PUBLICATIONS
Geochemistry, Geophysics, Geosystems
RESEARCH ARTICLE
10.1002/2015GC005944
Key Points:
It is the first time to investigate Cr
isotopic behaviors during subduction
We obtain the limited Cr isotopic
fractionation during metamorphic
dehydration
There are potential Cr isotopic
fractionations between hightemperature minerals
Correspondence to:
L. Qin,
[email protected]
Citation:
Shen, J., J. Liu, L. Qin, S.-J. Wang, S. Li,
J. Xia, S. Ke, and J. Yang (2015),
Chromium isotope signature during
continental crust subduction recorded
in metamorphic rocks, Geochem.
Geophys. Geosyst., 16, doi:10.1002/
2015GC005944.
Received 5 JUN 2015
Accepted 2 OCT 2015
Accepted article online 9 OCT 2015
Chromium isotope signature during continental crust
subduction recorded in metamorphic rocks
Ji Shen1, Jia Liu1, Liping Qin1,2, Shui-Jiong Wang3, Shuguang Li1,2, Jiuxing Xia1, Shan Ke2, and
Jingsui Yang4
1
CAS Key Laboratory of Crust—Mantle Materials and Environments, School of Earth and Space Sciences, University of
Science and Technology of China, Hefei, China, 2State Key Laboratory of Geological Processes and Mineral Resources,
University of Geosciences, Beijing, China, 3Isotope Laboratory, Department of Earth and Space Sciences, University of
Washington, Seattle, Washington, USA, 4State Key Laboratory for Continental Tectonics and Dynamics, Institute of
Geology, Chinese Academy of Geological Sciences, Beijing, China
Abstract The chromium isotope compositions of 27 metamorphic mafic rocks with varying metamorphic
degrees from eastern China were systematically measured to investigate the Cr isotope behavior during
continental crust subduction. The Cr isotope compositions of all samples studied were Bulk Silicate Earth
(BSE) like, with d53CrNIST979 of greenschists, amphibolites, and eclogites ranging from 20.06& to 20.17&,
20.05& to 20.27&, and 20.01& to 20.24&, respectively. The lack of resolvable isotopic variability among
the metamorphic rocks from different metamorphic zones indicated that no systematic Cr isotope fractionation was associated with the degree of metamorphism. However, the Cr isotopic variability among homologous samples may have reflected effects induced by metamorphic dehydration with a change of redox
state, rather than protolith heterogeneity (i.e., magma differentiation). In addition, the differences in d53Cr
(D53CrCpx-Gt) between coexisting clinopyroxene (Cpx) and garnet (Gt) from two garnet pyroxenites were
0.06& and 0.34&, respectively, indicating that significant inter-mineral Cr isotope disequilibria could occur
during metamorphism. To provide a basis for comparison with metamorphic rocks and to provide further
constraints on the potential Cr isotope heterogeneity in the mantle and in the protolith of some metamorphic rocks, we analyzed mantle-derived chromites and the associated peridotites from Luobusa, and we
obtained the following general order: chromite-free peridotites (20.21& to 20.11&) < chromite-bearing
peridotite (20.07&) < chromite (20.06&). These findings imply potential mantle heterogeneity as a result
of partial melting or fractional crystallization associated with chromite.
1. Introduction
C 2015. American Geophysical Union.
V
All Rights Reserved.
SHEN ET AL.
Chromium (Cr), an essential transport metal element, occurs naturally in terrestrial and oceanic reservoirs.
Chromium consists of four stable isotopes (50Cr, 52Cr, 53Cr, and 54Cr) with natural abundances of 4.35%,
83.79%, 9.50%, and 2.36%, respectively [Rossman and Taylor, 1998]. Cr is a redox-active element and typically
presents 13 and 16 valence states in most geological and biological processes. Cr61 is soluble and mobile
31
22
in the form of oxyanions CrO22
is usually insoluble and
4 (chromate) and Cr2O7 (bichromate), whereas Cr
immobile in the neutral pH range, and this valence state is common for Cr in natural rock-forming minerals
[Johnson and Bullen, 2004a,b]. Significant Cr isotope fractionation occurs when Cr61 is reduced to Cr31, and
the remaining Cr61 is enriched with heavy isotopes [Døssing et al., 2011; Ellis et al., 2002, 2004; Zink et al.,
2010]. In the past decade, Cr isotopes have been used as tracers for redox reactions associated with Cr during
surficial and environmental processes [Berna et al., 2009; Ellis et al., 2002; Frei et al., 2009, 2011, 2013; Wanner
et al., 2012a,b]. Under high-temperature and low-fo2 conditions, Cr21 might be dominant in planetary and
terrestrial basaltic melt, as well as in mantle olivine [Bell et al., 2014; Berry and O’Neill, 2004; Berry et al., 2006;
Eeckhout et al., 2007]. Until now, only a few studies have focused on high-temperature Cr isotopic behavior
because the degree of fractionation that occurs during high-temperature geological processes might be suppressed compared to that during low-temperature processes, as suggested by theoretical predictions [Johnson and Bullen, 2004b; Schauble et al., 2004]. High-precision Cr isotope analyses by TIMS and MC-ICP-MS
instruments could resolve small variations in Cr isotope fractionation resulting from middle to hightemperature metamorphic processes [Qin et al., 2010; Schiller et al., 2014; Trinquier et al., 2008].
Cr ISOTOPES OF SUBDUCTED CRUST
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Geochemistry, Geophysics, Geosystems
10.1002/2015GC005944
Due to the high compatibility of chromium during magma activity, ultramafic and mafic rocks are the major
reservoirs of Cr in Bulk Silicate Earth (BSE) [Faure, 1991]. Schoenberg et al. [2008] first studied the Cr isotope
compositions of a variety of mantle-derived rocks, including mantle xenoliths, ultramafic rocks, cumulates,
and oceanic and continental basalts. The Cr isotope composition (d53CrNIST979) of the investigated samples
was in a limited range of 20.21& to 20.02& with an average value of 20.12 6 0.10& (2SD), implying that
mantle-derived rocks have relatively homogenous Cr isotope compositions. Farkas et al. [2013] observed
that the global igneous chromites have slightly heavier Cr isotope compositions with an average d53CrNIST
979 value of 20.08 6 0.13& (2SD) compared with the BSE reported by Schoenberg et al. [2008] and then
inferred that chromite-bearing mantle might be isotopically heavier than mantle without chromites. This
phenomenon could be interpreted as being a result of chromite crystallization or isotope fractionation
(equilibrium or disequilibrium in nature) between coexisting minerals. The former explanation is preferred
because the chromites investigated in the study by Farkas et al. [2013] (mostly podiform and stratiform
within host dunites and harzburgites at arc settings) were derived from chromitites, which were thought to
result from crystallization or accumulation of chromite from the silicate melt mixed/interacted with nearby
lez-Jimenez et al. [2014a,b]). However, Farkas et al. [2013] also found that
melt/rock (see reviews by Gonza
some metamorphic Cr-rich minerals, such as fuchsites, uvarovites, Cr-tremolite, Cr-diopside, and Cr-pyrope,
displayed similar or higher d53Cr values (0.04& 0.50&) than chromites and were interpreted as enrichments of heavy Cr isotopes by metamorphism. As with the chromites in the same study, potential intermineral Cr isotope fractionation was not taken into account, and the net effects on whole-rock Cr isotope
composition during metamorphism were not fully assessed.
Iron, another transitional metal with multiple valence states, showed significant isotope fractionation at elevated temperatures, which was attributed to the different charges and coordination numbers among different minerals [Beard and Johnson, 2004; Macris et al., 2015; Young et al., 2015; Zhu et al., 2002]. As previously
noted, under middle-to-lower crust and mantle conditions, the major valence states of Cr are 21 and 31,
and Cr21 and Cr31 could substitute for Fe21 and Fe31, respectively, in mineral crystal lattices [Johnson and
Bullen, 2004a; Klein-BenDavid et al., 2011]. The similar properties of Cr and Fe suggest that there might also
be Cr isotope fractionation between coexisting minerals. If this is true, Cr isotope behavior during metamorphism needs to be further constrained on the basis of whole-rock Cr isotope compositions with varying
degrees of metamorphisms.
To avoid potential inter-mineral isotope fractionation when assessing Cr isotope fractionation behavior during metamorphism, we report herein high-precision whole-rock Cr isotope compositions for a set of wellcharacterized metamorphic mafic rocks from the South China Block and the Dabie-Sulu orogen, eastern
central China, with different degrees of metamorphisms ranging from greenschist to eclogite-facies [Wang
et al., 2014a] and retrograde metamorphisms from eclogite-facies to amphibolite-facies. The primary goal of
this work was to determine whether any Cr isotope fractionation is associated with middle to high
temperature metamorphism processes. The results will not only be helpful in understanding the behavior
of Cr isotopes during metamorphic dehydration related to the subduction of continental crust, but also provide constraints on the Cr isotope compositions of orogenic belts.
To provide the basis for comparison with metamorphic rocks and to provide further constraints on the
extent of Cr isotope heterogeneity in the mantle as well as in the igneous protolith of some metamorphic
rocks, we performed Cr isotope analyses of five deep mantle-derived rocks from the Luobusa chromite
deposit, including three chromite-free peridotites, one chromite-bearing peridotite, and one chromite from
chromitite.
2. Geological Background and Samples
2.1. Dabie Orogen and Meta-Basaltic Rocks
The Dabie and Sulu terranes, located at the eastern part of the east-west trending Qinling-Dabie-Sulu orogenic belt, formed during the continental collision between the South China Block (SCB) and the North
China Block (NCB) in the Triassic [Ames et al., 1996; Ayers et al., 2002; Hacker et al., 1998; Li et al., 1993, 2000].
The eastern Dabie orogen has been divided into five metamorphic belts according to degree of metamorphism: the Beihuaiyang zone (BZ), the North Dabie high-temperature and ultrahigh-pressure (UHP) metamorphic complex zone (NDZ), the Central Dabie middle-temperature and high-pressure metamorphic zone
SHEN ET AL.
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Geochemistry, Geophysics, Geosystems
10.1002/2015GC005944
(CDZ), the South Dabie low-temperature and high-pressure metamorphic zone (SDZ), and the Susong complex zone (SZ) [Li et al., 2001; Liu et al., 2007].
CDZ and SDZ are mainly composed of UHP metamorphic rocks including eclogite, gneiss, marble, and
mafic-ultramafic rocks. Eclogites from these two zones occur as lenses or blocks interbedded within orthogneisses, paragneisses, and marbles. Previous studies confirmed that the peak eclogite-facies metamorphic
temperature and pressure range between 6508C and 7508C and 2.5 and 3.0 GPa, respectively [Li et al., 1993;
Okay et al., 1989; Wang et al., 1989; Xu et al., 1992]. BZ is mainly composed of meta-sedimentary and metaigneous rocks as a passive-margin accretionary wedge formed during continental crust subduction [Li et al.,
2001; Zheng et al., 2005]. Metamorphic rocks from this area underwent low-grade greenschist to epidoteamphibolite facies metamorphism during Triassic SCB subduction [Faure et al., 2003; Li et al., 2001; Zheng
et al., 2005]. The metamorphic rocks from the three above mentioned zones have similar Neoproterozoic
protolith ages (mainly 740–830 Ma) associated with Neoproterozoic rift magmatism in the north margin of
the SCB [Hacker et al., 2000; Wu et al., 2007; Zheng et al., 2003, 2005].
Wudangshan is situated at the northern margin of the SCB and comprises abundant Precambrian suites,
including the Neoproterozoic Wudang and Yaolinghe groups [Ling et al., 2008, 2010]. The Wudang and Yaolinghe volcanic units were dated at 755 6 3 and 685 6 7 Ma, respectively. Both groups underwent Neoproterozoic greenschist-facies metamorphism and lack evidence of subsequent Triassic UHP metamorphism.
The Wudang group consists of fine-grain sedimentary beds intercalated with rift-related bimodal volcanic
sequences, which represent the protoliths for the meta-volcanic rocks in the Dabie orogen [Ling et al., 2010].
2.2. Luobusa Ophiolite
The chromite ore bodies formed in the Luobusa ophiolite in Tibet, which lay within the Yarlung-Zangbo
suture zone, the geological boundary between Asia and India. This ophiolite consists mainly of mantlederived harzburgite, dunite, and sparse lower-crustal cumulates accompanied by minor basaltic pillow lavas
and cherts [Zhou et al., 1996]. Diamonds discovered in Luobusa chromitite located the depth of the chromite formation in the mantle transition zone of 410 km to 660 km [Yang et al., 2014].
2.3. Samples
In the present study, we conducted Cr isotope analyses of 27 metamorphic mafic rocks from the Dabie orogen
and the Wudang terrane including seven greenschists from the Wudang Group, seven amphibolites from BZ,
five low-temperature eclogites from SDZ, and eight middle-temperature eclogites, and one garnet peridotite
(11BXL-2) from the Bixiling ultramafic complex in CDZ (Table 1). The chemical compositions and whole-rock Mg
isotope compositions of these samples have been reported by Wang et al. [2014a]. Furthermore, the Cr isotope
compositions of samples from different sections of an eclogite lens within the wall biotite paragneiss from
Shuanghe in CDZ (Figure 1a) were studied to elucidate the effects of retrograde metamorphism on isotope fractionation. Petrographic features revealed that this eclogite lens could be divided into five segments from the
core to the rim, recording peak eclogite-facies to amphibolite facies retrograde metamorphisms (09SH-6-1, 2,
3b, 3a, 4, see Figures 1a–1g). The coexisting garnets and clinopyroxenes separated from two garnet pyroxenites,
11MW-1 and 11MW-8, from the Maowu metamorphic ultramafic complex in CDZ were studied.
The five mantle-derived samples from Luobusa chromitite in this study consisted of three chromite-free
harzburgites (12YK2-5, 12YK2-8, and 12YK2-13), one chromite-bearing harzburgite (12YK1-42), and one
chromite (12YK1-20). The harzburgites investigated herein consisted of olivine, orthopyroxene, minor clinopyroxene, as well as accessory mineral assemblage of magnetite (1 chromite).
3. Analytical Methods
3.1. Sample Preparation and Chromatography
Except for the eclogite lenses from Shuanghe, all of the metamorphic mafic samples were the same as
those investigated in a previous study [Wang et al., 2014a]. Most chromium isotope analyses were performed at the Carnegie Institution of Washington, USA, whereas additional six samples were analyzed at
the University of Science and Technology of China (USTC), Hefei.
All rock samples (3–5 kg) were first crushed and then powdered in an agate mortar to ensure sample
homogeneity. Approximately 20 mg of rock powder was dissolved in a combination of ultrapure
SHEN ET AL.
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Table 1. Cr Isotopic Compositions of Metamorphic Rocks, Mantle-Derived Rocks, and Minerals
Sample
Cr (ppm)a
d53/52CrSRM 979
2SDb
Greenschists From Wudangshan
HWD-1
181
20.14
0.04
HWD-5
401
20.15
0.04
HWD-10
365
20.17
0.04
HWD-13
708
20.16
0.04
d
766
20.10
0.04
HWD-18
e
908
20.12
0.06
HWD-21
HWD-24
358
20.06
0.04
Average value
20.13
0.08
Greenschists and Amphibolites From Beihuaiyang
11GF-3
733
20.05
0.04
11GF-5
762
20.05
0.04
987
20.08
0.04
11GF-7d
1178
20.06
0.06
11GF-9e
11GF-11
1284
20.08
0.04
11LZ
522
20.16
0.04
106
20.27
0.06
11HJH-3e
Average value
20.11
0.16
Low-T Eclogites From South Dabie Zone
11HZ-3
717
20.03
0.04
11HZ-4
998
20.11
0.04
11HZ-5
856
20.11
0.04
11HZ-6
192
20.24
0.04
110
20.11
0.06
99MW-3e
Average value
20.12
0.15
Middle-T Eclogites and Retro-Metamorphic Amphibolites From Central Dabie Zone
11BXL-1
865
20.10
0.04
11BXL-3
920
20.11
0.04
865
20.05
0.06
11BXL-4e
11BXL-5
754
20.01
0.04
09SH-6-2
1,188
20.15
0.04
09SH-6-3a
1,031
20.14
0.04
09SH-6-3b
905
20.15
0.04
09SH-6-4
943
20.15
0.04
Average value
20.11
0.10
1,821
20.17
0.06
11BXL-2e
Luobusha Mantle-Derived Rocks
12YK2-5
1,186
20.11
0.04
12YK2-8
2,348
20.18
0.04
12YK2-12
3,431
20.21
0.04
12YK1-20
35,7267
20.06
0.04
12YK1-42
13,9983
20.07
0.04
Whole-Rock and Coexisting Minerals in Maowu Garnet Clinopyroxenef
11MW-1(WR)
194
20.10
0.04
11MW-1(Cpx)
72
0.04
0.06
11MW-1(Gt)
119
20.30
0.04
11MW-8(WR)
164
20.20
0.05
11MW-8(Cpx)
52
20.10
0.04
11MW-8(Gt)
174
20.16
0.04
LOI (%)c
Notes
2.49
7.76
7.40
9.65
6.66
5.92
5.41
4.17
2.74
6.98
8.03
8.89
6.63
3.28
2.84
1.33
1.31
1.28
1.40
1.39
1.15
0.58
0.92
3.06
2.93
2.13
1.54
Metamorphic gabbro cumulate
Metamorphic gabbro cumulate
Metamorphic gabbro cumulate
Metamorphic gabbro cumulate
Core: eclogitic facies
Mantles: transition from eclogite
to amphibolite facies
Rim: amphibolite facies
3.55
Garnet peridotite
Chromite-free peridotite
Chromite-free peridotite
Chromite-free peridotite
Chromite
Chromite-bearing peridotite
a
Concentrations were performed by isotope dilution using double spike method.
The uncertainties quoted for individual samples are 2SE (internal uncertainty) of single sample measurements (14 blocks and 30
cycles per block) or 2SD reproducibility of several SRM979 measurements in the same analytical session, or the long-term reproducibility of the standard, whichever is largest.
c
LOI data are from Wang et al. [2014a] and ICP-MS analyzed data.
d
The d53Cr were recalculated relative to NIST SRM 979 according to equation (2).
e
The Cr isotope compositions of the samples were performed by MC-ICP-MS, the uncertainties represents the large-term 2SD reproducibilities of the Cr standard SCP.
f
WR 5 whole rock; Opx 5 orthopyroxene; Cpx 5 clinopyroxene; Gt 5 garnet.
b
HF-HNO3-HCl in Savillex beakers. A capped beaker containing the sample-acid mixture was heated
overnight at 1308C on a hot plate in a laminar flow exhaust hood; after the sample was completely
dissolved, the solution was evaporated to dryness at 150–1608C. During evaporation, the beakers were
loosely capped to avoid cross-contamination and evaporation loss. The dried samples were then completely redissolved in 1 mL of 6 N HCl at 1608C, and the chromite-bearing peridotites and chromites
SHEN ET AL.
Cr ISOTOPES OF SUBDUCTED CRUST
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Figure 1. The field outcrop and photomicrographs of the eclogite lenses from Shuanghe in the Central Dabie Zone. (a) The eclogite occurs
as lenses within the country rock paragneisses; (b) sample 09SH-6-1 consists of coexisting mineral assemblages of garnet (Gt), omphacite
(Omp), rutile (Rt), and minor retrograde amphibole (Amp); (c) sample 09SH-6-2 with coexisting garnet and omphacite represents peak
eclogite facies metamorphism; (d) and (e) omphacite minerals are replaced by retrogression symplektite of amphibole 1 plagioclase (Pl) in
sample 09SH-6-3b and 09SH-6-3a; (f) and (g) replacement of all omphacites and part of garnets with retrogression symplektite of amphibole 1 plagioclase and rutiles with titanite (Ttn) occurs in sample 09SH-6-4.
were redissolved in a mixture of 20 lL concentrated HF and 1 mL 6 N HCl. After complete sample
digestion, the Cr concentrations of the sample solutions were analyzed using ICP-MS to ensure that
the aliquots to be taken out and mixed with 1 mL 50Cr-54Cr double spike (50Cr and 54Cr concentrations
of 2.716 and 1.742 nm/g, respectively) contained 1 lg Cr. The details of this double spike procedure
were reported in our previous study [Han et al., 2012]. The sample-spike mixture was dried completely,
then mixed with 0.2 mL 6 N HCl and heated at 1308C for 2–3 h in preparation for chromatographic
separation. Separation of Cr was achieved through a two-step cation exchange chromatography procedure that has been detailed in Trinquier et al. [2008] and Qin et al. [2010]. Bio-Rad 200–400 mesh
AG50-X8 resin was used in both columns. The procedure blanks were approximately 10 ng, which is
negligible.
3.2. Mass Spectrometry
Purified Cr samples were analyzed using a Thermo Finnigan Triton multicollector thermal-ionization mass
spectrometer (TIMS) at the Carnegie Institution of Washington. For these analyses, 400–800 ng Cr was
loaded in chloride form (3 N HCl) on outgassed Re filaments with silica gel and saturated boric acid [Qin
et al., 2010]. The standards and samples were measured at ionization temperatures between 12708C and
13908C to avoid possible interference at lower and higher ionization temperatures. Cr isotope signals
(50Cr1, 52Cr1, 53Cr1, and 54Cr1) were collected on the Faraday detectors designated L2, axial, H1, and H2,
SHEN ET AL.
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with the axial mass set at 52Cr. The
typical beam intensity of 52Cr is 1–5 V
using a 1011 X resistor. To monitor
interference of 50Ti1 and 50V1 on
50 1
Cr and 54Fe1 on 54Cr1, the ionic
intensities of 49Ti1, 51V1, and 56Fe1
were measured simultaneously at the
Faraday cups designated L3, L1, and
H4, respectively. Each analysis consisted of a total of 420 ratios, with
each ratio integrating the ion intensity
for 8 s. Chromium isotope data are
Figure 2. Long-term reproducibility of d53CrNIST 979 values for NIST SRM 979.
Uncertainties are the internal 2 standard errors of the single measurements. The
expressed in the usual d notation in
gray areas represent the 2 standard deviation (2SD) envelope for the average
per mil (&) units, which is the relative
53
d CrNIST 979 value.
deviation from the National Institute
of Standards and Technology (NIST) standard reference material (SRM) 979 multiplied by 1000:
"
#
ð53 Cr=52 CrÞsample
d53 Cr5 53 52
21 31000:
(1)
ð Cr= CrÞSRM979
3.3. Data Reproducibility
A spiked sample of NIST SRM 979 (a chromium chloride) was run 2–3 times at the beginning, middle,
and end of each analytical session. The 2SD value of these multiple standard measurements was typically <0.02&, except for one session where this value was 0.03&. The long-term (over 3 months) instrumental reproducibility was determined by repeated measurements of the spiked NIST SRM 979
standard. A compilation of all the standard results is displayed in Figure 2. The average value of 40
standards was 0.04& 6 0.04& (n 5 40, 2SD) relative to previous determined Cr isotope composition of
the same standard (SRM979) by another TIMS used in the double-spike calibration spreadsheet. This
small offset was subtracted from the sample measurements, and the reported d53Cr values are always
relative to SRM 979. Samples were typically analyzed once during each analytical session for 2 h. The
internal precision (2SE) on the measured 53Cr/52Cr ratio, based on 14 blocks of 30 ratios at 8 s integration per ratio was typically less than 0.03&. The error assigned to each sample was the largest value
among the following: 2 SE of the individual sample, 2SD of the standard runs obtained in the same session, and long-term instrumental reproducibility (0.04&) (Table 1). Another standard, NIST SRM 3112a,
was also measured during some analytical sessions for interlaboratory comparison. The average d53Cr
value of NIST SRM 3112a was 20.09 6 0.04& (n 5 4, 2SD) relative to NIST SRM 979. This value is consistent with the difference between these two standards of 20.07 6 0.05& that was reported in Schoenberg et al. [2008].
Chromium isotope analyses of six additional metamorphic rock samples were performed using a Neptune
MC-ICP-MS at the CAS Key Laboratory of Crust-Mantle Materials and Environments (USTC); an analytical protocol similar to that used for the Triton TIMS was performed. The long-term precision was better than
60.06& (2SD) (n 5 103) for the 53Cr/52Cr ratios of the Cr elemental standard (SCP). The Cr isotope composition of sample HWD-18 measured by the Neptune MC-ICP-MS (20.09 6 0.06&) was consistent with that
measured by the Triton TIMS (20.10 6 0.04&). In addition, the d53Cr value of another Cr standard NIST SRM
3112a relative to SCP obtained using the Neptune MC-ICP-MS (20.07 6 0.06&, n 5 29) were identical with
those obtained using the Triton TIMS (20.08 6 0.04&, n 5 3). The Cr isotope data are finally adjusted relative to NIST SRM 979 according to the difference between SRM 979 and 3112a by TIMS.
3.4. Major and Trace Element Concentrations of Whole Rocks and Minerals of the Shuanghe
Eclogite Lenses
The major elemental compositions of whole rocks were analyzed at the Hebei Institute of Regional Geology
and Mineral Resources, China, using wavelength dispersive X-ray fluorescence spectrometry [Gao et al.,
1995]. Analytical uncertainties were better than 1%. Trace element concentrations were analyzed using
solution quadrupole ICP-MS (PerkinElmer ElanDRCII) at the CAS Key Laboratory of Crust-Mantle Materials
SHEN ET AL.
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and Environments. Analyses of United
States Geological Survey rock standards
(BCR-2, BHVO-1, and AGV-1) indicated precision and accuracy better than 10% for
trace elements and REEs.
The major elemental concentrations of the
minerals were determined using an EPMA
1600 (Shimadzu) electron microprobe at
USTC. The coupled major and trace elements in the minerals were analyzed using
LA-ICP-MS at the State Key Laboratory
of Geological Processes and Mineral
Resources, China University of Geoscience,
Wuhan. The concentrations of major elements obtained using these two methods
for the same minerals generally agreed
within 5% uncertainty [Liu et al., 2008]. The
accuracies for the USGS standards (BHVO2G, BIR-1G, and BCR-2G) were better than
5–10% for most trace elements.
4. Results
Chromium concentrations and isotope
compositions of 27 metamorphic rocks, 5
mantle-derived rocks, and the coexisting
minerals from two garnet pyroxenites are
shown in Table 1, along with loss on ignition (LOI) data for metamorphic mafic
rocks from this work and from Wang et al.
[2014a].
The d53Cr values of seven greenschists from
Wudangshan, seven amphibolites from BZ,
Figure 3. d53CrNIST 979 values for metamorphic mafic rocks, mantle-derived
five low-temperature eclogites from SDZ,
rocks, metamorphic minerals, and mantle minerals. Error bars in this and all
of the following figures represent 2 standard deviations (2SD). The vertical
and eight middle-temperature eclogites
shaded area indicates the suggested d53CrNIST 979 value for the Bulk Silicate
and retrograde metamorphic amphibolites
Earth (BSE) values reported by Schoenberg et al. [2008].
from CDZ were 20.17& to 20.06&,
20.27& to 20.05&, 20.24& to 20.03&, and 20.15& to 20.01& relative to NIST SRM 979 (Table 1 and Figure 3), respectively. In summary, the metamorphic samples showed similar Cr isotope compositions (within
uncertainties) to previously published data for mantle-derived rocks [Schoenberg et al., 2008] (Figure 3). Notably,
there were up to 0.2& variations in d53Cr within individual sample groups. Two garnet pyroxenites had
whole-rock d53Cr of 20.10& (11MW-1) and 20.20& (11MW-8), whereas the Cr isotope compositions of coexisting garnets and clinopyroxenes were 20.30& and 0.04& from 11MW-1 and 20.16& and 20.10& from
11MW-8. Three chromite-free peridotites from Luobusa had d53CrNIST 979 values varying from 20.21& to
20.11&, and chromite-bearing peridotites and chromite samples had d53Cr of 20.07& and 20.06&, respectively (Figure 3).
The major and trace element concentrations of the Shuanghe eclogite lenses are presented in Table 2. Table
3 shows the average Cr content of the major minerals and the estimated mineral/rock ratios in volume (%).
5. Discussion
In a previous study, no resolvable difference in Cr isotope compositions between basalts and mantle xenoliths was observed [Schoenberg et al., 2008]. Thus, the protoliths of the investigated metamorphic mafic
SHEN ET AL.
Cr ISOTOPES OF SUBDUCTED CRUST
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Geochemistry, Geophysics, Geosystems
Table 2. Major and Trace Element Compositions of the Shuanghe Eclogite
Lenses
Sample
Number
SiO2
Al2O3
Fe2O3
FeO
CaO
MgO
Na2O
K2O
TiO2
MnO
P2O5
H2O
LOI
P
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Ba
Cs
Rb
Sr
Cr
V
Nb
Ta
Zr
Hf
Th
U
Mg#
09SH-6-2
09SH-6-3b
09SH-6-3a
09SH-6-4
47.42
10.27
2.87
12.12
6.96
13.10
1.64
0.06
1.60
0.200
0.10
1.06
3.06
99.39
18.7
35.5
4.55
18.4
4.21
1.23
4.32
0.72
4.27
0.76
2.18
0.3
1.78
0.25
157
0.22
2.9
198
1230
335
2.8
0.2
100
3
4.47
2.3
61.6
47.53
11.87
2.36
11.86
8.78
10.54
2.14
0.05
1.36
0.222
0.10
0.73
2.93
99.74
6.6
14.6
2.19
10.1
3.26
1.07
4.29
0.77
4.67
0.86
2.45
0.33
1.98
0.29
39.7
0.23
2.8
201
1010
309
3
0.
120
3.8
2.64
1.1
57.6
50.92
12.40
4.15
10.54
6.21
9.41
1.88
0.05
1.53
0.226
0.31
1.41
2.13
99.73
8.1
17.7
3.13
14.8
4.38
1.29
5.0
0.81
4.87
0.87
2.55
0.36
2.15
0.3
117.5
0.16
2.2
120
1120
283
4.8
0.3
130
4
0.66
0.93
54.3
52.32
14.25
3.64
7.42
8.08
7.49
3.39
0.35
0.93
0.156
0.13
1.22
1.54
99.69
2.8
5.6
0.9
4.4
1.72
0.69
3.05
0.58
3.63
0.66
1.87
0.24
1.48
0.21
37.9
0.13
5.2
107
1030
238
5.5
0.5
100
3
0.19
0.44
55.7
Table 3. Mineralogical Characteristics of Shuanghe Eclogite Lenses
Estimated Modes for Mineralsa
Sample ID
09SH-6-2
Mineral/whole rock (vol %)
Average Cr contents (ppm)
09SH-6-3a
Mineral/whole rock (vol %)
Average Cr contents (ppm)
09SH-6-3b
Mineral/whole rock (vol %)
Average Cr contents (ppm)
09SH-6-4
Mineral/whole rock (vol %)
Average Cr contents (ppm)
Omp
Grt
Amp
Rt
35
2420
50
1129
24
1298
48
823
10
4
3092
1
22
1114
45
719
12
4
2899
1
5
28
739
35
966
3
2907
2
392(Ttn)
214(ilm)
5
3694
a
Omp 5 omphacite, Grt 5 garnet, Amp 5 amphibole, Rt 5 rutile,
Ttn 5 titanite, and ilm 5 ilmenite.
SHEN ET AL.
Ttn 1 ilm
Cr ISOTOPES OF SUBDUCTED CRUST
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rocks, which were widely accepted as
Neoproterozoic rift magma, were
expected to share similar Cr isotope
composition with the mantle. In the
present study, the key observations
include: (1) all metamorphic rocks
showed no systematic difference in Cr
isotope composition from their mantlederived protoliths, indicating a lack of
systematic Cr isotope fractionation
associated with different degrees of
dehydration accompanied by continental crust subduction; (2) there are
resolvable Cr isotope variations within
the individual metamorphic belts.
5.1. The Effect of the Degree of
Metamorphic Dehydration
Metamorphic dehydration is a critical
process in controlling the isotope fractionation of water-soluble elements,
such as Li and Mg [Li et al., 2014; Teng
et al., 2007; Wang et al., 2014a]. The
dehydration process releases H2O and
other volatiles (such as CO2), as well as
water-soluble elements, into the overlying slab and mantle wedge. The LOI
value is typically used as an index quantifying the amounts of structurally
bound water (H2O or OH2) and volatile
gases (e.g., CO2) in this process. The LOI
values of the samples were expected to
decrease with increasing degrees of
metamorphic dehydration. For example, the eclogites generally displayed
lower LOI values than the greenschists
and amphibolites, as shown in Figure
4a. However, the Cr concentrations and
isotope compositions of the metamorphic rocks studied herein showed no
systematic variations from greenschists
to eclogites (Figures 4a–4c), indicating
that there were no systematic variations
of elemental loss and isotope fractionation associated with different degrees
of dehydration.
The major oxidation state of Cr in typical
terrestrial rock-forming mineral is Cr31,
which is usually not water soluble. However, some recent studies have suggested that Cr31 could be significantly
mobile in Cl2-enriched fluids under
middle-to-lower crust and upper mantle
conditions [Klein-BenDavid et al., 2011;
8
Geochemistry, Geophysics, Geosystems
10.1002/2015GC005944
Figure 4. (a) Cr content versus LOI; (b) d53Cr versus LOI; (c) d53Cr versus Cr content for metamorphic rocks from the Dabie orogen.
Marshall et al., 2003; Sobolev et al., 2009; Watenphul et al., 2014]. Given the low Cl2 content of subducted
continental crust (e.g., 180ppm) [Rudnick and Gao, 2003], dissolution loss of Cr31 should be limited if
there is no change in the oxidation state of the system. Cr31 released from the breakup of hydrous minerals could be consumed by formation of new minerals. For example, mica group minerals, amphibole,
rutile, garnet, and omphacite are all known as Cr31-hosting minerals in mafic rocks undergoing different degrees of metamorphism [Johan et al., 1983; Meinhold, 2010; Sobolev et al., 1997; Spandler et al.,
2011]. Thus, limited Cr31 loss might be the main explanation for why no systematic elemental loss and
isotope fractionation were present.
Moreover, Shuanghe eclogite lenses revealed an integrated retrograde sequence: peak eclogite facies
(core)-transition facies (mantle)-amphibolite facies (rim) (Figure 1). The LOI contents of these rocks tend to
decrease (Figure 5a), indicating the loss of significant amounts of water. Combined with a lack of compositional zoning in these major minerals, a significant loss of Cr was not expected to occur. The Cr content of
whole rocks and major minerals (Figure 5b) was most likely inherited from their protoliths. The whole profile
was isotopically uniform (20.14& to 20.15&) regardless of variations in the LOI content, chemical composition, and relative proportions of major Cr-bearing minerals (e.g., rutile, amphibole, garnet, and omphacite
in Figures 5a–5c). This evidence further supports no systematic Cr isotope fractionation associated with the
degree of dehydration.
In this study, the clinopyroxenes from two metamorphic garnet pyroxenites were 0.06& and 0.34& heavier
than the coexisting garnets (Figure 3), suggesting inter-mineral Cr isotope disequilibrium in some metamorphic rocks. Because the investigated metamorphic rocks had BSE-like Cr isotope composition, the isotopically heavier metamorphic minerals (e.g., Cr-bearing pyroxene and garnet) reported in a previous study
[Farkas et al., 2013] might have resulted from inter-mineral isotope disequilibrium or fractionation and were
not necessarily representative of the whole-rock composition.
5.2. The Causes for Cr Isotope Variations Among the Homologous Metamorphic Rocks
Although no systematic variations of Cr isotope composition were present, the Cr isotope compositions of
the metamorphic rocks within the same metamorphic belt varied significantly (up to 0.2&).
Figure 5. (a) d53Cr versus LOI; (b) Cr contents in whole rocks and major Cr-rich minerals; (c) mineral proportions relative to the whole rocks in volume (%). This figure displays Cr elemental and isotopic behaviors during eclogite facies to amphibolite facies retrograde metamorphism (data from Tables 1–3).
SHEN ET AL.
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Geochemistry, Geophysics, Geosystems
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Figure 6. (a) Fe21/RFe versus LOI; (b) d53Cr versus Fe21/RFe; (c) d53Cr versus Mg# value; (d) d53Cr versus MgO content. Fe21/RFe and Mg#
values are from Wang et al. [2014a] and Table 2. Solid lines in Figure 6d represent calculated Cr isotope compositions of residual
melts during fractional crystallization, assuming a Rayleigh fractionation process with average crystal-melt fractionation factors
(amineral-melt) of 0.99995, 0.9999, 0.9998, and 0.9997, corresponding to isotope fractionation between mineral and melt (Dd53Crcrystal53
53
melt 5 d Crcrystal 2 d Crmelt) of 20.05, 20.1, 20.2, and 20.3&, respectively. The yellow star represents a Mg-rich primary mafic melt
with MgO 5 10.7 wt % [Teng et al., 2008] and d53Cr 5 20.18&. The f value represents the fraction of Cr in the residual melt and is displayed alongside the fractionation curves as 0.9–0.3.
Nonrepresentative sampling could be ruled out because 3–5 kg of materials for each whole rock was
crushed [Wang et al., 2014a]. Previous works have proposed that interactions between metamorphic rocks
and wall marble could alter both Mg and O isotope compositions of some metamorphic rocks [Wang et al.,
2014a,b]. This process is unlikely to influence the Cr isotope composition, as the Cr concentration in marbles
is very low (<15 ppm, see supplementary material of Wang et al. [2014a,b]). Processes that may have contributed to the Cr isotope variability with individual metamorphic zones are discussed in the following
section.
5.2.1. Metamorphism Dehydration Accompanied by the Change of Oxidation State
In Figures 6a and 6b, LOI and d53Cr are plotted against the redox-sensitive Fe21/RFe ratio for all of the
investigated metamorphic rocks (see Table 2 and the supplementary material of Wang et al. [2014a] for
major element data). The significant variability in the whole-rock Fe21/RFe ratios and the roughly negative
correlation of Fe21/RFe with d53Cr for individual metamorphic zones indicate that a change of oxidation
state may be the cause of the Cr isotope variations within individual metamorphic zones.
Dehydration of the subducted crust most likely leads to changes in the oxidation state for a water-rock
system, regardless of the degree of metamorphism. Previous studies have revealed that the release of
redox volatiles from subducted oceanic slab can significantly change the redox state of the residual slab
[Groppo and Castelli, 2010; Song et al., 2009]. Despite differences in the volatile compositions between
oceanic and continental crusts, changes in valence state can be expected during continental crust
subduction.
SHEN ET AL.
Cr ISOTOPES OF SUBDUCTED CRUST
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Geochemistry, Geophysics, Geosystems
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Recently, many studies have reported that
Cr21 is frequently observed in the mafic
melt and minerals from Earth and other
terrestrial planets over a wide fo2 (oxygen
fugacity) range (with logfo2 from 0 to
216) at high temperature [Bell et al., 2014;
Berry and O’Neill, 2004; Berry et al., 2006;
Eeckhout et al., 2007; McKeown et al.,
2014]. Moreover, although Cr21 is favored
in the melt (fluid) relative to Cr31, the
Cr21/Cr31 values of the fluid phase are
strongly dependent on the oxidation state
[Delano, 2001]. Model calculations suggested that Cr21-bearing minerals, such as
olivine, are isotopically lighter than Cr31bearing minerals [Moynier et al., 2011]. The
apparent increase in d53Cr with decreasing Fe21/RFe within each metamorphic
Figure 7. Combined plot of Cr isotope and Mg isotope systematics of metagroup, except in the Shuanghe retrograde
morphic mafic rocks from the Dabie orogen. Mg isotope data are from Wang
et al. [2014a].
eclogite lenses, is consistent with the
increased Cr21/Cr31 ratio in the released
fluid/melt and the decreased Cr21/Cr31 in the residual rocks when the residual rocks were shifted to more
oxidizing conditions. Moreover, the overall lack of correlation between LOI values and Fe21/RFe ratios indicates that the redox state was not correlated with the degree of dehydration (Figure 6a).
5.2.2. Heterogeneous Protolith: Potential Isotope Fractionation During Magmatic Differentiation
Alternatively, isotope heterogeneity in the protolith could account for the variations in Cr isotope composition within individual metamorphic zones. It has been demonstrated that changes in fo2 during magma
evolution could cause isotope fractionation of redox-sensitive elements, such as Fe [Teng et al., 2008]. Wang
et al. [2014a] proposed that there was no significant MgO loss during metamorphism in these metamorphic
rocks, thus the Mg contents in the metamorphic rocks most likely reflect those in their protolith. The
roughly negative correlation between d53Cr and Mg# (MgO) suggests that the Cr isotope variability could
be due to the protolith heterogeneity, likely as a result of magmatic differentiation (Figures 6c and 6d).
To further test this hypothesis, the effects of fractional crystallization were modeled by Rayleigh fractionation with average mineral-melt fractionation factors (amineral-melt) of 0.9997–0.99995, corresponding to fractionation between mineral and melt (Dd53Crmineral-melt) of 20.3& to 20.05& (Figure 6d). Although no
experimentally calibrated equilibrium fractionation factor between minerals and melts is currently available,
theoretical calculations by Moynier et al. [2011] suggested that Cr21-dominated olivine is isotopically lighter
than Cr31-bearing minerals and melt. Thus, crystallization of olivine can potentially increase the Cr isotope
composition of the residue. According to our model, f values as low as 0.3 are required to interpret the
spread of our data. However, unlike Fe, Cr is relatively incompatible in olivine (less than 100 ppm). To
achieve a low f value of 0.3, an unreasonable degree of olivine crystallization is required. Furthermore, combined with the limited isotope fractionation between mantle peridotites and terrestrial basalts [Schoenberg
et al., 2008], the mineral-melt fractionation factor of 20.3& also seems unrealistic. Thus, fractional crystallization occurring in the protolith alone is not sufficient to explain Cr isotope heterogeneity within individual
metamorphic groups.
5.3. Comparison of Cr and Mg Isotope Systematics of Subducted Continental Crust
A compilation of Cr and Mg isotope data for the metamorphic mafic rocks is presented in Figure 7. Wang
et al. [2014a] obtained similar Mg isotope compositions for greenschists, amphibolites, and eclogites, and
they proposed that Mg isotope fractionation during continental crust subduction is limited similarly to that
of Cr isotope. Furthermore, the Mg isotope compositions of amphibolites and eclogites vary significantly,
which may result from source contamination, crustal assimilation, or protolith heterogeneity [Wang et al.,
2014a]. No correlation was observed between d53Cr and d26Mg, suggesting that different factors control Mg
SHEN ET AL.
Cr ISOTOPES OF SUBDUCTED CRUST
11
Geochemistry, Geophysics, Geosystems
10.1002/2015GC005944
and Cr isotope fractionation according to their different physical and chemical properties. Specifically, Cr is
a trace and redox-sensitive element, whereas Mg is a major element and is not redox-sensitive.
5.4. Cr Isotope Compositions of the Mantle-Derived Rocks and Potential Isotope Fractionation
During Chromite Crystallization
To this point, the mantle-derived rocks that have been studied for Cr isotope composition mainly include
mantle peridotite xenoliths, ultramafic cumulates, and basalts [Schoenberg et al., 2008]. Farkas et al. [2013]
observed that the Cr isotope compositions of 30 globally distributed chromites ((Fe, Mg)Cr2O4) were slightly
heavier than that of BSE. According to the authors, nearly all chromites were separated from host chromitites within ultramafic rocks at arc settings rather than directly from peridotites. Although the cause of the
chromitite formation was still debated, the process of crystallization/precipitation of chromite with minor
lez-Jimenez et al.,
silicate and platinum-group minerals from the melt/fluid was widely accepted [Gonza
2014a,b; Leblanc and Ceuleneer, 1991; Leblanc and Nicolas, 1992; Zhou et al., 1996, 2001]. Thus, chromites
dominate the Cr elemental contents and isotope compositions in chromitites, compared to rare silicate
phases and platinum-group minerals. The Cr isotope variation between the chromites and BSE might reflect
isotope fractionation between chromite and melt.
To further investigate the possible difference between the Cr isotope compositions of mantle-derived peridotites and those of chromites, we evaluated the influence of chromites in mantle-derived peridotites controlling Cr isotope fractionation in the Luobusa chromite ore deposit and the harzburgites from western
China. The d53Cr values of the studied samples increased in the following order: chromite-free peridotites
(20.21& to 20.11&) < chromite-bearing peridotites (20.07&) < chromites (20.06&) (Figure 3). Two
important points should be noted: (1) Cr isotope compositions of deep mantle rocks are identical to those
of upper mantle rocks and mantle-derived silicates, suggesting homogeneous isotope compositions of the
mantle sources at varying depths, even up to the mantle transition zone [Yang et al., 2014]; (2) the observation of the heavy Cr isotope compositions of the chromites relative to those of the chromite-free peridotites
indicates that Cr isotope fractionation may have occurred during crystallization of the chromite. This finding
could be interpreted as resulting from the difference in oxidation states between chromites (mostly Cr31)
and residual ultrabasic or basic melt (dominated by Cr21). Moreover, this observation is in accord with theoretical calculations by Moynier et al. [2011]. Given the limited range of variation in the Cr isotope composition of chromites from various sources observed in both this study and previous studies [Farkas et al., 2013;
Schoenberg et al., 2008], the potential Cr isotope fractionation between chromites and magma melt, if any,
should be limited.
The main findings of this study are that there was little isotopic variability among the bulk rocks of metamorphic samples from the subducted continental crust and that their Cr isotope compositions were largely
consistent with the value for BSE as defined by Schoenberg et al. [2008]. Chromium isotope fractionation
during the metamorphic dehydration appeared to be limited owing to low activity of Cr31 in subductionrelated fluids. Therefore, despite of possible small variations associated with the changes in the oxidation
state, the largely BSE-like Cr isotopic characters in subducted continental crust could substantially persist to
the mantle, or back to the surface, which would result in limited Cr isotope variations of orogenic orthometamorphites, or intraplate basalts.
6. Conclusions
The Cr isotope composition of metamorphic mafic rocks from different metamorphic zones of the Dabie
orogen has been investigated. The BSE-like Cr isotope compositions of the studied homologous greenschists, amphibolites, and eclogites indicated no systematic fractionation associated with different degrees
of metamorphic dehydration. However, the Cr isotopic variability of samples within the same metamorphic
belt may reflect the effects of redox states changed by dehydration rather than protolith heterogeneity (i.e.,
magmatic differentiation). The heavier Cr isotope compositions of the metamorphic minerals that have
been reported in previous studies reflected inter-mineral Cr isotope disequilibrium during metamorphism.
Overall, our results confirm that the subducted continental crust could largely retain BSE-like Cr isotope
compositions despite undergoing UHP prograde metamorphic dehydration and subsequent exhumation.
The influence on Cr isotopes of the incorporation of subduction-related metamorphic rock or fluid into the
mantle might be not significant.
SHEN ET AL.
Cr ISOTOPES OF SUBDUCTED CRUST
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The Cr isotope compositions of cogenetic Luobusa chromitites and peridotites increased in the following
order: chromite-free peridotites (20.21& to 20.11&) < chromite-bearing peridotite (20.07&) < chromitite
(20.06&). Our data are consistent with the previous observation that mantle-derived chromites are isotopically heavier than BSE, implying potential isotope fractionation between chromite and silicate melt. The similarity in Cr isotope composition between the metamorphic rocks and the mantle-derived rocks provides
further support for the suggestion that there is no resolvable Cr isotope fractionation during continental
subduction.
Acknowledgment
We would like to thank Yongsheng He,
Sheng’ao Liu, Hongjie Wu, and
Chuanwei Zhu for help in the clean
lab. We also thank Mary Horan and
Tim Mock for laboratory assistance at
the Department of Terrestrial
Magnetism (DTM) of the Carnegie
Institution of Washington. The efficient
editorial handing of Cin-Ty Lee is
greatly appreciated. The manuscript
benefited greatly from the review
comments of Ronny Schoenberg and
Justin Simon. The chemical
composition and Mg isotope
composition data of the metamorphic
rocks for this paper are available at
http://dx.doi.org/10.1016/j.gca.2014.03.
029. This work was funded by the
Chinese Ministry of Science and
Technology (2015CB856102), the
National Nature Science Foundation of
China (41273076 and 41473066), the
‘‘111’’ project and the Fundamental
Research Funds for the Central
Universities to Liping Qin and the
Chinese Universities Scientific Fund
(WK2080000059) to Ji Shen.
SHEN ET AL.
References
Ames, L., G. Z. Zhou, and B. C. Xiong (1996), Geochronology and isotopic character of ultrahigh-pressure metamorphism with implications
for collision of the Sino-Korean and Yangtze cratons, central China, Tectonics, 15(2), 472–489.
Ayers, J. C., S. Dunkle, S. Gao, and C. F. Miller (2002), Constraints on timing of peak and retrograde metamorphism in the Dabie Shan
Ultrahigh-Pressure Metamorphic Belt, east-central China, using U–Th–Pb dating of zircon and monazite, Chem. Geol., 186, 315–331.
Beard, B. L., and C. M. Johnson (2004), Inter-mineral Fe isotope variations in mantle-derived rocks and implications for the Fe geochemical
cycle, Geochim. Cosmochim. Acta, 68(22), 4727–4743.
Bell, A., P. Burger, L. Le, C. Shearer, J. Papike, S. Sutton, M. Newville, and J. Jones (2014), XANES measurements of Cr valence in olivine and
their applications to planetary basalts, Am. Mineral., 99, 1404–1412.
Berna, E. C., T. M. Johnson, R. S. Makdisi, and A. Basu (2009), Cr stable isotopes as indicators of Cr (VI) reduction in groundwater: A detailed
time-series study of a point-source plume, Environ. Sci. Technol., 44(3), 1043–1048.
Berry, A. J., and H. S. C. O’Neill (2004), A XANES determination of the oxidation state of chromium in silicate glasses, Am. Mineral., 89(5-6),
790–798.
Berry, A. J., H. S. C. O’Neill, D. R. Scott, G. J. Foran, and J. Shelley (2006), The effect of composition on Cr21/Cr31 in silicate melts, Am.
Mineral., 91(11-12), 1901–1908.
Delano, J. W. (2001), Redox history of the Earth’s interior since 3900 Ma: Implications for prebiotic molecules, Origins Life Evol. Biosphere,
31(4-5), 311–341.
Døssing, L. N., K. Dideriksen, S. L. S. Stipp, and R. Frei (2011), Reduction of hexavalent chromium by ferrous iron: A process of chromium
isotope fractionation and its relevance to natural environments, Chem. Geol., 285(1), 157–166.
Eeckhout, S. G., N. Bolfan-Casanova, C. McCammon, S. Klemme, and E. Amiguet (2007), XANES study of the oxidation state of Cr in lower
mantle phases: Periclase and magnesium silicate perovskite, Am. Mineral., 92(5-6), 966–972.
Ellis, A. S., T. M. Johnson, and T. D. Bullen (2002), Chromium isotopes and the fate of hexavalent chromium in the environment, Science,
295(5562), 2060–2062.
Ellis, A. S., T. M. Johnson, and T. D. Bullen (2004), Using chromium stable isotope ratios to quantify Cr (VI) reduction: Lack of sorption effects,
Environ. Sci. Technol., 38(13), 3604–3607.
Farkas, J., V. Chrastny, M. Novak, E. Cadkova,
J. Pasava, R. Chakrabarti, S. B. Jacobsen, L. Ackerman, and T. D. Bullen (2013), Chromium isotope variations (d53/52Cr) in mantle-derived sources and their weathering products: Implications for environmental studies and the
evolution of d53/52Cr in the Earth’s mantle over geologic time, Geochim. Cosmochim. Acta, 123, 74–92.
Faure, G. (1991), Principles and Applications of Geochemistry, John Wiley, N. Y.
Faure, M., W. Lin, U. Sch€arer, L. Shu, Y. Sun, and N. Arnaud (2003), Continental subduction and exhumation of UHP rocks. Structural and
geochronological insights from the Dabieshan (East China), Lithos, 70(3-4), 213–241.
Frei, R., C. Gaucher, S. W. Poulton, and D. E. Canfield (2009), Fluctuations in Precambrian atmospheric oxygenation recorded by chromium
isotopes, Nature, 461(7261), 250–253.
Frei, R., C. Gaucher, L. N. Døssing, and A. N. Sial (2011), Chromium isotopes in carbonates—A tracer for climate change and for reconstructing the redox state of ancient seawater, Earth Planet. Sci. Lett., 312(1–2), 114–125.
Frei, R., C. Gaucher, D. Stolper, and D. E. Canfield (2013), Fluctuations in late Neoproterozoic atmospheric oxidation—Cr isotope
chemostratigraphy and iron speciation of the late Ediacaran lower Arroyo del Soldado Group (Uruguay), Gondwana Res., 23(2),
797–811.
Gao, S., B.-R. Zhang, X.-M. Gu, Q.-L. Xie, C.-L. Gao, and X.-M. Guo (1995), Silurian-Devonian provenance changes of South Qinling basins:
Implications for accretion of the Yangtze (South China) to the North China cratons, Tectonophysics, 250(1), 183–197.
Gonzalez-Jim
enez, J. M., W. L. Griffin, F. Gervilla, J. A. Proenza, S. Y. O’Reilly, and N. J. Pearson (2014a), Chromitites in ophiolites: How, where,
when, why? Part I. A review and new ideas on the origin and significance of platinum-group minerals, Lithos, 189, 127–139.
Gonzalez-Jim
enez, J. M., W. L. Griffin, J. A. Proenza, F. Gervilla, S. Y. O’Reilly, M. Akbulut, N. J. Pearson, and S. Arai (2014b), Chromitites in
ophiolites: How, where, when, why? Part II. The crystallization of chromitites, Lithos, 189, 140–158.
Groppo, C., and D. Castelli (2010), Prograde P–T evolution of a lawsonite eclogite from the Monviso meta-ophiolite (Western Alps):
Dehydration and redox reactions during subduction of oceanic FeTi-oxide gabbro, J. Petrol., 51(12), 2489–2514.
Hacker, B. R., L. Ratschbacher, L. Webb, T. Ireland, D. Walker, and S. W. Dong (1998), U/Pb zircon ages constrain the architecture of the
ultrahigh-pressure Qinling-Dabie Orogen, China, Earth Planet. Sci. Lett., 161(1-4), 215–230.
Hacker, B. R., L. Ratschbacher, L. Webb, M. O. McWilliams, T. Ireland, A. Calvert, S. W. Dong, H. R. Wenk, and D. Chateigner (2000),
Exhumation of ultrahigh-pressure continental crust in east central China: Late Triassic-Early Jurassic tectonic unroofing, J. Geophys. Res.,
105(B6), 13,339-13,364.
Han, R., L. Qin, S. T. Brown, J. N. Christensen, and H. R. Beller (2012), Differential isotopic fractionation during Cr (VI) reduction by an
aquifer-derived bacterium under aerobic versus denitrifying conditions, Appl. Environ. Microbiol., 78(7), 2462–2464.
Johan, Z., H. Dunlop, L. LeBel, J. Robert, and M. Volfinger (1983), Origin of chromite deposits in ophiolitic complexes-evidence for a volatilerich and sodium-rich reducing fluid phase, paper presented at Fortschritte der Mineralogie, E Schweizerbart’sche Verlags Naegele U
Obermiller, Stuttgart, Germany.
Johnson, T. M., and T. D. Bullen (2004a), Selenium, iron and chromium stable isotope ratio measurements by the double isotope spike
TIMS method, in Handbook of Stable Isotope Methods, Elsevier, Amsterdam.
Johnson, T. M., and T. D. Bullen (2004b), Mass-dependent fractionation of selenium and chromium isotopes in low-temperature environments, Rev. Mineral. Geochem., 55(1), 289–317.
Cr ISOTOPES OF SUBDUCTED CRUST
13
Geochemistry, Geophysics, Geosystems
10.1002/2015GC005944
Klein-BenDavid, O., T. Pettke, and R. Kessel (2011), Chromium mobility in hydrous fluids at upper mantle conditions, Lithos, 125(1–2),
122–130.
Leblanc, M., and G. Ceuleneer (1991), Chromite crystallization in a multicellular magma flow: Evidence from a chromitite dike in the Oman
ophiolite, Lithos, 27(4), 231–257.
Leblanc, M., and A. Nicolas (1992), Ophiolitic chromitites, Int. Geol. Rev., 34(7), 653–686.
Li, S. G., et al. (1993), Collision of the North China and Yangtse Blocks and formation of coesite-bearing eclogites: Timing and processes,
Chem. Geol., 109(1-4), 89–111.
Li, S. G., E. Jagoutz, Y. Chen, and Q. Li (2000), Sm-Nd and Rb-Sr isotopic chronology and cooling history of ultrahigh pressure metamorphic
rocks and their country rocks at Shuanghe in the Dabie Mountains, Central China, Geochim. Cosmochim. Acta, 64(6), 1077–1093.
Li, S. G., F. Huang, Y. H. Nie, W. L. Han, G. Long, H. M. Li, S. Q. Zhang, and Z. H. Zhang (2001), Geochemical and geochronological constraints
on the suture location between the North and South China blocks in the Dabie Orogen, Central China, Phys. Chem. Earth, Part A,
26(9-10), 655–672.
Li, W. Y., F. Z. Teng, B. A. Wing, and Y. Xiao (2014), Limited magnesium isotope fractionation during metamorphic dehydration in metapelites from the Onawa contact aureole, Maine, Geochem. Geophys. Geosyst., 15, 408–415, doi:10.1002/2013GC004992.
Ling, W., B. Ren, R. Duan, X. Liu, X. Mao, L. Peng, Z. Liu, J. Cheng, and H. Yang (2008), Timing of the Wudangshan, Yaolinghe volcanic
sequences and mafic sills in South Qinling: U-Pb zircon geochronology and tectonic implication, Chin. Sci. Bull., 53(14), 2192–2199.
Ling, W., R. Duan, X. Liu, J. Cheng, X. Mao, L. Peng, Z. Liu, H. Yang, and B. Ren (2010), U-Pb dating of detrital zircons from the Wudangshan
Group in the South Qinling and its geological significance, Chin. Sci. Bull., 55(22), 2440–2448.
Liu, Y., Z. Hu, S. Gao, D. G€
unther, J. Xu, C. Gao, and H. Chen (2008), In situ analysis of major and trace elements of anhydrous minerals by
LA-ICP-MS without applying an internal standard, Chem. Geol., 257(1-2), 34–43.
Liu, Y. C., S. G. Li, and S. T. Xu (2007), Zircon SHRIMP U-Pb dating for gneisses in northern Dabie high T/P metamorphic zone, central China:
Implications for decoupling within subducted continental crust, Lithos, 96(1-2), 170–185.
Macris, C. A., C. E. Manning, and E. D. Young (2015), Crystal chemical constraints on inter-mineral Fe isotope fractionation and implications
for Fe isotope disequilibrium in San Carlos mantle xenoliths, Geochim. Cosmochim. Acta, 154, 168–185.
Marshall, D., L. Groat, G. Giuliani, D. Murphy, D. Mattey, T. S. Ercit, M. A. Wise, W. Wengzynowski, and W. D. Eaton (2003), Pressure, temperature and fluid conditions during emerald precipitation, southeastern Yukon, Canada: Fluid inclusion and stable isotope evidence, Chem.
Geol., 194(1), 187–199.
McKeown, D. A., A. C. Buechele, R. Tappero, T. J. McCoy, and K. G. Gardner-Vandy (2014), X-ray absorption characterization of Cr in forsterite
within the MacAlpine Hills 88136 EL3 chondritic meteorite, Am. Mineral., 99(1), 190–197.
Meinhold, G. (2010), Rutile and its applications in earth sciences, Earth Sci. Rev., 102(1-2), 1–28.
Moynier, F., Q.-Z. Yin, and E. Schauble (2011), Isotopic evidence of Cr partitioning into Earth’s core, Science, 331(6023), 1417–1420.
Okay, A. I., S. Xu, and A. M. C. Sengor (1989), Coesite from the Dabie Shan eclogites, central China, Eur. J. Mineral., 1(4), 595–598.
Qin, L., C. M. D. Alexander, R. W. Carlson, M. F. Horan, and T. Yokoyama (2010), Contributors to chromium isotope variation of meteorites,
Geochim. Cosmochim. Acta, 74(3), 1122–1145.
Rossman, K. J. R., and P. D. P. Taylor (1998), Isotopic composition of the elements, Pure Appl. Geophys., 70, 217–236.
Rudnick, R. L., and S. Gao (2003), Composition of the continental crust, Treatise Geochem., 3, 1–64.
Schauble, E., G. R. Rossman, and H. P. Taylor Jr. (2004), Theoretical estimates of equilibrium chromium-isotope fractionations, Chem. Geol.,
205(1), 99–114.
Schiller, M., E. Van Kooten, J. C. Holst, M. B. Olsen, and M. Bizzarro (2014), Precise measurement of chromium isotopes by MC-ICPMS,
J. Anal. At. Spectrom., 29, 1406–1416.
Schoenberg, R., S. Zink, M. Staubwasser, and F. Von Blanckenburg (2008), The stable Cr isotope inventory of solid Earth reservoirs
determined by double spike MC-ICP-MS, Chem. Geol., 249(3), 294–306.
Sobolev, N., A. Logvinova, and E. Efimova (2009), Syngenetic phlogopite inclusions in kimberlite-hosted diamonds: Implications for role of
volatiles in diamond formation, Russ. Geol. Geophys., 50(12), 1234–1248.
Sobolev, N. V., F. V. Kaminsky, W. L. Griffin, E. S. Yefimova, T. T. Win, C. G. Ryan, and A. I. Botkunov (1997), Mineral inclusions in diamonds
from the Sputnik kimberlite pipe, Yakutia, Lithos, 39(3-4), 135–157.
Song, S., L. Su, Y. Niu, Y. Lai, and L. Zhang (2009), CH4 inclusions in orogenic harzburgite: Evidence for reduced slab fluids and implication
for redox melting in mantle wedge, Geochim. Cosmochim. Acta, 73(6), 1737–1754.
Spandler, C., T. Pettke, and D. Rubatto (2011), Internal and external fluid sources for eclogite-facies veins in the Monviso meta-ophiolite,
Western Alps: Implications for fluid flow in subduction zones, J. Petrol., 52(6), 1207–1236.
Teng, F., W. McDonough, R. Rudnick, and B. Wing (2007), Limited lithium isotopic fractionation during progressive metamorphic dehydration in metapelites: A case study from the Onawa contact aureole, Maine, Chem. Geol., 239(1-2), 1–12.
Teng, F. Z., N. Dauphas, and R. T. Helz (2008), Iron isotope fractionation during magmatic differentiation in Kilauea Iki Lava Lake, Science,
320(5883), 1620–1622.
Trinquier, A., J.-L. Birck, and C. J. Allègre (2008), High-precision analysis of chromium isotopes in terrestrial and meteorite samples by
thermal ionization mass spectrometry, J. Anal. At. Spectrom., 23(12), 1565–1574.
Wang, S.-J., F. Teng, S. G. Li, and J. A. Hong (2014a), Magnesium isotopic systematics of mafic rocks during continental subduction, Geochim. Cosmochim. Acta, 143, 34–48.
Wang, S.-J., F. Teng, and S. G. Li (2014b), Tracing carbonate–silicate interaction during subduction using magnesium and oxygen isotopes,
Nat. Commun., 5, 5328, doi:10.1038/ncomms6328.
Wang, X. M., J. G. Liou, and H. K. Mao (1989), Coesite-bearing eclogite from the Dabie Mountains in Central China, Geology, 17(12), 1085–1088.
Wanner, C., S. Zink, U. Eggenberger, and U. M€ader (2012a), Assessing the Cr (VI) reduction efficiency of a permeable reactive barrier using
Cr isotope measurements and 2D reactive transport modeling, J. Contam. Hydrol., 131(1), 54–63.
Wanner, C., U. Eggenberger, D. Kurz, S. Zink, and U. M€ader (2012b), A chromate-contaminated site in southern Switzerland–Part 1: Site
characterization and the use of Cr isotopes to delineate fate and transport, Appl. Geochem., 27(3), 644–654.
Watenphul, A., C. Schmidt, and S. Jahn (2014), Cr(III) solubility in aqueous fluids at high pressures and temperatures, Geochim. Cosmochim.
Acta, 126, 212–227.
Wu, Y.-B., Y.-F. Zheng, J. Tang, B. Gong, Z.-F. Zhao, and X. Liu (2007), Zircon U–Pb dating of water–rock interaction during Neoproterozoic
rift magmatism in South China, Chem. Geol., 246, 65–86.
Xu, S. T., W. Su, Y. C. Liu, L. L. Jiang, S. Y. Ji, A. I. Okay, and A. M. C. Sengor (1992), Diamonds from high-pressure metamorphic rocks in Eastern Dabie Mountain, Chin. Sci. Bull., 37(2), 140–145.
Yang, J.-S., P. T. Robinson, and Y. Dilek (2014), Diamonds in ophiolites, Elements, 10(2), 127–130.
SHEN ET AL.
Cr ISOTOPES OF SUBDUCTED CRUST
14
Geochemistry, Geophysics, Geosystems
10.1002/2015GC005944
Young, E. D., C. E. Manning, E. A. Schauble, A. Shahar, C. A. Macris, C. Lazar, and M. Jordan (2015), High-temperature equilibrium isotope
fractionation of non-traditional stable isotopes: Experiments, theory, and applications, Chem. Geol., 395, 176–195.
Zheng, Y. F., B. Fu, B. Gong, and L. Li (2003), Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie-Sulu orogen in China: Implications for geodynamics and fluid regime, Earth Sci. Rev., 62(1-2), 105–161.
Zheng, Y. F., J. B. Zhou, Y. B. Wu, and Z. Xie (2005), Low-grade metamorphic rocks in the Dabie-Sulu orogenic belt: A passive-margin accretionary wedge deformed during continent subduction, Int. Geol. Rev., 47(8), 851–871.
Zhou, M.-F., P. T. ROBINSON, J. Malpas, and Z. Li (1996), Podiform chromitites in the Luobusa ophiolite (Southern Tibet): Implications for meltrock interaction and chromite segregation in the upper mantle, J. Petrol., 37(1), 3–21.
Zhou, M.-F., P. Robinson, J. Malpas, J. Aitchison, M. Sun, W.-J. Bai, X.-F. Hu, and J.-S. Yang (2001), Melt/mantle interaction and melt evolution
in the Sartohay high-Al chromite deposits of the Dalabute ophiolite (NW China), J. Asian. Earth Sci., 19(4), 517–534.
Zhu, X. K., et al. (2002), Mass fractionation processes of transition metal isotopes, Earth Planet. Sci. Lett., 200(1–2), 47–62.
Zink, S., R. Schoenberg, and M. Staubwasser (2010), Isotopic fractionation and reaction kinetics between Cr (III) and Cr (VI) in aqueous
media, Geochim. Cosmochim. Acta, 74(20), 5729–5745.
SHEN ET AL.
Cr ISOTOPES OF SUBDUCTED CRUST
15