According to a number of recent researches Cr-spinel re-
mains almost unaffected during serpentinization, although it
becomes variably altered during low-grade metamorphism
(e.g. Mellini et al. 2005; Mikuš & Spišiak 2007; Farahat
2008; González-Jiménez et al. 2009; Merlini et al. 2009;
Gervilla et al. 2012; Singh & Singh 2013). According to
Bach et al. (2006) common low-T serpentinization, taking
place during ocean-floor metamorphism cannot cause any
changes in Cr-spinel composition. Furthermore, Merlini et
al. (2009) suggest that Cr-spinels that were not subjected to
any higher-T metamorphic overprints after serpentinization
retain their mantle/igneous composition. In addition, Barnes
(2000) suggested that only prograde metamorphism might
have a significant impact on Cr-spinel chemistry at a post-
magmatic stage of evolution.
Overall observations indicate that Cr-spinel undergoes dis-
solution along fractures and grain boundaries, being partially
replaced by ferrian chromite. Although well-developed cli-
nochlore growths are common adjacent to the altered parts of
Cr-spinel grains, zoning in the latter is texturally limited to
their boundaries and fractures. The irregular development of
ferrian chromite along grain boundaries and fractures further
indicates that no crystallographic orientation has been fol-
lowed for its formation and that alteration is heterogeneous
even on grain scale. Moreover, it indicates that alteration pro-
cess had not occurred uniformly from all directions (e.g. Bliss
& MacLean 1975; Mukherjee et al. 2010). Thus, textural evi-
dence suggests that the alteration rims of zoned Cr-spinel rep-
resent only the initial stages of its compositional modification.
According to Candia & Gaspar (1997) complete metamorphic
re-equilibration takes place only when P
H2O
= P
total
. Under
P
H2O
< P
total
, relict primary textures are preserved. Moreover,
Merlini et al. (2009) suggested the following reaction for ferrian
chromite formation: 2(Mg
0.60
Fe
0.40
)(Cr
1.30
Al
0.70
)O
4
+ 3/2
( M g
2 . 5 7
A l
0 . 3 2
F e
0 . 1 1
) S i
2
O
5
( O H )
4
+H
2
O + 1 / 1 2 O
2
7 / 6
( M g
0 . 4 0
F e
0 . 6 0
) ( C r
1 . 8 5
F e
0 . 0 8
A l
0 . 0 7
) O
4
+ 1 / 2
(Mg
9.18
Fe
0.34
Al
1.60
Cr
0.88
)(Al
2
Si
6
)O
20
(OH)
16
, and supported
that replacement of Cr-spinel by ferrian chromite is com-
monly partial, because the reaction between Cr-spinel and
serpentine during metamorphism does not convert all of the
reactants and the environment acts as a closed system with
the exception of fluids. Cr-spinel textures show that the ini-
tial size of the grains remained unchanged after alteration.
However, a mass loss took place during ferrian chromite for-
mation (indicated by the reaction above) resulting in the for-
mation of pores.
The occurrence of ferrian chromite in hydrated forearc
mantle peridotites is very rare (e.g. Tsujimori et al. 2004;
Saumur & Hattori 2013). On the other hand, the formation of
ferrian chromite in serpentinites requires heating after ser-
pentinization (e.g. Cerny 1968). Although high-T phases,
such as antigorite, tremolite and talc are present in the stud-
ied hydrated peridotites, the presence of texturally ‘imma-
ture’ ferrian chromite is in accordance with affection of
peridotites by a short-lived thermal event, which did not al-
low complete development of thick ferrian chromite rims.
The occurrence of ferrian chromite in the examined rocks in-
dicates that they have been affected by a brief heating event
uncommon in cases of typical serpentinite exhumation. Field
observations support that serpentinites display foliation
along well-developed shear zones, which provides further
evidence that the studied peridotites were affected by regional
metamorphism. These shear zones have been developed dur-
ing the upward protrusion of the investigated peridotites
(Rassios & Moores 2006), from the base of the mantle
wedge through its hotter interior towards cooler shallow
crustal levels along major thrust zones. Probably the passage
of these already hydrated peridotites through the hot mantle
wedge interior caused a thermal event allowing ferrian
chromite formation to take place.
Ferrian chromite formation
Cr-spinel replacement by secondary phases like ferrian
chromite and Cr-magnetite is direct evidence that even Cr-Fe-
oxides may become unstable in the post-magmatic environ-
ment (e.g. Farahat 2008; González-Jiménez et al. 2009;
Mukherjee et al. 2010). Textural and compositional varia-
tions in the studied zoned Cr-spinel grains suggest that alter-
ation to ferrian chromite took place after serpentinization.
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During that metamorphic episode primary Cr-spinel (now
preserved in the core of zoned Cr-spinels) lost Al
2
O
3
and
MgO, whereas it became enriched in FeO
t
and residually in
Cr
2
O
3
(Fig. 5). Kimball (1990) suggested that during Cr-spinel
breakdown, Cr is preferentially incorporated in Cr-spinel and
Al in chlorite and that explains the higher Cr and lower Al
content in altered spinel. Mg and Al are fixed in the coexist-
ing layer silicates (chrysotile/antigorite) to promote the for-
mation of clinochlore. In addition, the core has lower
concentrations of MnO and SiO
2
compared to the rim
(Fig. 5). Mn commonly substitutes for Fe
2+
in Cr-spinel lat-
tice (e.g. Singh & Singh 2013). However, Mn is more sus-
ceptible to leaching (Stanton 1972), thus MnO increase from
core to rim is in accordance with Mn release from the sur-
rounding olivine upon serpentinization (e.g. Barnes 2000).
Furthermore, the SiO
2
content detected in ferrian chromite
rims should be ascribed to septochlorite intergrowths within
the pores (e.g. Mellini et al. 2005; Derbyshire et al. 2013). It
is worth mentioning that although high ZnO contents may be
common in altered Cr-spinel from metamorphosed ultramafic
rocks (e.g. Barnes 2000; Singh & Singh 2013) the present
ferrian chromite compositions are depleted in Zn. High Zn
contents in ferrian chromite might be explained by Cr-spinel
re-equilibration with olivine prior to serpentinization, be-
cause Zn is commonly concentrated in olivine. However, the
olivine in the examined rocks does not contain ZnO (unpub-
lished data), which further explains the absence of Zn from
the ferrian chromite.
The examined ferrian chromite compositions differ from
those commonly reported in the literature, especially in terms
of Fe
3+
#, which is suggestive of their compositional ‘immatu-
rity’. In the vast majority of the studied zoned Cr-spinels
Fe
3+
# shows a weak increase from core to rim, having low
values ( < 0.12). However, two ferrian chromite analyses
show higher Fe
3+
# (0.38 and 0.40). Their slight Fe
3+
enrich-
ment suggests the local passage to relatively more oxidizing
alteration conditions. Gervilla et al. (2012) claimed that
Fe-bearing fluid circulation takes place in such conditions,
assisted by the already formed interconnected network of
pores in Cr-spinel, causing dissolution of the silicates (mainly
chlorite) in the pores and promoting diffusion of Fe
2+
and
Fe
3+
into ferrian chromite, according to the following reaction:
(Fe
0.6
Mg
0.4
)Cr
2
O
4
+ Fe
3
O
4
2(Fe
0.8
Mg
0.2
)CrFeO
4
. The pres-
ent data suggest that the amount of magnetite component
added in ferrian chromite was very restricted and the reac-
tion proposed above remained incomplete.
Metamorphic evolution path
Projection of Cr-spinel analyses on the triangular diagram
Al
3+
—Cr
3+
—Fe
3+
indicates that ferrian chromite compositions
plot mainly within the compositional field of Cr-spinel
Fig. 5. BSE image (white square on Fig. 2h), showing limited Cr-spinel replacement by ferrian chromite (indicated by the black dashed line)
and clinochlore and profile line analyses illustrating elemental variations between clinochlore, ferrian chromite and unaltered Cr-spinel.
Abbreviations: Chl – clinochlore, Fe-chr – ferrian chromite, Ser – serpentine, Spn – Cr-spinel.
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formed during the greenschist facies metamorphism (Fig. 6).
In addition, two ferrian chromite analyses plot within the
field of spinel affected by low amphibolite facies metamor-
phism. T is supposed to range between 200 and 400 °C dur-
ing greenschist facies metamorphism (Ernst 1993).
Moreover, according to Merlini et al. (2009) T needs to be
strictly above 300 °C to allow metamorphism to provoke
significant changes in Cr-spinel chemistry. The presence of
tremolite and talc in the secondary assemblage of the Pindos
hydrated peridotites provides direct evidence that T rose
above 400 °C (Evans & Frost 1975) during metamorphism.
Additionally, the occurrence of clinochlore/antigorite inter-
growths in the altered silicate groundmass of the studied
rocks, combined with the absence of metamorphic olivine
(after dehydration of antigorite) implies that T did not exceed
the range of lower amphibolite facies metamorphism (Caruso
& Chernosky 1979).
Therefore, the following two-stage post-magmatic evolu-
tion scenario is proposed in order to explain Cr-spinel alter-
ation in the Pindos serpentinized peridotites. During
ocean-floor hydrothermal alteration, low T serpentinization
did not produce any compositional or textural change in
Cr-spinel, as it took place under reducing conditions (indi-
cated by the low Fe
3+
# values), probably at T’s below 300 °C
where chrysotile is stable (e.g. Schwartz et al. 2013). The
second stage mainly has to do with the formation of ferrian
chromite under higher T ( > 300 °C) hydrous fluid-saturated
conditions, and involves the dissolution—precipitation reac-
tion of primary Cr-spinel with serpentine to produce cli-
nochlore and FeO- and Cr
2
O
3
-rich, Al
2
O
3
-poor Cr-spinel,
according to the reactive mechanism proposed by Merlini et
al. (2009). These hydrous MgO- and SiO
2
-rich fluids (proba-
bly derived from first stage low-T serpentinization of olivine
and pyroxene) had the opportunity to carry out element dif-
fusion exchanges, promoting a compositional gradient
across Cr-spinel grains, which is now being viewed as zon-
ing. Cr-spinel and serpentine re-equilibration to form ferrian
chromite and clinochlore, respectively, took place around the
transition from mid- to advanced greenschist facies meta-
morphism (at T’s > 300 °C), which is also supposed to stabi-
lize antigorite.
Generally, the present ferrian chromite compositions indi-
cate metamorphic alteration of Cr-spinel under elevated
greenschist facies conditions. However, two ferrian chromite
analyses, characterized by the highest Fe
3+
contents (Fig. 6),
are suggestive of more oxidizing conditions. These ferrian
chromite compositions in the studied peridotites imply the
passage to low-grade amphibolite facies metamorphism
(550 °C < T < 600 °C). However, such metamorphic grade was
only locally achieved, as it is indicated by the coincidental
presence of ferrian chromite with high Fe
3+
# values.
Conclusions
Variably serpentinized peridotites occur in the Dramala
Unit of the Pindos Ophiolite Complex in Greece. Accessory
Cr-spinel in these rocks occasionally displays limited alter-
ation to an opaque phase along fractures and grain bound-
Fig. 6. Compositional changes in Cr-spinels from the Pindos serpen-
tinized peridotites expressed in a triangular Al—Fe
3+
—Cr plot with
special reference to the fields of the different metamorphic facies de-
fined for Cr-spinels by Purvis et al. (1972), Evans & Frost (1975) and
Suita & Streider (1996). Solvus determined at 600, 550 and 500 °C
by Shack & Ghiorso (1991) for chromite coexisting with olivine
containing 90% forsterite. Symbols: black squares – harzburgitic
Cr-spinel cores, open circles – dunitic Cr-spinel cores, grey
squares – ferrian chromite.
aries. Cr-spinel cores preserve their primary composition,
which further indicates that the studied rocks represent resid-
ual mantle peridotites that were produced by variable de-
grees of melting through a two-stage melting process, initiated
in a MOR setting that was evolved in a SSZ. Cr-spinel is al-
tered to a FeO- and Cr
2
O
3
-rich but MgO- and Al
2
O
3
-poor
spinel phase, referred to as ferrian chromite, which is ar-
moured by clinochlore overgrowing mesh serpentine. The
extent and frequency of Cr-spinel replacement by ferrian
chromite does not correlate with the degree of rock serpenti-
nization and appears to be the result of a short-lived thermal
event. Overall data show that alteration of Cr-spinel took
place after serpentinization, mainly during an episode of ad-
vanced greenschist facies metamorphism.
Acknowledgments: This paper is based in part on the Ph.D.
Thesis of A. Kapsiotis at the University of Patras, Greece. Drs.
D. Lenaz, P. Uher and T. Mikuš are gratefully acknowledged
for their constructive criticism on a preliminary version of the
manuscript. Special thanks are also due to Dr. I. Broska for his
editorial comments. The author is thankful to Dr. K. Hatzipa-
nagiotou for his encouragement and those colleagues from the
Department of Geology at the University of Patras who did
not tire in sharing ideas. V. Kotsopoulos of the Laboratory of
Electron Microscopy and Microanalysis, University of
Patras, is also acknowledged for his assistance with the mi-
croanalyses and SEM micrographs. Research was partly sup-
ported by the University of Patras, ‘K. Karatheodoris’
program and Pythagoras I Project, which is co-funded by the
European Social Fund and national resources (EPEAK). A.
Kapsiotis was also supported by the State Scholarship Foun-
dation of Greece (IKY) during his Ph.D. study.
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