Ore Geology Reviews 110 (2019) 102926
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Ore Geology Reviews
journal homepage: www.elsevier.com/locate/oregeorev
Initial subduction of Neo-Tethyan ocean: Geochemical records in chromite T
and mineral inclusions in the Pozantı-Karsantı ophiolite, southern Turkey
Xia Liua,b,c,⁎,1, Ben-Xun Sua,b,c, Yan Xiaob,d, Chen Chena,b,c, Ibrahim Uysale, Jie-Jun Jinga,b,c,
Peng-Fei Zhangf, Yang Chub,d, Wei Linb,c,d, Patrick Asamoah Sakyig
a Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
b Institutions of Earth Science, Chinese Academy of Sciences, Beijing 100029, China
cUniversity of Chinese Academy of Sciences, Beijing 100049, China
d State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
e Department of Geological Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey
fDepartment of Earth Sciences, the University of Hong Kong, Pokfulam Road, Hong Kong, China
g Department of Earth Science, University of Ghana, P.O. Box LG 58, Legon-Accra, Ghana
A R T I C L E I N F O A B S T R A C T
Keywords: Chromitites in the Pozantı-Karsantı ophiolite in Turkey mainly occur as podiform chromitites within mantle
Chromite harzburgite and stratiform-like chromitites in mantle-crust transition zone. Chromites in chromitites have varing
Trace elements Cr# from 62.8 to 80.3 and can be divided into two types, namely; intermediate (Cr#: 62.8 – 69.2) and high-Cr
Mineral inclusions (Cr#: 73.9 – 80.3) types. Major elements of the high-Cr chromitite have an affinity with boninite, whereas the
Parental magma intermediate chromitite shows transitional features between MORB and boninite. The compositional differences
Subduction initiation
in clinopyroxene inclusions between intermediate- and high-Cr chromitite, coupled with the relatively high trace
element contents (e.g. V, Ga) in the high-Cr chromitite, indicate distinctive parental magmas. Trace elemental
profile analysis of a nodular chromite grain in one nodular chromitite sample PK14-41 demonstrates significant
but non-systematic variations from the core to the rim, which also confirmed the compositional heterogeneity of
the parental magmas. The presence of primary hydrous mineral inclusions such as amphibole in chromite, to-
gether with Ca-rich minerals (e.g. calcite), reflect the water-rich and Ca-rich characteristics of the parental
magma. The higher fO2 of high-Cr chromitite evidenced by the lower V/Mn values may be due to more oxidized
fluids released from downgoing crustal materials. Thus, we conclude that the parental magmas of the Pozantı-
Karsantı chromitite were derived from a proto-forearc mantle and evolved to higher fO2 with the subduction
initiation at that time, but were water- and Ca-rich in general.
1. Introduction divided into two types based on its occurrences, namely; podiform
chromitite within mantle peridotite and the banded or layered chro-
Ophiolites are commonly regarded as remnants of perished oceanic mitite associated with ultramafic cumulate along the crust-mantle
lithosphere (e.g. Coleman, 1977; Pearce et al., 1984), which are key boundary (Paktunc, 1990). Podiform chromitites, only occurring –1 to
indicators of petrotectonic and ancient plate-tectonic activity (e.g. 1.5 km below the petrographic Moho, are pervasively surrounded by
Stern, 2005 and references therein). In the 1980s, Pearce et al. (1984) mantle harzburgites with dunite envelopes. Banded or layered chro-
came up with the concept of “suprasubduction zone” ophiolite due to mitite interlayering with dunite cumulate is interpreted as igneous
the similarity of some ophiolite lava compositions to convergent mar- cumulate charged by magma fractionation similar to stratiform chro-
gins. Therefore, ophiolites that form as a result of subduction initiation mitites within layered complex (e.g. Kinnaird et al., 2002).
may preserve imprints of initiation of subduction and formation of Chromitites are divided into two categories according to the Cr# of
some critical mineral resources such as chromitite (e.g. Aitchison et al., chromite, namely; high-Cr (Cr# > 60; Cr#=100×Cr/(Cr+Al) and
2000; Whattam and Stern, 2011; Stern, 2004, Stern et al., 2012; Chen high-Al (Cr# < 60) chromitites (e.g. Arai, 1994; Zhou et al., 1994,
et al., 2018, 2019; Uysal et al., 2018). Ophiolitic chromitites can be 1998, 2014; Robinson et al., 2015; Arai and Miura, 2016). However,
⁎ Corresponding author.
E-mail address: liuxia@mails.iggcas.ac.cn (X. Liu).
1 Present address: No. 19, Beitucheng Western Road, Chaoyang District, 100029, Beijing, PR China.
https://doi.org/10.1016/j.oregeorev.2019.05.012
Received 5 February 2019; Received in revised form 21 April 2019; Accepted 10 May 2019
Available online 11 May 2019
0169-1368/ © 2019 Elsevier B.V. All rights reserved.
X. Liu, et al. Ore Geology Reviews 110 (2019) 102926
studies in recent years have identified three varieties of chromitites as correspondingly change from high-Al to intermediate and ultimately to
high-Cr (Cr# > 70), high-Al (Cr# < 50) and intermediate high-Cr varieties (e.g. Chen et al., 2018, 2019; Uysal et al., 2018).
(50 < Cr# < 70) (Chen et al., 2018; Uysal et al., 2016; Uysal et al., Nonetheless, the physical property and chemical composition of the
2018). Researchers have proposed that high-Al and high-Cr chromitites evolving parental magmas that formed ophiolitic chromitites especially
are formed by interaction between mantle peridotite and MORB-like the intermediate type remain puzzling for geoscientists, and therefore,
melt and boninitic melt, respectively (Zhou et al., 1994, 1998, 2014; requires a comprehensive study to constrain them.
Uysal et al., 2009; González-Jiménez et al., 2011; Stern et al., 2012; Trace element compositions of chromite (Sc, Ti, V, Mn, Co, Ni, Zn
Akmaz et al., 2014; Robinson et al., 2015; Arai and Miura, 2016). The and Ga) are regarded as useful tools of revealing the compositions of the
Cr# of chromite is considered as a petrogenetic indicator of mafic-ul- parental magmas of chromitite and their formation settings (Pagé and
tramafic rocks and their tectonic settings. Currently, some studies on Barnes, 2009; Colás et al., 2014; Zhou et al., 2014; Avcı et al., 2017;
Turkish ophiolites have pointed out that intermediate chromitite could Uysal et al., 2018; Su et al., 2019). Furthermore, various primary mi-
be closely related to the inception of subduction (e.g. Chen et al., 2018, neral and fluid inclusions entrapped in chromite are also invaluable in
2019; Uysal et al., 2018; Saka et al., 2019). With ongoing subduction documenting characteristics of chromitite parental magmas and con-
initiation, melts that are generated by decompression melting of up- ditions such as temperature and oxygen fugacity responsible for the
welling asthenospheric mantle in the “subduction zone” might evolve agglomeration of chromite grains (e.g. Lorand and Ceuleneer, 1989;
from MORB-like to boninitic melt accompanied by transformation of Melcher et al., 1997; Borisova et al., 2012; Zhou et al., 2014; Avcı et al.,
the composition of the parental magmas; thus, chromitite would 2017; Lian et al., 2017a; Johan et al., 2017; Liu et al., 2018). Studies of
Fig. 1. (a) Distribution of ophiolites in Turkey (after Robertson, 2002; Chen et al., 2018). (b) Geological map (after GDMRE, 2002; Su et al., 2018) and (c) vertical
section (after Dilek and Thy, 2009; Parlak et al., 2009; Su et al., 2018) of the Pozantı-Karsantı ophiolite.
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X. Liu, et al. Ore Geology Reviews 110 (2019) 102926
these primary inclusions entrapped in chromite provide important in- sequence of mafic-ultramafic cumulates, isotropic gabbros and over-
formation about the characteristics of parental magma and the forma- lying pelagic sediments (Fig. 1c). The mantle peridotites are dominated
tion setting of chromitites. by harzburgite and subordinate dunite and lherzolite, and continue to
In this study, we present a systematic study of major and trace overly ultramafic cumulates of dunite, pyroxenite and wehrlite
element compositions of chromite in chromitite in the Pozantı-Karsantı (Fig. 1c). Peridotites occurring in mantle and crustal sequences are
ophiolite, southern Turkey. These data, together with mineral inclu- commonly crosscut by mafic dikes. Geochronological studies on the
sions in chromite, are used to constrain the origin and nature of par- mafic dikes and metamorphic soles indicate ages of 107–83Ma (Thuizat
ental magmas from which chromitite formed. Previously published Ca- et al., 1981; Dilek et al., 1999; Çelik and Chiaradia, 2008) and
enrichment characteristics of parental magmas of chromitites (Liu et al., 92–87Ma (Dilek et al., 1999; Lian et al., 2017b), respectively. Chro-
2018) will receive further documentation and constraint. mitites in the Pozantı-Karsantı ophiolite, mainly occurring in mantle
peridotite and cumulate rocks, form the bulk of the economic deposits
2. Geological setting and sample collection of the Pozantı-Karsantı area.
2.1. Geological setting 2.2. Sample collection and petrology of chromitites
The geologic terrane of Turkey is made up mainly of five major There is large-scale occurrence of chromitites in the Pozantı-
tectonic units, arranged from north to south, as follows; the Pontides Karsantı ophiolite that are now being mined mainly in open pits
domain, the Anatolian terrane, the Central Taurus terrane, the South (Fig. 2a). They occur as podiform within mantle peridotites (Fig. 2b)
Taurides exotic unit, and the Peri-Arabian domain (e.g. Bozkurt and and as banded in ultramafic cumulate rocks (Fig. 2c). Podiform chro-
Mittwede, 2001; Moix et al., 2008; Okay, 2008). Ophiolites are spread mitites, forming the bulk economic deposits of the Pozantı-Karsantı
widely in these tectonic units (Fig. 1a), and represent the relics of chrome ore mine, are commonly surrounded by dunite envelopes with
disappeared Neo-Tethyan oceanic lithosphere and record the process of sharp or contiguous boundaries hosting harzburgites, while the envel-
collision between the Arabian plate and Eurasian plate. The South opes might be perished by the fault in some cases (Fig. 2b). Banded
Tauride exotic unit is traditionally divided into three contiguous parts, chromitites, showing rhythmic layers with dunite, is inferred to be close
namely; the western, central and eastern Taurides (Özgül, 1976). From to the petrological Moho (Fig. 2c; Parlak et al., 2000, 2002; Su et al.,
west to east, the exotic unit, made up of many well-preserved ophiolites 2018). These chromitites in different structural levels are variable in
with Cretaceous ages (e.g. Lycian nappes, Pozantı-Karsantı, Kızıldağ, morphology from massive and disseminated to nodular, anti-nodular
Guleman). and banded chromitites (Fig. 2d). In this study, we selected 23 various
The Pozantı-Karsantı ophiolite is located in the eastern Tauride belt types of chromitites, including 6 sparsely disseminated chromitites, 2
with an exposed area of approximately 1300 km2 (Fig. 1b). It has been dense chromitites, 7 massive chromitites, 6 banded chromitites and 1
suggested that the Pozantı-Karsantı ophiolite separated from the Mersin nodular chromitite. Detailed petrologic observation, major and trace
ophiolite by the Ecemiş left strike-slip (e.g. Lytwyn and Casey, 1995; element analyses and BSE (backscattered electrons) images were con-
Polat et al., 1996; Parlak et al., 2000, 2002). The ophiolite rests on a ducted on these selected samples.
thin sheet of metamorphosed oceanic crust termed metamorphic sole Petrology of the selected chromitites from the Pozantı-Karsantı
tectonically underlain by an ophiolitic mélange that contains ophiolitic ophiolite was determined via microscopic observation of thin-sections
fragments (Fig. 1c). During the Cretaceous, the Pozantı-Karsantı under transmitted and reflected lights. The studied chromitites consist
ophiolite, together with the underlying metamorphic sole and ophiolitic mainly of chromite and olivine, rarely with anhedral clinopyroxene
mélange, overthrusted on the Late Devonian to Early Cretaceous car- occurring as silicate matrix of chromitites. Individual chromite grains
bonate platform basement (Fig. 1c; Dilek et al., 1999; Robertson, 2002). from both podiform chromitite and ultramafic cumulate are mostly
The ophiolite is made up mainly of mantle peridotites and crustal subhedral to euhedral, with varying diameters from several μm to
Fig. 2. (a). General view of an open pit of chromite mining site in the Pozantı-Karsantı ophiolite; (b) Podiform chromitite with very thin dunite envelope in
harzburgite; (c) Inter-layered dunite and chromitite; (d) Field photo showing different types of chromitites in the Pozantı-Karsantı ophiolite.
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Table 1
Major oxides (wt.%) of chromite in two types of chromitites from the Pozantı-Karsantı ophiolite, south Turkey.
Sample Texture SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO NiO Na2O K2O Total Mg# Cr#
Intermediate chromitite
PK14-01 S 0.36 0.28 16.2 47.0 22.9 0.24 12.2 0.10 0.10 0.23 0.04 99.7 58.7 66.0
0.18 0.30 15.5 47.8 24.3 0.24 11.1 0.02 0.15 0.04 0.01 99.5 53.3 67.4
0.08 0.27 15.6 47.3 25.5 0.26 11.0 0.00 0.06 0.00 0.00 100.0 52.2 67.1
PK14-25 D 0.02 0.07 17.6 48.9 14.7 0.25 14.5 0.01 0.11 0.00 0.00 96.1 69.6 65.0
0.05 0.09 17.7 48.6 14.7 0.25 14.5 0.03 0.06 0.02 0.00 96.0 69.6 64.8
0.03 0.10 17.2 48.9 15.1 0.30 14.0 0.01 0.09 0.00 0.00 95.7 68.1 65.7
0.01 0.09 16.9 49.3 15.4 0.26 13.9 0.02 0.10 0.03 0.00 95.9 67.5 66.3
0.04 0.09 17.4 52.2 14.8 0.22 14.7 0.00 0.12 0.00 0.00 99.5 68.7 66.8
0.05 0.08 17.7 52.2 14.8 0.23 14.9 0.03 0.09 0.03 0.00 100.1 69.1 66.5
0.04 0.06 17.7 51.9 14.7 0.21 14.7 0.03 0.13 0.00 0.00 99.5 68.5 66.3
0.02 0.11 17.0 52.1 14.7 0.23 14.6 0.01 0.10 0.00 0.01 98.8 68.8 67.3
0.06 0.08 17.3 51.9 14.5 0.22 14.7 0.00 0.09 0.00 0.00 98.8 68.9 66.8
0.06 0.08 17.0 52.2 14.6 0.25 14.5 0.04 0.09 0.02 0.00 99.0 68.5 67.2
PK14-02 M 0.02 0.31 18.5 46.6 15.9 0.24 14.4 0.02 0.14 0.02 0.00 96.1 68.9 62.8
0.16 0.29 18.2 46.4 15.8 0.25 14.5 0.05 0.14 0.04 0.00 95.9 69.5 63.1
0.06 0.26 18.5 46.8 15.8 0.24 14.6 0.01 0.14 0.01 0.00 96.3 69.4 62.9
0.02 0.31 17.9 49.1 16.6 0.14 16.0 0.00 0.14 0.00 0.00 100.2 72.7 64.8
0.03 0.28 17.9 49.4 16.8 0.20 15.8 0.01 0.14 0.00 0.01 100.6 72.0 64.9
0.03 0.27 17.9 49.7 16.6 0.15 15.8 0.02 0.15 0.03 0.00 100.5 72.2 65.1
0.04 0.28 17.0 50.8 16.7 0.14 15.2 0.03 0.12 0.01 0.01 100.3 70.1 66.7
0.02 0.28 17.0 50.5 16.8 0.18 15.1 0.01 0.17 0.00 0.01 100.1 69.8 66.5
0.02 0.31 17.7 50.2 16.4 0.12 15.3 0.02 0.16 0.00 0.00 100.3 70.1 65.5
0.02 0.26 18.1 49.6 16.7 0.17 15.1 0.00 0.14 0.01 0.01 100.2 69.5 64.7
0.12 0.27 18.0 49.4 16.7 0.15 15.1 0.02 0.15 0.00 0.01 99.9 69.3 64.9
PK14-56 M 0.00 0.30 15.9 47.9 18.0 0.18 13.4 0.02 0.16 0.02 0.00 95.8 65.1 66.9
0.02 0.32 15.8 47.9 18.1 0.18 13.1 0.00 0.12 0.00 0.00 95.6 64.0 67.0
0.04 0.37 15.5 48.0 18.2 0.17 13.2 0.02 0.10 0.00 0.00 95.6 64.1 67.4
0.02 0.32 15.3 51.2 19.2 0.22 14.4 0.02 0.12 0.00 0.01 100.8 66.7 69.2
High-Cr chromitite
PK14-12 S 0.04 0.18 11.3 58.7 17.5 0.32 12.8 0.01 0.07 0.00 0.01 101.0 61.4 77.7
0.07 0.16 11.2 58.7 17.6 0.31 12.9 0.03 0.08 0.00 0.00 101.2 61.8 77.8
0.03 0.17 11.2 59.1 17.2 0.31 13.3 0.00 0.05 0.00 0.02 101.4 63.1 77.9
0.04 0.19 11.2 58.7 18.5 0.33 12.3 0.01 0.04 0.00 0.00 101.3 58.8 77.9
PK14-27 S 0.06 0.20 12.2 54.0 15.7 0.30 13.3 0.02 0.08 0.01 0.00 95.8 65.9 74.8
0.11 0.20 12.3 53.6 16.0 0.30 13.1 0.01 0.08 0.01 0.00 95.7 64.8 74.6
0.12 0.17 12.5 53.6 15.9 0.31 12.8 0.01 0.09 0.00 0.01 95.5 63.8 74.2
0.08 0.20 12.3 58.1 15.9 0.25 13.8 0.03 0.11 0.00 0.00 100.8 65.1 76.0
0.08 0.17 12.1 58.2 15.6 0.25 13.8 0.03 0.10 0.02 0.00 100.3 65.9 76.4
0.09 0.21 12.3 58.0 15.6 0.26 13.7 0.04 0.09 0.02 0.00 100.2 65.1 76.0
0.13 0.20 12.4 58.5 15.9 0.27 13.7 0.04 0.11 0.00 0.01 101.2 64.4 76.0
PK14-44 S 0.12 0.14 9.92 59.7 20.1 0.34 10.4 0.03 0.07 0.01 0.00 100.8 50.8 80.1
0.05 0.14 10.1 60.4 18.3 0.34 11.5 0.01 0.06 0.00 0.00 101.0 55.9 80.0
0.20 0.14 9.93 59.7 18.6 0.34 11.2 0.02 0.04 0.00 0.00 100.1 54.6 80.1
0.04 0.15 10.3 60.0 18.5 0.32 11.7 0.01 0.05 0.00 0.00 101.0 56.4 79.6
0.06 0.13 9.87 60.2 18.3 0.34 11.5 0.03 0.04 0.00 0.00 100.4 56.0 80.3
PK14-46 S 0.04 0.12 10.8 55.6 18.2 0.22 11.0 0.01 0.03 0.02 0.00 96.1 55.8 77.5
0.04 0.14 10.7 55.3 18.7 0.25 10.5 0.01 0.06 0.03 0.00 95.7 53.6 77.6
0.03 0.16 10.8 55.0 18.2 0.22 10.6 0.00 0.08 0.01 0.00 95.1 54.2 77.4
0.04 0.14 10.8 54.8 18.5 0.25 10.4 0.02 0.01 0.01 0.00 95.0 53.6 77.3
PK14-57 S 0.08 0.22 10.0 58.7 18.6 0.18 13.3 0.00 0.07 0.00 0.00 101.3 63.3 79.7
0.05 0.20 9.87 59.0 18.5 0.21 13.2 0.01 0.09 0.01 0.00 101.1 62.9 80.0
0.07 0.20 10.3 59.2 18.0 0.28 12.9 0.03 0.09 0.01 0.00 101.1 61.9 79.4
0.18 0.21 10.1 59.1 18.5 0.29 12.1 0.05 0.08 0.00 0.00 100.7 58.4 79.6
0.16 0.19 10.2 59.3 17.8 0.30 13.0 0.07 0.10 0.00 0.00 101.1 62.0 79.5
PK14-64 D 0.05 0.19 11.2 55.7 15.6 0.16 13.0 0.02 0.06 0.03 0.00 96.0 64.8 77.0
0.11 0.22 11.1 55.3 15.9 0.20 12.9 0.01 0.07 0.01 0.00 95.8 64.1 76.9
0.02 0.20 11.1 55.4 15.7 0.13 12.8 0.02 0.06 0.01 0.00 95.5 64.2 77.0
0.02 0.15 11.2 59.9 16.1 0.27 13.3 0.00 0.06 0.00 0.01 100.9 63.3 78.2
0.04 0.19 10.6 60.0 16.6 0.28 12.0 0.00 0.08 0.00 0.00 99.8 58.3 79.1
0.06 0.19 11.0 59.5 16.0 0.26 13.1 0.00 0.04 0.04 0.00 100.1 63.0 78.4
0.02 0.21 10.8 59.6 15.8 0.26 13.2 0.00 0.04 0.00 0.00 100.0 63.6 78.7
PK14-03 M 0.01 0.21 13.4 56.5 15.8 0.19 14.6 0.02 0.13 0.01 0.00 100.9 68.5 73.9
0.32 0.22 13.0 56.9 15.3 0.18 13.3 0.02 0.09 0.00 0.00 99.4 63.4 74.6
PK14-09 M 0.01 0.17 11.6 54.7 15.6 0.30 13.5 0.02 0.11 0.01 0.01 96.0 67.3 76.0
0.03 0.18 11.3 54.8 15.3 0.28 13.5 0.00 0.13 0.00 0.01 95.5 67.7 76.5
0.06 0.20 11.5 57.2 15.4 0.20 14.1 0.00 0.05 0.03 0.01 98.8 68.0 77.0
0.04 0.17 11.0 57.1 16.5 0.20 13.5 0.08 0.10 0.04 0.01 98.7 65.7 77.6
(continued on next page)
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X. Liu, et al. Ore Geology Reviews 110 (2019) 102926
Table 1 (continued)
Sample Texture SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO NiO Na2O K2O Total Mg# Cr#
0.03 0.22 11.6 57.2 15.2 0.20 14.5 0.00 0.07 0.01 0.02 99.0 69.3 76.7
PK14-23 M 0.00 0.10 12.4 57.0 13.6 0.01 14.2 0.02 0.15 0.01 0.01 97.4 68.9 75.4
0.00 0.12 12.7 56.4 13.4 0.00 14.2 0.01 0.14 0.01 0.06 97.1 69.6 74.9
0.00 0.10 12.5 56.5 13.9 0.00 14.4 0.01 0.16 0.00 0.00 97.6 69.6 75.2
PK14-28 M 0.01 0.10 12.8 59.3 14.2 0.22 14.5 0.03 0.17 0.02 0.01 101.3 68.4 75.7
PK14-31 M 0.06 0.11 12.4 58.2 13.4 0.21 14.7 0.00 0.15 0.03 0.00 99.3 70.2 75.9
0.02 0.13 11.9 58.3 14.1 0.25 14.1 0.01 0.11 0.00 0.00 98.9 68.0 76.7
0.03 0.15 12.2 58.3 13.6 0.21 14.8 0.01 0.14 0.00 0.00 99.4 70.7 76.2
0.05 0.11 12.7 58.3 13.4 0.21 14.7 0.00 0.10 0.00 0.00 99.5 70.0 75.5
PK14-51 B 0.04 0.17 10.6 59.9 15.7 0.32 14.4 0.00 0.08 0.00 0.00 101.1 68.1 79.2
0.07 0.19 10.5 60.5 15.8 0.26 14.0 0.01 0.07 0.00 0.00 101.4 66.0 79.4
0.05 0.24 10.8 60.1 15.5 0.26 14.4 0.01 0.15 0.00 0.01 101.5 68.1 78.9
0.08 0.18 10.7 59.4 18.1 0.33 12.5 0.02 0.03 0.00 0.01 101.4 59.7 78.8
PK14-52 B 0.13 0.18 10.2 55.0 16.2 0.19 12.9 0.03 0.06 0.01 0.01 94.9 65.1 78.3
0.04 0.16 10.3 55.2 16.4 0.21 13.2 0.02 0.05 0.02 0.01 95.6 66.3 78.2
0.04 0.17 10.4 54.9 16.2 0.18 13.1 0.02 0.05 0.00 0.00 95.1 66.0 78.0
0.15 0.19 10.5 59.0 16.3 0.29 13.7 0.01 0.06 0.00 0.00 100.1 65.4 79.1
0.19 0.18 10.2 58.2 16.1 0.30 13.5 0.06 0.09 0.05 0.03 98.9 66.1 79.3
PK14-60 B 0.09 0.20 11.1 58.6 15.8 0.29 12.7 0.02 0.11 0.01 0.00 98.9 62.0 78.0
0.04 0.19 11.0 58.2 16.2 0.30 12.5 0.03 0.07 0.00 0.00 98.6 61.1 78.0
0.03 0.21 11.3 58.9 16.1 0.29 12.8 0.01 0.10 0.03 0.01 99.8 61.9 77.7
PK14-67 B 0.04 0.16 10.3 59.0 16.5 0.20 13.3 0.03 0.07 0.01 0.00 99.6 64.5 79.3
0.05 0.16 10.3 58.6 16.6 0.18 13.3 0.02 0.09 0.00 0.00 99.3 64.7 79.2
PK14-69 B 0.08 0.12 10.8 55.7 21.0 0.23 12.1 0.02 0.04 0.02 0.00 100.0 58.5 77.6
0.02 0.13 11.0 56.2 20.4 0.23 12.6 0.02 0.07 0.02 0.00 100.69 60.5 77.4
PK14-75 B 0.06 0.17 10.0 56.1 15.3 0.22 13.5 0.03 0.08 0.03 0.00 95.5 69.0 79.0
0.05 0.20 9.9 55.9 16.0 0.19 13.1 0.00 0.07 0.00 0.01 95.5 69.0 79.1
0.01 0.17 10.0 55.8 14.9 0.16 14.0 0.02 0.06 0.02 0.01 95.2 69.0 78.9
PK14-41 N 0.03 0.15 11.0 55.3 16.7 0.23 12.4 0.03 0.05 0.01 0.00 95.9 62.1 77.1
0.02 0.17 10.7 55.4 16.7 0.21 12.5 0.00 0.07 0.02 0.00 95.8 62.9 77.6
0.01 0.19 10.9 55.1 16.9 0.17 12.3 0.01 0.08 0.00 0.00 95.7 61.9 77.3
Note: S: sparsely disseminated; D: disseminated; SM: semi-massive; M: massive; B: banded; N: Nodular; Mg#=Mg2+/(Mg2++Fe2+); Cr#=Cr3+/(Cr3++Al3+).
1 – 2 cm. Almost all the samples are composed of fresh chromite, with 193 nm Coherent COMPex Pro ArF Excimer laser coupled to an Agilent
the exception of some chromite grains showing alteration features 7500a (LA-ICP-MS) at the IGGCAS. Each analysis was performed using
along the margins and fractures of the crystals. In some cases, the color 80 μm-diameter ablating spots at 6 Hz with an energy of –100mJ per
of chromite grains is variable from reddish to black under transmitted pulse for 45 s after measuring the gas blank for 20 s. Standard refer-
light. Various mineral inclusions enclosed in chromite grains are com- ences materials NIST610 and NIST612 (GeoReM: http://georem.mpch-
plex in type and morphology (e.g. Avcı et al., 2017; Lian et al., 2017a; mainz.gwdg.de/) were used as external standards to produce calibra-
Liu et al., 2018). tion curves. Every eight analyses were followed by two analyses of the
standards to correct for time-dependent drift. Calibration was per-
3. Analytical methods formed using NIST612 as an external standard, together with Mg as
internal standard elements for chromite. Off-line data processing was
3.1. Major oxide analysis performed using the GLITTER 4.0 program. Profile analyses of a nod-
ular chromite grain in one nodular chromitite sample PK14-41 were
Major element compositions of the minerals were determined by also conducted by LA-ICP-MS. All results are presented in Table 3.
wavelength dispersive spectrometry using a JEOL JXA8100 electron
probe microanalyzer (EPMA) at the Institute of Geology and 4. Results
Geophysics, Chinese Academy of Sciences (IGGCAS), Beijing. The
EPMA analyses were carried out at an accelerating voltage of 15 kV and 4.1. Major oxide contents of chromite
10 nA beam current, 5 μm beam spot and 10–30 s counting time on
peak. Natural and synthetic minerals were used for standard calibra- Chromite grains in the Pozantı-Karsantı chromitites have major
tion. A program based on the ZAF procedure was used for matrix cor- element compositions of Cr O from 46.4 to 60.5 wt.%, Al O from 9.87
rections. Typical analytical accuracy for all the elements analyzed is 2 3 2 3to 18.5 wt.%, with generally low minor-element oxide contents (TiO :
better than 1 to 2%. The major oxide compositions of chromite in the 20.06 – 0.37wt.%; NiO:< 0.17 wt.%; MnO:<0.34 wt.%). Their Cr#
collected samples from the Pozantı-Karsantı ophiolite are presented in values are highly variable, ranging from 62.8 to 80.3, and made up of
Table 1. In order to examine the major element distribution in a single two groups namely; subordinate to intermediate- (62.8 – 69.2) and
chromite grain, profile EPMA analyses were carried out on one nodular high-Cr (73.9 – 80.3) varieties, which are similar to chromitites from
chromite in chromitite sample PK14-41 (Table 2). the Guleman and Kızıldağ ophiolites (Chen et al., 2015, 2019; Uysal
et al., 2018). In the Cr# versus Mg# diagram, the high-Cr chromites
3.2. Trace element analysis plot close to the boninitic end-member (Fig. 3a), while the intermediate
chromites have rather transitional compositions between chromite
Trace element concentrations of chromite were determined with a crystallized from boninite and MORB (Fig. 3a). Furthermore, in the Cr#
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Table 2
Major oxides (wt.%) profile of a nodular chromite grain in chromitite sample PK14-41.
Comment SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO NiO Na2O K2O Total Mg# Cr#
Rim 0.08 0.22 11.2 59.3 16.7 0.26 13.1 0.01 0.10 0.11 0.01 101.1 63.1 78.1
0.08 0.19 11.1 59.6 16.7 0.29 13.0 0.02 0.08 0.14 0.03 101.2 63.0 78.3
0.28 0.20 8.3 59.9 19.5 0.35 11.2 0.00 0.05 0.01 0.01 99.9 55.1 82.8
0.11 0.18 10.9 60.0 17.0 0.30 12.5 0.02 0.10 0.05 0.02 101.1 60.4 78.8
0.05 0.19 11.0 60.1 17.0 0.28 12.5 0.01 0.10 0.00 0.00 101.2 59.7 78.6
0.21 0.16 10.1 60.2 17.2 0.33 12.3 0.01 0.13 0.00 0.01 100.6 59.4 80.1
0.10 0.18 10.8 60.2 17.3 0.30 12.5 0.01 0.05 0.05 0.00 101.5 59.9 78.9
0.14 0.16 11.1 59.7 16.4 0.27 12.8 0.00 0.12 0.00 0.00 100.7 61.2 78.3
0.10 0.13 11.1 59.8 17.1 0.28 12.6 0.00 0.02 0.00 0.01 101.1 60.1 78.3
0.08 0.20 10.9 59.7 17.2 0.28 12.1 0.00 0.07 0.00 0.00 100.5 58.2 78.6
0.08 0.16 11.1 59.9 16.8 0.31 12.7 0.01 0.06 0.00 0.01 101.1 60.8 78.4
0.05 0.16 10.8 60.2 17.0 0.30 12.9 0.00 0.09 0.00 0.00 101.5 61.4 78.9
0.20 0.17 11.0 60.0 16.8 0.28 12.6 0.00 0.07 0.00 0.00 101.1 60.0 78.5
0.07 0.21 10.9 60.4 16.5 0.30 12.8 0.00 0.09 0.00 0.02 101.4 61.2 78.7
0.10 0.15 11.0 59.8 16.6 0.25 12.9 0.01 0.10 0.01 0.00 101.0 61.6 78.4
0.16 0.22 11.0 59.7 16.7 0.30 12.5 0.01 0.07 0.00 0.00 100.6 59.9 78.5
0.38 0.16 10.6 59.5 16.4 0.27 12.3 0.02 0.06 0.00 0.00 99.7 59.3 79.0
0.07 0.17 11.0 59.8 17.3 0.33 12.6 0.00 0.06 0.00 0.00 101.4 60.1 78.5
0.12 0.21 10.8 59.6 17.6 0.31 12.2 0.01 0.10 0.06 0.00 101.0 58.8 78.8
0.33 0.19 8.1 61.9 17.7 0.34 11.5 0.01 0.03 0.00 0.01 100.1 56.3 83.7
0.09 0.17 11.0 59.8 17.1 0.30 12.5 0.00 0.10 0.00 0.02 101.1 60.0 78.5
0.06 0.16 11.2 59.8 16.8 0.29 12.8 0.02 0.09 0.01 0.01 101.2 61.3 78.2
0.10 0.17 10.9 59.5 16.6 0.30 12.9 0.00 0.11 0.01 0.00 100.7 62.0 78.5
0.10 0.17 11.0 59.6 16.4 0.29 12.9 0.00 0.10 0.00 0.00 100.5 62.0 78.5
0.07 0.19 11.2 59.8 16.1 0.26 13.1 0.00 0.09 0.00 0.00 100.8 62.6 78.2
0.06 0.21 11.1 60.0 15.9 0.28 13.6 0.00 0.04 0.01 0.01 101.2 64.4 78.3
0.13 0.17 11.0 60.2 15.6 0.26 13.4 0.04 0.10 0.00 0.01 100.9 64.1 78.6
0.10 0.16 10.9 60.3 16.0 0.29 13.5 0.00 0.11 0.00 0.00 101.4 63.9 78.7
0.09 0.21 11.2 60.1 15.6 0.24 13.6 0.00 0.09 0.00 0.00 101.1 64.3 78.2
1.39 0.18 9.6 56.7 16.7 0.30 13.5 0.00 0.15 0.04 0.01 98.5 63.3 79.8
0.08 0.19 11.0 60.2 16.4 0.29 12.9 0.00 0.12 0.02 0.00 101.3 61.8 78.5
0.72 0.19 11.0 57.5 16.5 0.25 13.0 0.02 0.07 0.02 0.00 99.2 61.7 77.8
0.03 0.21 11.1 59.9 16.8 0.28 12.8 0.01 0.11 0.00 0.01 101.3 61.3 78.3
0.11 0.18 11.0 59.8 16.8 0.25 12.6 0.01 0.07 0.03 0.01 101.0 60.5 78.5
0.05 0.19 10.5 60.2 17.5 0.35 12.1 0.00 0.03 0.00 0.00 100.9 58.2 79.3
0.04 0.18 11.2 59.8 16.6 0.30 12.7 0.01 0.08 0.03 0.00 100.9 61.0 78.2
0.05 0.17 10.7 60.1 17.3 0.28 12.3 0.01 0.04 0.16 0.01 101.2 60.1 79.1
0.08 0.14 11.1 59.6 17.2 0.30 12.7 0.01 0.05 0.00 0.00 101.1 60.5 78.3
0.02 0.20 11.2 60.0 16.5 0.28 13.2 0.00 0.09 0.00 0.00 101.5 62.9 78.3
0.88 0.18 10.6 58.7 16.5 0.33 13.6 0.01 0.09 0.00 0.01 100.9 63.0 78.7
0.05 0.16 11.1 60.1 16.4 0.29 12.8 0.00 0.08 0.03 0.00 101.0 61.4 78.5
0.01 0.17 11.1 59.8 16.1 0.27 13.2 0.00 0.12 0.02 0.00 100.9 63.4 78.3
0.05 0.20 11.1 60.0 16.0 0.30 13.3 0.00 0.13 0.01 0.00 101.1 63.6 78.4
0.04 0.19 11.1 59.7 16.2 0.30 13.4 0.00 0.08 0.00 0.00 101.0 63.9 78.4
Core 0.04 0.18 11.2 59.9 16.2 0.26 13.2 0.00 0.14 0.03 0.01 101.2 63.4 78.2
versus TiO2 diagram, chromites in intermediate chromitite show a 41 shows significant heterogeneous variations from core to rim (Fig. 5),
compositional affinity with island arc tholeiite (IAT) (Fig. 3b). in contrast to the trace elemental profile feature of the Guleman chro-
Analyses conducted on a chromite grain in nodular chromitite mite (Uysal et al., 2018). Titanium and Mn show a decreasing trend
sample PK14-41 revealed homogeneous major element compositions from core to rim, while Sc and Ni display a reverse trend (Fig. 5). The
from core to rim, with limited variations in Cr# (77.8 – 80.1), Mg# core of the nodular chromite has higher V, Zn and Co contents than the
(58.2 – 64.4), Cr2O3 (56.7 – 61.9 wt.%), Al2O3 (8.1 – 11.2 wt.%), FeO rim.
(15.6 – 19.5 wt.%) and MgO (11.2 – 13.6 wt.%) (Table 2; Fig. 4).
5. Discussion
4.2. Trace element compositions of chromite
5.1. Insights into compositional heterogeneity of parental magmas of the
Chromite grains in intermediate chromitites in the Pozantı-Karsantı chromitites
ophiolite have variable Ni content from 580 to 967 ppm, Ga from 18.6
to 25.0 ppm and Sc from 4.95 to 11.0 ppm (Table 3). These chromites 5.1.1. Constrains from trace elements of high-Cr and intermediate
are also characterized by restricted compositional ranges of Co chromitites
(156 – 265 ppm), Zn (420 – 675 ppm) and V (899 – 1004 ppm) Although chromite with Cr# of 60 is considered to represent a
(Table 3). The trace element variations of the high-Cr chromite re- classic boundary between high-Cr and high-Al chromitites (e.g.
semble the intermediate chromite, except the lower contents of V Arai,1994; Arai and Miura, 2016; Zhou et al., 1994, 1998, 2014), three
(313 – 665 ppm) and Ga (7.31 – 21.6 ppm) (Table 3). categories of high-Cr (Cr# >70), intermediate (Cr#: 50–70) and high-
Trace elemental profile of a nodular chromite grain in sample PK14- Al (Cr# < 50) chromitites may be more reasonable in the study of the
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X. Liu, et al. Ore Geology Reviews 110 (2019) 102926
Table 3 mineralization of chromitites (e.g. Uysal et al., 2016, 2018; Chen et al.,
Trace element (ppm) of chromite and a profile of a nodular chromite grain in 2018, 2019). Intermediate chromitites have been mainly successively
chromitite sample PK14-41. discovered in the Kızıldağ (Chen et al., 2018, 2019), Guleman (Uysal
Sample Texture Sc Ti V Mn Co Ni Zn Ga et al., 2018) and Pozantı-Karsantı ophiolites in southern Turkey. In
major oxides of chromite, we hardly find other significant differences
Intermediate chromitite between the high-Cr and intermediate chromitites, except the classical
PK14-01 S 8.18 1473 931 1639 255 709 624 20.5
8.31 1453 928 1631 255 698 641 20.5 distinguish indicator of Al2O3 and Cr2O3 as well as slightly higher TiO2
11.0 1443 923 1675 266 655 676 21.2 content in the intermediate chromitites (0.06 – 0.37wt.%) than the
PK14-25 D 5.68 463 899 885 158 581 421 18.6 high-Cr chromitites (0.1 – 0.2 wt.%). However, in addition to the dif-
5.36 466 906 878 159 623 430 19.5 ference in Cr# between the intermediate chromitite and typical high-Cr
5.63 466 909 870 156 580 424 18.8 chromitite, trace element Ga can also be considered as a perfect in-
PK14-02 M 5.01 1531 943 1073 174 967 456 23.6
4.95 1517 957 1076 174 960 459 24.2 dicator in distinguishing between them (Fig. 3d). The correlation dia-
5.03 1539 947 1076 174 952 450 23.2 gram of trace element for olivine and chromite denotes that chromite is
PK14-56 M 7.13 1771 968 1138 181 770 632 23.5 the reservoir of Ga in ophiolitic chromitite consisting of olivine and
6.55 1845 983 1147 186 766 651 24.6 chromite, whereas Mn does not show obvious differentiation between
6.84 1835 1004 1140 184 802 668 25.0 chromite and olivine both in high-Cr and intermediate chromitites
High-Cr chromitite (Fig. 6a). Therefore, combining Ga with Ga/Mn, the transitional feature
PK14-12 S 7.14 1260 697 1839 345 698 331 20.6 of the intermediate chromitite straddling areas between typical high-Al
8.23 1270 698 1796 329 733 329 20.6
8.98 1263 700 1870 347 685 354 21.5 and high-Cr chromitite can be demonstrated more clearly (Fig. 6b). On
PK14-27 S 6.55 1002 538 1043 182 549 402 14.9 the other hand, all chromites in chromitites we analyzed were per-
6.35 1005 547 1062 187 587 414 15.6 formed on the core of unaltered grains, so that we are able to claim that
6.64 993 544 1069 190 522 408 15.4 the composition of intermediate chromitite can reflect the transitional
PK14-44 S 10.5 906 807 2050 366 559 482 20.0
6.27 876 812 2249 446 732 684 19.9 feature of its parental magma, which is not related to either MORB-like
9.64 911 810 2087 380 480 455 18.7 or boninitic-like varieties.
PK14-46 S 8.4 861 539 1451 270 294 785 13.6
7.48 957 561 1594 284 305 871 14.6 5.1.2. Controls on trace elemental profile of chromite
7.06 838 504 1371 251 274 766 12.6 The variation in the composition of chromitite can be equally in-
PK14-57 S 7.22 1145 586 1480 253 520 387 15.1
7.38 1158 584 1517 252 567 400 16.2 dicated by the compositional heterogeneity of one chromite grain
7.12 1167 590 1493 256 554 399 16.1 (Fig. 5). Although the chemical profile of major elements across the
PK14-64 D 6.35 1005 567 1157 204 412 430 15.5 nodular chromite grain shows no remarkable variations, the composi-
6.20 965 554 1098 198 427 417 15.0 tion of trace elements of the same grain varies irregularly from core to
6.47 994 567 1155 205 407 454 15.1
PK14-03 M 4.47 1229 521 1218 156 597 313 13.1 rim. In order to further constrain the origin of these compositional
4.86 1263 531 1196 155 673 342 14.6 variations within this chromite grain, it is important to know the par-
4.36 1254 533 1210 156 682 337 14.3 tition coefficients (D) of trace elements determined in the chromite.
PK14-09 M 5.84 964 558 1042 180 562 377 14.6 Partition coefficients (D) of trace elements between chromite and melts
5.92 955 550 1015 178 563 370 14.1 under different circumstances have been determined or estimated by
5.96 974 558 1020 176 567 375 14.0
PK14-28 M 4.51 512 705 997 150 756 416 15.4 many studies as presented in Table 4. This data demonstrate that Rb, Zr
5.06 509 698 904 144 866 374 15.7 and Nb are highly incompatible in chromite, whereas Mn, Ti and Sc are
5.37 496 684 886 143 858 362 15.3 moderately incompatible in chromite (DMn= 0.88 – 0.98;
PK14-31 M 6.05 629 505 809 143 734 356 15.6 DTi= 0.05 – 0.75; DSc= 0.18 – 0.57; Table 4). It is noticeable that
5.39 641 502 784 144 695 360 14.5
6.47 647 509 826 147 756 367 15.3 certain elements (such as Sc and Mn) behave similarly in chromite of
PK14-51 B 14.4 1420 657 1773 345 884 338 21.6 boninite and MORB, which is illustrated by their analogous partition
6.72 1434 662 1773 346 760 330 20.1 coefficient and might be considered as perfect indicators reflecting the
7.11 1470 665 1768 337 758 323 21.0 characteristics of chromite crystallized from parental magma. The
PK14-52 B 6.80 986 594 1264 225 591 320 14.2 highly compatible trace elements are Co and Zn with partition coeffi-
6.22 996 601 1295 225 542 322 14.1
6.61 992 593 1217 205 567 309 13.3 cients of 2.1 – 8.3 and 3.6 – 7.9 (Table 4), respectively. Other elements
PK14-60 B 5.99 1006 486 1180 206 446 399 14.3 such as Ni, Cu and Ga shared an affinity of moderate compability
6.34 1020 495 1196 213 442 420 14.8 (DNi= 1.3 – 11; DCu= 3.1; DGa= 1.83 – 3.75; Table 4). V is compatible
7.24 1037 502 1255 220 402 418 14.7 in chromite in many cases such as in lunar mare basalt (DV= 38) and
PK14-67 B 5.97 862 398 999 173 500 340 11.0
6.23 889 398 989 178 511 360 11.4 garnet pyroxenite, basalt (Dv= 1.3; Elkins et al., 2008), whereas the Dv
6.43 889 405 1002 179 498 382 12.0 ranges from 0.02 to 1.62 in chromite from alkali olivine basalt
PK14-69 B 7.44 761 647 1394 211 402 541 13.1 (Table 4). The variation of Dv for chromite is a reflection of changes in
7.53 761 651 1365 207 435 542 13.8 oxygen fugacity (e.g. Canil, 2002; Dare et al., 2009). Parameterization
7.99 754 652 1369 206 429 548 13.9 of V partitioning into spinel as a function of fO2 has been summarizedPK14-75 B 5.32 930 422 993 177 509 365 11.6
5.53 947 426 1007 181 478 367 11.7 by Canil (2002), which shows that an increase in fO2 results in de-
5.65 944 429 1058 189 501 377 12.3 creasing compatibility of V in spinel. Moreover, V is more compatible in
PK14-41 Rim 4.20 699 313 697 115 343 141 7.38 chromite with higher Cr content for a given fO2 (Lee et al., 2005).
4.20 690 314 650 111 339 142 7.31 With the constrain of partition coefficients of trace elements for
4.55 724 315 653 110 353 139 7.87 chromite, we can better understand the variation of trace elements in
4.40 709 325 688 114 360 144 7.84
4.43 737 330 742 126 351 156 7.71 sample PK14-41. These variations from core to rim among incompatible
4.39 753 331 810 146 300 192 8.14 elements (Sc, Ti and Mn) and compatible elements (Co, Zn and Ga) of
4.04 751 328 802 131 353 167 8.11 the nodular chromite are not consistent. The decreasing contents of
Core 3.50 772 353 919 157 299 212 8.09 compatible elements (Co and Zn) from core to rim may indicate
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X. Liu, et al. Ore Geology Reviews 110 (2019) 102926
Fig. 3. Correlation diagrams of (a) Cr# vs. Mg#, (b)
Cr# vs. TiO2 (wt.%), (c) Al2O3 vs. Cr2O3 (wt.%) and
(d) Ga (ppm) vs. Cr# for chromite in the chromitite
from the Pozantı-Karsantı, Kızıldağ (Chen et al.,
2019) and Guleman (Uysal et al., 2018) ophiolites.
Fields of typical high-Cr and high-Al chromitite are
from Zhou et al., 2014. Fields for MORB (mid-Oo-
cean ridge basalts) and boninite according to
Rollinson (2005). Representative analyses of a bo-
ninite chromite (white square with blue rim), a
MORB chromite (plus) and Thetford Mines Ophiolite
(TMO) (hexagon) are from Pagé and Barnes (2009).
Data of Pozantı-Karsantı chromitite (white rhombus
with green rim) is from Avcı et al. (2017). IAT: is-
land arc tholeiite; Dn: dunite; Hz: harzburgite; Lz:
Lherzolite; FMM: Fertile MORB mantle. (For inter-
pretation of the references to color in this figure
legend, the reader is referred to the web version of
this article.)
fractional crystallization of chromite from homogeneous parental into the high-Mg, water-rich characteristics of the parental magma of
magma, which could also explain the increasing contents of in- the Pozantı-Karsantı chromitite, which is in accordance with other ty-
compatible element Sc away from the core (Fig. 5). However, fractional pical chrome ore deposit associated with ophiolite (e.g. Zhou et al.,
crystallization alone cannot explain the compositional variations of 2014; Xiong et al., 2016, 2018). However, with further classification of
incompatible elements such as Mn and Ti, which decrease steadily to- chromite in chromitite in this study, we found that the composition of
wards the rim (Fig. 5). In addition, the decreasing Ga content away mineral inclusions (e.g. clinopyroxene) varies with the type of chro-
from core is another strong evidence. Therefore, we strongly infer that mite, namely intermediate chromite and high-Cr chromite (Fig. 6c).
the variation of these trace elements within single chromite grain in Although clinopyroxene inclusions show a positive correlation between
sample PK14-41 represents the continuous compositional variation of Cr2O3 and Al2O3 content both in the intermediate chromite and high-Cr
parental magma during the formation of chromite, which may be re- chromite, clinopyroxene included in the intermediate chromite has
lated to participation of exotic material such as sediments from sub- higher Al2O3 content than in the high-Cr chromite for a given Cr2O3
ducting oceanic crust. (Fig. 6c). These mineral inclusions found in the Pozantı-Karsantı chro-
mitite are primary inclusions and exposed away from fractures in
5.1.3. Constraints from mineral inclusions in chromite chromite grain. In addition, elemental mapping images of these mineral
Hitherto, most properties of parental magma of chromitite, such as inclusions demonstrate the homogeneous compositional feature (Fig. 7
water- and Mg-enrichments were deduced from primary mineral or in Liu et al., 2018), which can eliminate the effect of elemental diffu-
fluid inclusions entrapped in chromite (e.g. Melcher et al., 1997; Zhou sion between silicate inclusions and chromite. We thus emphasize that
et al., 2014; Johan et al., 2017). Various types of mineral inclusions in these inclusions were crystallized simultaneously or earlier than chro-
chromite from a variety of chromitite ores in the Pozantı-Karsantı mite from the same parental magma. Therefore, compositional varia-
ophiolite have also been discovered (e.g. Lian et al., 2017a; Liu et al., tions of clinopyroxene inclusions perfectly indicate that the chemical
2018). Silicate minerals found in the chromites are mainly olivine, property of the parental magma was not homogenous during the crys-
clinopyroxene and amphibole, which consist of single phase or multiple tallization of chromite.
phase assemblages in a single inclusion (Liu et al., 2018; Fig. 5). Olivine
mostly occurs as single euhedral or subhedral grain with diameters up 5.2. Variations of physical properties and Ca-rich characteristics of parental
to 50 μm, whereas other minerals such as clinopyroxene may occur as magmas
small irregular inclusion in the border zone around the olivine
(Fig. 7a–c). Clinopyroxene inclusions are commonly and randomly 5.2.1. Unraveling the difference of fO2 of parental magmas
distributed within the chromite, showing a rounded to subhedral, and Oxygen fugacity (fO2), which describes the activity of O2 within a
“lamellae” outline in BSE images (Fig. 7b–e), similar to hydrous in- system (e.g. Carmichael, 1991; Kress and Carmichael, 1991), plays an
clusion of amphibole (Fig. 7f; Liu et al., 2018). It is worth noting that important role in many geological processes such as ore genesis, at-
the first ever report of calcite inclusion (Liu et al., 2018) occurred to- mospheric evolution and trace-element partitioning (Lee et al., 2005).
gether with chlorite filling negative crystals in chromite grain from the In many cases, fO2 is determined indirectly by some parameters (e.g.
Pozantı-Karsantı ophiolite (Fig. 7g, h). Chemical compositions of these Fe3+/Fe3++Fe2+) in whole rocks, and is controlled by the different
primary silicate inclusions from each sample provide further insights valence state of these redox-sensitive elements. However, the ratio of
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X. Liu, et al. Ore Geology Reviews 110 (2019) 102926
Fig. 4. Major oxide (wt.%), Cr# and Mg# profile analyses of a chromite grain Fig. 5. Trace elemental profile analyses (ppm) of a chromite grain in nodular
in nodular chromitie sample PK14-41. chromitite sample PK14-41.
that intermediate chromite tends to have higher V/Mn compared to
redox-sensitive to redox-insensitive elements such as V/Sc and Zn/FeT high-Cr chromites from the same ophiolite such as the Pozantı-Karsantı
(where Fe 2+ 3+T= Fe +Fe ) might better “see through” early differ- in this study and the Guleman in Uysal et al. (2018). The study of
entiation processes in a magma to retain a memory of the original partition coefficient of trace elements in spinel by Canil (2002) con-
melting conditions (e.g. Lee et al., 2005, 2010). Herein, we propose the firms that the V/Mn variation among intermediate and high-Cr chro-
application of V/Mn as another fO2 proxy for chromite to determine the mite might indicate the difference in fO2 conditions for their parental
property of parental magma. The reasons why we chose V/Mn are as magma where higher fO2 is accompanied by higher Cr# and lower V/
follows: Mn is a moderately incompatible element (DMn=0.88 – 0.98), Mn of chromite.
which doesn’t show remarkable difference in the composition of chro-
mite between boninite and MORB (Pagé and Barnes, 2009; Zhou et al.,
2014). On the other hand, the ratio of Mn content between olivine and 5.2.2. Effect of hydrous fluids on the mineralization of chromitite
chromite in various types of chromitite is close to 1, as demonstrated in The presence of amphibole inclusions in chromite demonstrates that
Fig. 6a, where Mn lies on the diagonal path. However, V is a redox- fluids were involved in the formation of chromitite and the alkali-
sensitive element and its partition coefficient in chromite is predicted to bearing nature of parental magma (e.g. Melcher et al., 1997; Johan
depend on the fO2 conditions of parental magmas (Canil and et al., 1983, 2017), and so does the Pozantı-Karsantı chromitites in this
Fedortchouk, 2000; Canil, 2002). Therefore, the effects of magmatic study. The involvement of fluids is also demonstrated in the study of Li
differentiation process on the content of V in parental magma can be isotopes of olivine in the Pozantı-Karsantı chromitite. Besides, Li is
reduced by using V/Mn ratio rather than the absolute contents of V much more incompatible in chromite than in olivine (Seitz and
alone. Studies on the correlation of Cr# and V/Mn in chromite show Woodland, 2000; Ottolini et al., 2004), and therefore inter-mineral
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Fig. 6. Correlation diagrams of (a) trace elements
(ppm) for chromite and olivine, (b) Ga/Mn vs. Ga
(ppm) for chromite, (c) Al2O3 vs. Cr2O3 (wt.%) for
clinopyroxene inclusions in chromite and (d) V/Mn
vs. Cr2O3for chromite. Symbols are same as in Fig. 3.
Olivine data of the Kızıldağ chromitite in Fig. 6a and
clinopyroxene inclusion in Fig. 6c from Chen et al.
(2019) and Liu et al. (2018), respectively.
diffusion between olivine and chromite in the Pozantı-Karsantı chro- Ca-rich character of the parental magmas of chromitite (Liu et al.,
mitite can be ignored. Therefore, Li isotope of olivine from chromitite 2018). We argue that CaO is of crucial importance to the formation of
records its process of formation and the characteristics of parental chromitite occurring in ophiolite, as exemplified below.
magmas (e.g. Jeffcoate et al., 2007; Su et al., 2016, 2018; Chen et al., An experimental study of the effect of melts composition on Cr2+/
2019; Zhang et al., 2019). The olivine in chromitite from the Pozantı- Cr3+ in silicate melts has revealed a negative correlation between logγ#
Karsantı ophiolite displays variable but systematical increase of δ7Li Cr3+O1.5 and XCaO, indicating stabilization of Cr3+ with increasing XCaO
values along the increased ratio of chromite and olivine from dis- (Berry et al., 2006). This phenomenon is consistent with CaO-rich melts,
seminated to banded and massive chromitite (Fig. 8). Since 7Li is ex- which tend to have more octahedral sites than CaO-poor melts (Berry
ceedingly mobile in fluids relative to 6Li and preferentially enters fluids et al., 2006). Therefore, we propose the possible involvement of a Ca-rich
(e.g. Huang, 2011; Xiao et al., 2015), we attribute this progressive component in parental melts of chromite that facilitated the transporta-
variation of δ7Li in olivine to the effect of the gradually enriched hy- tion of Cr3+ in magma conduits and consequently deposited when the
drous fluids released from the oceanic sedimentary cover or subducting prevailing physical conditions were suitable. However, it should be
slab into parental magma prior to the formation of chromitite. stressed that Ca cannot be abundantly accommodated by the lattice of
We also emphasize that the formation of high-Cr chromitite may be chromite, which insinuates why Ca-rich components in melts are more
related to more oxidized fluids released from the subducting slabs as likely to enter the lattice of Ca-bearing silicate inclusions such as clin-
evidenced by higher fO2 in high-Cr chromitite than intermediate opyroxene or participate in the formation of Ca-rich mineral inclusion
chromitite. Studies of MORBs and arc lavas have shown that the asth- (e.g. calcite). This inference was confirmed by higher CaO content in
enosphere in sub-arc mantle is more oxidized than the ambient mantle chromite from the barren Purang ophiolite compared with the miner-
(Christie et al., 1986; Carmichael, 1991; Su et al., 2015), whereas Lee alized Luobusa ophiolite (Su et al., 2019). The Ca-rich characteristics may
et al. (2005, 2010) claimed that the fO2 of asthenospheric mantle is, on have been derived from refertilized mantle source, namely cpx-bearing
the average, surprisingly homogeneous. However, irrespective of harzburgite, similar to the high-Ca boninitic melts of Luobusa high-Cr
whether the fO2 of asthenospheric mantle is homogenous or not, the chromitites (Zhang et al., 2019). However, experimental studies of
role of oxidized fluids in the formation of chromite from parental melts limestone melting conditions have showed that carbonate rocks can melt
cannot be overlooked. The positive effect of higher volume of hydrous at relatively low temperature (e.g. 740 °C at 1 Kbar; Wyllie and Tuttle,
fluids emanating from the top of subducting lithospheric slab on the 1960) with the addition of volatile such as water (e.g. Lentz, 1999;
genesis of gradually Cr-enriched chromitites is also confirmed by other Durand et al., 2015). Based on experimental studies, in conjunction with
studies (Derbyshire et al., 2013; Tzamos et al., 2017). the widespread occurrence of upper Triassic and lower Cretaceous car-
bonate rocks around the Pozantı-Karsantı ophiolite (Tekeli et al., 1984),
5.2.3. Effects of Ca-rich component in parental magma on mineralization we suggest that the Ca-rich components are more likely to originate from
Ca-rich inclusions such as calcite, wollastonite, apatite and uvaro- flux melting of carbonate rocks during subduction initiation. These car-
vite, together with high-CaO silicate minerals (clinopyroxene and am- bonate rocks involved in the chromite ore mineralization may also in-
phibole) and slightly higher CaO of olivine than matrix olivine, indicate fluence the Li isotope of olivine in chromitites with high δ7Li values from
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Table 4 Avcı et al., 2017) or represent the product of melt-rock interaction
Partition coefficients of trace elements between chromite and melts under during subduction initiation (Chen et al., 2018; Su et al., 2018). In this
different conditions. study, the similarity of high-Cr chromitite with typical high-Cr chro-
Partition coefficient Reference mitite from Luobusa, Purang and Kızıldağ ophiolites that crystallized
from boninite-type melts may be in consistent with the conclusion from
Sc Avcı et al. (2017). However, the transitional feature of intermediate
Alkali olivine basalt 0.36 – 0.57 Horn et al., 1994
Synthetic CaO-FeO-MgO-Al O - 0.0478 Nagasawa et al., 1980 chromitite ranging from typical MORB to boninites, together with the2 3
SiO progressive and continuous changes of major and trace elements from2
MORB 0.18 Pagé and Barnes, 2009 intermediate chromitites to high-Cr chromitites, indicate that the tec-
Boninite 0.2 Pagé and Barnes, 2009 tonic setting of subduction initiation is more reasonable. In addition,
Ti studies of the representative subduction zone, namely IBM (Izu-Bonin-
Basalt 0.048 McKenzie and O'Nions, Mariana) zone, have unraveled a change of composition in magmas
1991 from MORB-like to boninitic, represented by the transitional feature of
MORB 0.34 Pagé and Barnes, 2009
Boninite 0.75 Pagé and Barnes, 2009 composition for lavas between MORB-like, forearc basalt (FAB) and
boninite (Reagan et al., 2010). The presence of different types of
V magmas is reckoned as sources of various chromites ranging from
Garnet pyroxenite, basalt 1.3 Elkins et al., 2008
Lunar mare basalt 38 Ringwood and Essene, MORB-like to boninitic found in IBM (Morishita et al., 2011; Zhang
1970 et al., 2016; Xiong et al., 2018; Zhang et al., 2019). These discoveries
Synthetic CaO-FeO-MgO-Al2O3- 3.56 – 19.29 Canil, 1999 about chromitite in the IBM zone are in perfect accordance with the
SiO2 chromitite within the Pozantı-Karsantı ophiolite, reinforcing our in-
Synthetic komatiite, basalt 5.08 – 31.22 Canil, 1999
MORB 4.49 Pagé and Barnes, 2009 ference that the parental magma of chromitite in our study was mod-
Boninite 7.73 Pagé and Barnes, 2009 ified during the subduction initiation. Other evidences supporting the
Alkali olivine basalt 0.02 – 1.62 Horn et al. 1994 idea that various chromitites were formed during the process of sub-
Mn duction initiation are as follows:
MORB 0.88 Pagé and Barnes, 2009
Boninite 0.98 Pagé and Barnes, 2009 (1) The remarkable but inconsistent variations of trace elements in a
Co nodular chromite grain reflect the compositional or physical prop-
Alkali olivine basalt 4.7 – 8.3 Horn et al., 1994 erty changes of the parental magma instead of the effect of frac-
Lunar basalt 2.1 Klemme et al., 2006 tional crystallization from the same source (Fig. 5).
MORB 4.49 Pagé and Barnes, 2009
Boninite 3.83 Pagé and Barnes, 2009 (2) The Ga content in chromites from chromitites in this study de-
creases from intermediate to high-Cr chromitite (Figs. 3d, 6b),
Ni
MORB 6 – 11 Li et al., 2008 which sheds more lights on a switch from a proto-forearc to forearc
3.2 Pagé and Barnes, 2009 setting (e.g. Alabaster et al., 1982; Pagé and Barnes, 2009).
Boninite 1.31 Pagé and Barnes, 2009 (3) The compositional difference in primary clinopyroxene inclusions
Cu within two types of chromite provides strong evidence for the
Lunar basalt 3.1 Klemme et al., 2006 variations of parental magma crystallizing chromite, which implies
Zn evolving melts accompanying the on-going subduction of des-
Alkali olivine basalt 3.6 – 4.5 Horn et al., 1994 cending lithospheric slab.
MORB 7.92 Pagé and Barnes, 2009 (4) The negative relationship between Cr# and V/Mn of chromite reflects
Boninite 6.9 Pagé and Barnes, 2009 that the oxygen fugacity of the parental magma of high-Cr chromitite
Ga is higher than parental magma of intermediate chromitite, which may
Alkali olivine basalt 2.49 – 3.75 Horn et al., 1994 be due to the involvement of hydrous liquids characterized by high
MORB 3.28 Pagé and Barnes, 2009 fO2 in subduction initiation (Carmichael, 1991).Boninite 1.83 Pagé and Barnes, 2009
Rb Although the dynamic mechanism of how the subduction initiation
Garnet pyroxenite, basalt 0.029 Elkins et al., 2008 was triggered is debatable, a three-stage process may serve as a perfect
Zr model for subduction initiation in subduction zone such as the IBM
Alkali olivine basalt 0.03 – 0.14 Horn et al., 1994
Lunar basalt 0.56 Klemme et al., 2006 zone (Whattam and Stern, 2011; Stern, 2004, Stern et al., 2012). Sub-
Lunar mare basalt 0.046 – 0.05 McKay et al., 1986 duction initiation may form across a zone of weakness (e.g. transform
Nb fault or fracture zone), where the downgoing thicker and denser slab
Alkali olivine basalt 0.02 – 0.15 Horn et al., 1994 allows the upwelling of the asthenosphere and decompression melting
Garnet pyroxenite, basalt 0.0006 Elkins et al., 2008 to occur, producing MORB-like forearc basalt without interaction with
Lunar basalt 0.86 Klemme et al., 2006 slab-derived fluid in the early stage (Fig. 9a). At this stage, sparse
chromitite with a high-Al compositional characteristic was formed due
to the MORB-like affinity of parental magma. Consequently, high fO2
15‰ to 50‰ (Košler et al., 2001). fluids derived from marine sediments such as carbonate covering on the
continued subducting lithosphere or slab-derived fluids penetrated the
5.3. Tectonic implications upwelling mantle and triggered melting, producing the parental melts
ranging from MORB-like to boninitic composition that initiated the
Previous studies have revealed that the chromitites of the Pozantı- formation of a proto-arc crust (Fig. 9b). The second stage represent a
Karsantı ophiolite were formed in subduction zone from boninite-type crucial stage for the precipitation of intermediate chromitites via the
melts originating from high-degree partial melting (Saka et al., 2014; reaction between “transitional” melts with harzburgite left behind in
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Fig. 7. Back-scattered electron images and energy dispersive spectromer analyses showing some typical silicate and Ca-rich inclusions with different shapes and sizes
in chromite from chromitite in the Pozantı-Karsantı ophiolite. Ol: olivine; Cpx: clinopyroxene; Amp: Amphibole; Cal: calcite; Chl: chlorite; Phl: phlogopite.
higher fO2 and CaO-enrichment that similar to boninitic melts. It is
noteworthy that the magmatic arc was formed at this stage, signaling
the commencement of true subduction.
6. Conclusions
Chromitites in the Pozantı-Karsantı ophiolite have two categories,
namely; intermediate (Cr#: 62.8 – 69.2) and high-Cr (Cr#: 73.9 – 80.3)
chromitite. Their differences in major and trace elements, together with
variations in trace elemental profile of nodular sample PK14-41, sug-
gest the heterogeneity of the parental magmas of chromitites in the
Fig. 8. Variations of δ7Li for olivine in disseminated, banded and massive Pozantı-Karsantı ophiolite. The compositional features of the inter-
chromitites from the Pozantı-Karsantı ophiolite (data from Su et al., 2018). mediate and high-Cr chromitite indicate that they had IAT and boninite
affinity, respectively, which were likely produced during the subduc-
tion initiation. The presence of hydrous inclusions such as amphibole in
the first stage (e.g. Ahmed and Arai, 2002; Arai et al., 2004; Zhou et al., the chromite reveals that fluids were involved in the formation of the
2014). In the third stage, hydrous fluids from the sinking lithosphere chromitite, which also increased the oxygen fugacity of the parental
and carrying marine sediments increased gradually with time and in- magmas. Thus together with the presence of Ca-rich mineral inclusions
duced a higher degree of melting of the harzburgitic mantle to generate in chromite, we conclude that parental magmas of the chromitite were
boninitic melts that erupted to form the boninitic lavas (Fig. 9c). In water- and Ca-rich but heterogeneous in composition and physical
addition, more Ca-rich components such as marine carbonate were property such as oxygen fugacity. Besides, the authors reckon that more
most likely involved in the formation of the parental magmas, so typical oxidized fluids released from the subducting slab were responsible for
high-Cr chromitites were crystallized from melts characterized by a the higher fO2 of high-Cr chromitite than intermediate chromitite.
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Fig. 9. Three-stage formation model of the Pozantı-Karsantı ophiolite and associated chromitites during subduction initiation (modified after Stern et al., 2012; Chen
et al., 2019).
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