H ratios of nominally anhydrous minerals from ultrahigh-pressure eclogites of the Dabie orogen, eastern China

Geochimica et Cosmochimica Acta 71 (2007) 2079–2103 www.elsevier.com/locate/gca

H2O contents and D/H ratios of nominally anhydrous minerals from ultrahigh-pressure eclogites of the Dabie orogen, eastern China Ying-Ming Sheng a, Qun-Ke Xia a

a,*

, Luigi Dallai b, Xiao-Zhi Yang a, Yan-Tao Hao

a

CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China b CNR, Istituto di Geoscienze e Georisorse Via G. Moruzzi, 1, I-56124 Pisa, Italy Received 6 March 2006; accepted in revised form 22 January 2007; available online 30 January 2007

Abstract Garnet and omphacite from ultrahigh-pressure eclogites from the Dabie orogen, eastern China were investigated by MicroFTIR. The results show that all garnet and omphacite grains contain structural water occurring as hydroxyl (OH), with H2O contents varying from 14 to 1915 ppm (H2O wt) and from 105 to 695 ppm, respectively. Within the same sample, the water contents are either homogeneous at the grain scale or vary systematically from higher in the core to lower in the rim. Low water contents at crystal rims possibly result from hydroxyl exsolution after pressure decrease upon rock exhumation. The dD values of omphacites are between 108.4& and 114.2&, and independent of water contents. Heterogeneous water contents of garnet occur at the centimeter scale and fluid mobility during UHP metamorphism was very limited. The estimated whole-rock water content based on mineral H2O contents is between 260 and 750 ppm, thereby implying that eclogitic rocks formed during continental subduction have the potential to recycle (at least) several hundreds ppm water to mantle depths. The preserved chemical differences likely indicate that the eclogitic rocks resided at mantle conditions for a limited time span, and imply that they were exhumed shortly after subduction. The water released during decompression might represent the early-stage retrograde fluid.  2007 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Constituent minerals of UHP eclogites (i.e. garnet, clinopyroxene, rutile, etc.) contain trace to minor amounts of water occurring as OH defects in the structure (e.g., Rossman et al., 1989; Rossman and Smyth, 1990; Langer et al., 1993; Katayama and Nakashima, 2003; Katayama et al., 2006). The amount of water in these nominally anhydrous minerals (NAMs) is much lower than that retained in hydrous phases occurring in upper mantle domains. However, garnet and clinopyroxene are the main constituents of UHP eclogites and together probably represent the most

*

Corresponding author. Fax: +86 551 360 7386. E-mail address: [email protected] (Q.-K. Xia).

0016-7037/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2007.01.018

voluminous H2O reservoir at mantle depths. Although the amount of water incorporated into eclogitic rocks and brought down to the mantle is relatively small, it can play an important role in the geodynamics of the deep Earth: it can affect the physical-chemical properties of minerals (e.g. depress the solidus temperature of peridotite, and change the composition of the melt produced) and modify the overall extent of melting and magma production at mantle conditions (e.g., Gaetani et al., 1993; Inoue, 1994; Hirose, 1997; Gaetani and Grove, 1998; Asimow and Langmuir, 2003). Even at the ppm level, water can dramatically affect the transport properties of minerals, such as diffusion (e.g. Graham and Elphick, 1991; Wang et al., 2004), deformation (e.g. Mackwell et al., 1985; Karato et al., 1986; Mei and Kohlstedt, 2000), viscosity (e.g. Dixon et al., 2004), electrical conduction (e.g. Karato, 1990) and thermal

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Y.-M. Sheng et al. / Geochimica et Cosmochimica Acta 71 (2007) 2079–2103

conduction (Hofmeister, 2004). Moreover, water can influence the properties of seismic discontinuities (e.g. Wood, 1995) and anisotropy (e.g. Jung and Karato, 2001) within the Earth’s mantle. Recent seismological and electrical conductivity studies suggest that several hundred ppm of H2O may exist in the transition zone (Wood, 1995; Van der Meijde et al., 2003; Huang et al., 2005). The content and distribution of water in coexisting minerals from UHP rocks were recently investigated in garnet, rutile and omphacite minerals from Kokchetav eclogites (Katayama and Nakashima, 2003; Katayama et al., 2006) and in eclogitic garnet from the Dabie orogen, eastern China (Su et al., 2004; Xia et al., 2005). In both cases significant amounts of water were detected (up to 870 ppm H2O wt in omphacite and 1735 ppm H2O wt in garnet). In this paper, the content and distribution of water in garnet and omphacite from 24 UHP eclogites of the Dabie orogen were investigated by Fourier transform infrared

spectroscopy (FTIR) with the aim of constraining the nature of the fluids and evaluating their possible role during subduction and exhumation of continental plate. This data set is the most extensive so far on NAMs from UHP rocks. The hydrogen isotope composition of selected omphacite grains was also determined in order to compare dD values with those reported for hydrous silicates of the same region (e.g. Zheng et al., 2003). A number of studies have proven that the UHP rocks cropping out in this low-d18O type-locality experienced pre-subduction meteoric–hydrothermal alteration and the isotopic signature of meteoric water was preserved in high pressure mineral phases upon transport to mantle depths and subsequent exhumation. Primary oxygen and hydrogen isotope compositions of hydrothermal fluids have by and large not been retained by hydrosilicate minerals, apart for a few phengitic eclogites (n = 3; Zheng et al., 2003). In fact, the measured dD values of mica and epidote largely reflect retrograde isotopic equilibration

Fig. 1. Simplified geological map of eastern Dabie and localities of samples (modified after Jahn et al., 2003).

Water in UHP minerals

among OH-bearing phases. Isotopic re-equilibration occurred among these paragenetic phases irrespective of whether the rocks experience closed- and/or open-system behavior. With our investigation we attempt to characterize the dD of the pre-metamorphic fluids by using mineral phases that are more abundant in eclogitic rocks, and less susceptible to retrograde isotopic re-equilibration. If confirmed by further studies, this approach may be extended to other mafic rocks that experienced UHP metamorphic conditions. 2. GEOLOGICAL SETTING The Dabie metamorphic complex lies at the eastern end of the extensive Qinling–Dabie orogenic belt. It is truncated at its eastern end by the Tanlu fault, which offsets the UHP complex by more than 500 km northward to the Shandong peninsula (Sulu orogen). The Dabie-Sulu metamorphic complex is composed of a variety of gneisses and schists along with marbles, eclogites, amphibolites, migmatites, and minor ultramafic rocks. Metamorphic grades range from UHP eclogite facies to greenschist facies. Eclogites occur as 2 cm to 20 m in diameter blocks or boudins hosted in marbles and/or foliated quartz-schists and/or biotite gneisses. They rarely form cm-scale layers and lenses interbedded in the above-mentioned rocks. The formation of UHP rocks from the Dabie-Sulu orogen has been ascribed to the continental collision between the Sino-Korean and Yangtze cratons (e.g., Eide, 1995; Cong, 1996; Liou et al., 1996; Zheng et al., 2003). Geochronological studies have shown that the UHP metamorphism took place during the Triassic (Li et al., 1999, 2000; Xie et al., 2004) and affected the Neoproterozic meta-igneous protoliths (Zheng et al., 2003, 2004). The general geology of the Dabie orogen has been described in several recent publications (e.g., Wang et al., 1995; Cong, 1996; Liou et al., 1996; Zheng et al., 2005) and may be summarized as follows. The Dabie orogen is composed of five major tectonic units: (1) the Beihuaiyang low-grade metamorphic unit; (2) the North Dabie HT granulite facies unit, (3) the Central Dabie UHP eclogite facies unit, (4) the South Dabie HP eclogite facies unit, and (5) the Susong LT/HP blueschist facies unit (Fig. 1). The recovery of micro-diamond in eclogites at Huangwei and Baizhangya in the North Dabie unit (Xu et al., 2005), and the occurrence of coesite pseudomorph in eclogites at Huangzhen in the South Dabie unit (Li et al., 2004) constrain the minimum pressure estimates for peak metamorphism in these two units to 3.3 GPa. On the basis of mineral paragenesis and phase chemistry (e.g., Xiao et al., 2002), the main metamorphic stages inferred for the Dabie eclogites are: (1) Prograde eclogite-facies metamorphism (phengite + omphacite inclusions within garnet porphyroblasts), (2) Coesite eclogite-facies metamorphism (coesite + K-rich omphacite inclusions within garnet), (3) Recrystallized eclogite-facies metamorphism represented by coarse-grained garnet and omphacite, (4) High-amphibolite-facies retrograde metamorphism (pyroxene symplectites after garnet, formation of amphibole and plagioclase), and (5) Late retrograde epidote–

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amphibole–facies stage (amphibole kelyphites around garnet and/or symplectic pyroxene, biotite after phengite, and coarse-grained epidote + plagioclase crystallization). 3. SAMPLES This study includes 22 eclogites from Bixiling (eight samples, labeled as BXL), Shuanghe (six samples, labeled as SH), Wumiao (four samples, labeled as WM), Maowu (MW1) and Xindian (XD1, 05XD1 and 05XD2) in the Central Dabie unit, and two eclogites from Baizhangya (BZY1) and Huangwei (HW1) in the North Dabie unit. All the eclogites from Bixiling and Shuanghe are ‘‘fresh’’ without strong retrograde alteration. The major constituents of these samples are garnet and omphacite, with <5% to 15% quartz. Some samples contain 1% to 2% accessory rutile, apatite and zircon. Garnet and omphacite are coarsegrained (>0.7 mm) and equigranular in texture. Amphibole + plagioclase symplectites are in places observed around the margin of omphacite and garnet. No coarsegrained hydrous minerals (amphibole, biotite, epidote, etc.) were found in any sample, suggesting that the retrograde epidote–amphibole-facies metamorphism did not affect these eclogites. Samples WM2 from Wumiao and 05XD1 and 05 XD2 from Xindian are fresh and similar to the eclogites from Bixiling and Shuanghe, except their large garnet grains (up to 2–3 mm in size). Other samples from Wumiao (WM1, WM5 and WM6), Xindian (XD1) and Maowu (MW1) are strongly retrogressed: omphacites are replaced by amphiboles and preserved only as inclusions in garnet and/or quartz. Samples from Baizhangya (BZY1) and Huangwei (HW1) in the North Dabie unit are strongly retrogressed to garnet amphibolites and most garnets are fresh with fine grain size (<0.3 mm). 4. ANALYTICAL METHODS 4.1. FTIR analysis Doubly polished thin sections 2 · 1 cm in size and 0.15 to 0.35 mm thick were prepared from each sample. The cleaning procedure included 24 h immersion of sections in acetone, followed by repeated cleaning with ethanol and distilled water. To remove the surface absorbed water, sections were heated in an oven at 150 lm for >6 h. Infrared spectra were obtained by a Nicolet 5700 FTIR spectrometer equipped with a Continulm IR microscopy at the University of Science and Technology of China in Hefei, and a Brucker Equinox 55 FTIR spectrometer equipped with a Hyperion 2000 super-microscopy at Tongji University in Shanghai. Measurements were carried out by unpolarized radiation from an IR light source, a KBr beam-splitter and a MCT-A liquid N2cooled detector at room temperature. 128 or 256 scans at a resolution of 2 or 4 cm1 were averaged for each analysis. The background was recorded before analyzing each section. IR spectra were measured by 50 · 50 lm aperture on ‘‘clean’’ areas without visible inclusions and cracks; however, as shown by FTIR spectra (see 5.1.1),

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Table 1 Chemical compositions of garnets in UHP eclogites of eastern Dabie Sample Points

37.55 21.59 0.12 0.03 26.89 0.00 0.03 4.92 8.32 0.06 0.00 0.02 99.53

Cations per 12 oxygen atoms Si 2.959 Al 2.006 Ti 0.002 Cr 0.002 Fe 1.772 Ni 0.000 Mn 0.002 Mg 0.577 Ca 0.702 Na 0.009 K 0.000 P 0.014 End members Prp Alm + Sps Grs + And + Uva T (C)

19 57 23 634

SH5 8 37.38 21.52 0.07 0.02 24.20 0.01 0.02 5.98 10.32 0.03 0.00 0.02 99.58 2.927 1.986 0.001 0.001 1.585 0.000 0.001 0.699 0.866 0.005 0.000 0.009 23 47 29 702

SH6 3 37.25 22.80 0.03 0.04 23.11 0.00 0.01 6.92 9.35 0.01 0.00 0.01 99.53 2.891 2.086 0.000 0.002 1.500 0.000 0.001 1.801 0.777 0.001 0.000 0.007 26 50 25 799

SH9 3 38.32 22.20 0.00 0.05 22.76 0.04 0.05 9.06 6.97 0.02 0.01 0.04 99.51 2.943 2.010 0.000 0.003 1.462 0.002 0.003 1.038 0.574 0.003 0.001 0.024 34 47 19 823

SH14 8 38.04 21.81 0.09 0.01 23.25 0.00 0.01 7.94 8.35 0.02 0.00 0.02 99.56 2.944 1.991 0.002 0.001 1.505 0.000 0.001 0.917 0.693 0.003 0.000 0.008 31 46 23 684

01BXL 8 36.93 21.48 0.06 0.26 26.82 0.00 0.04 5.33 8.41 0.04 0.01 0.02 99.41 2.920 2.002 0.004 0.016 1.774 0.000 0.003 0.629 0.712 0.006 0.001 0.001 21 55 24 627

01BXL01 3 38.35 22.82 0.08 0.04 17.84 0.05 0.02 7.77 12.54 0.04 0.01 0.01 99.57 2.926 2.052 0.005 0.002 1.138 0.003 0.001 0.883 1.025 0.007 0.001 0.001 29 38 33 735

01BXL04 3 37.57 21.53 0.02 0.04 25.86 0.03 0.07 5.48 8.84 0.02 0.00 0.01 99.47 2.953 1.995 0.001 0.002 1.700 0.002 0.005 0.642 0.745 0.003 0.000 0.001 21 54 25 608

BXL01 8

BXL04 3

BXL08 3

BXL15 13

36.94 21.35 0.07 0.24 26.78 0.01 0.07 5.33 8.35 0.04 0.02 0.01 99.18

40.46 23.35 0.05 0.06 13.89 0.03 0.06 17.75 3.96 0.02 0.01 0.01 99.65

38.49 22.68 0.16 0.04 15.74 0.02 0.02 8.99 13.32 0.04 0.01 0.04 99.55

40.48 23.45 0.10 0.06 12.10 0.00 0.02 15.82 7.85 0.03 0.01 0.02 99.91

2.921 1.991 0.001 0.015 1.772 0.001 0.004 0.629 0.707 0.006 0.002 0.000 21 55 24 480

2.946 2.004 0.003 0.004 0.846 0.002 0.003 1.928 0.309 0.003 0.001 0.000 64 26 10 757

2.916 2.025 0.009 0.003 0.997 0.001 0.001 1.015 1.082 0.006 0.001 0.003 33 31 36 830

3.090 2.110 0.002 0.004 0.773 0.000 0.001 1.800 0.642 0.004 0.001 0.011 57 23 20 730

Notes. Chemical compositions are homogeneous within single grains and among the different grains in individual samples, so average values of all analytical points from same sample are shown. T calculated temperature using garnet–clinopyroxene geothermometer of Powell (1985) at assumed pressure of 30 kbar.

Y.-M. Sheng et al. / Geochimica et Cosmochimica Acta 71 (2007) 2079–2103

SiO2 Al2O3 TiO2 Cr2O3 FeO NiO MnO MgO CaO Na2O K2O P2O5 Total

SH4 3

Water in UHP minerals

many garnets contain submicroscopic fluid inclusions (<1–2 lm). Water content (expressed as H2O wt in this paper) of garnet and omphacite were calculated using the Beer–Lambert law A = e * c * t * c. Absorbance A is expressed as the integrated absorbance of structural OH; the integrated molar absorption coefficient, e, from Bell et al. (1995) is 1.39/ (ppm H2O cm2) for garnet and 7.09/(ppm H2O cm2) for clinopyroxene. Because the precision of thickness ‘‘t’’ for the different domains of the same section is <10% (n of measured points covering the whole section for all samples P35), the average value was used for different spots from the same sample. The orientation factor c is 1 for garnet and 1/3 for omphacite (Paterson, 1982). Three band groups in the 3100–3800 cm1 range (and sometimes the weak 3710 cm1 band) were fitted to the Gaussian shape after baseline correction, and the corresponding peak areas were then calculated. The group III band in garnet (interpreted in terms of submicroscopic fluid inclusions, see 5.1.1) and the 3710 cm1 band in garnet and omphacite (likely resulting from the subtraction of the background spectrum slightly contaminated by the presence of water-vapor) were

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not included in the total integrated absorbance used to calculate structural water contents (Tables 4 and 5). The contribution of these two bands, together with (1) baseline correction, (2) possible differences in crystal thickness within the same section and (3) the use of an absorption coefficient calibrated on pyrope and augite (Bell et al., 1995) for the Dabie pyrope-almandine garnets and omphacites, may therefore affect the accuracy of water content estimation. In particular, the influence of the absorption coefficient may be negligible for garnet (<10% among even pure endmembers) (Rossman and Aines, 1991; Bell et al., 1995) but significant for omphacite, due to the different absorption bands of augite (Bell et al., 1995). In addition, although unpolarized analysis of the optically isotropic garnet can yield accurate water content data, similar analysis of optically anisotropic omphacite could lead to a non-systematic biasing of results (Libowitzky and Rossman, 1996). We use the average of more than 10 grains from the same thin section (with at least 3 spots per grain) to represent ‘‘true’’ water content of omphacite in that sample. The overall uncertainty is estimated to be <30% (<10% baseline correction, <5% thickness variation and <10% absorption

Table 2 Chemical compositions of omphacites in UHP eclogites of eastern Dabie, China Sample points

SH4 3

SH5 8

SH6 3

SH9 3

SH14 8

01BXL 8

01BXL1 4

01BXL04 3

BXL01 8

BXL04 5

BXL08 3

BXL15 13

SiO2 Al2O3 TiO2 Cr2O3 FeO NiO MnO MgO CaO Na2O K2O P2O5

54.29 8.92 0.07 0.00 7.73 0.03 0.04 8.26 12.83 7.08 0.01 0.02

54.09 5.73 0.06 0.10 8.15 0.01 0.02 10.20 15.90 5.23 0.00 0.03

54.84 10.53 0.08 0.01 6.46 0.00 0.02 7.71 12.11 7.49 0.01 0.00

55.41 9.13 0.02 0.03 7.18 0.02 0.00 8.23 13.03 6.87 0.01 0.01

54.98 9.41 0.04 0.05 5.68 0.01 0.02 9.01 13.21 6.93 0.01 0.03

55.40 8.52 0.06 0.04 7.12 0.02 0.00 8.30 13.06 6.75 0.02 0.01

56.21 11.52 0.06 0.08 1.97 0.04 0.02 9.12 13.48 6.99 0.01 0.02

55.79 12.41 0.02 0.04 5.34 0.02 0.03 6.51 10.24 8.88 0.00 0.02

56.24 11.49 0.06 0.19 1.72 0.03 0.05 9.28 13.69 6.86 0.02 0.01

54.78 2.95 0.07 0.09 2.85 0.01 0.03 17.45 19.24 1.74 0.11 0.00

56.45 11.75 0.00 0.09 2.05 0.00 0.04 9.07 12.80 7.10 0.02 0.04

55.53 9.24 0.07 0.18 1.67 0.03 0.05 11.08 15.62 5.82 0.01 0.02

Total

99.29

99.53

99.25

99.94

99.36

99.30

99.52

99.32

99.62

99.33

99.40

99.33

Cations per six oxygen atoms Si 1.987 1.994 Al 0.385 0.249 Ti 0.001 0.001 Cr 0.000 0.003 Fe 0.237 0.251 Ni 0.001 0.000 Mn 0.001 0.001 Mg 0.451 0.561 Ca 0.503 0.628 Na 0.503 0.374 K 0.000 0.000 P 0.001 0.001 VM2 0.006 0.003 End members Jd 51 Ae 16 Wef 33

36 18 46

1.988 0.450 0.001 0.000 0.196 0.000 0.000 0.416 0.470 0.527 0.000 0.000 0.003 59 10 31

2.003 0.389 0.000 0.001 0.217 0.001 0.000 0.443 0.505 0.481 0.000 0.001 0.013 53 13 34

1.989 0.401 0.000 0.001 0.172 0.000 0.001 0.486 0.512 0.486 0.000 0.001 0.002 54 11 35

2.015 0.365 0.002 0.001 0.216 0.001 0.000 0.450 0.509 0.476 0.001 0.000 0.014 50 15 35

1.989 0.481 0.002 0.002 0.058 0.001 0.001 0.481 0.511 0.480 0.000 0.001 0.009 65 0 35

1.999 0.524 0.001 0.001 0.160 0.000 0.001 0.348 0.393 0.617 0.000 0.001 0.011 64 11 24

1.987 0.478 0.002 0.005 0.051 0.001 0.001 0.489 0.518 0.470 0.001 0.000 0.011 64 0 36

1.982 0.126 0.002 0.003 0.086 0.000 0.001 0.941 0.746 0.122 0.005 0.000 0.127 25 0 75

1.996 0.489 0.000 0.002 0.060 0.000 0.001 0.478 0.485 0.487 0.001 0.001 0.028 67 0 33

1.982 0.388 0.002 0.005 0.050 0.001 0.001 0.589 0.597 0.403 0.001 0.001 0.001 55 2 43

Notes. VM2: vacancy in M2 site (=1-Ca–Na–K). Chemical compositions are homogeneous within single grains and among the different grains in individual samples, so average values of all analytical points from same sample are shown.

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Sample

Weight (mg)

Session 1 bxl03 bxl07 bxl07 bxl10 bxl10

249.0 251.4 252.4 251.2 250.1

Session 2 bxl01 bxl01 01BXL-1 01BXL-1 bxl03 Session 3 bxl15 bxl15 bxl06

Pbellow (mbar)

mVcup8

H2O equiv. (mg)

H2O (lmol)

H2Owt. (ppm)

ppm FTIR

7.7 5.5 5.9 8.2 7.8

1059 756 811 1128 1073

0.2655 0.1897 0.2034 0.2828 0.2690

15 11 11 16 15

1066 754 806 1126 1075

115

500.9 500.1 499.7 465.1 500.2

13.8 15.7 9.3 8.2 11

1897 2159 1128 1279 1513

0.4759 0.5414 0.3207 0.2828 0.3793

26 30 18 16 21

950 1082 642 608 758

249.7 251.6 250.8

3.6 3.6 5

495 495 688

0.1241 0.1241 0.1724

7 7 10

497 493 688

ppm IRMS

% blank

dDmeas

dDcorr_FTIR

dDcorr_Pbellow

780

0.87

145.75

111.0

133.8

180 380

1016 625

0.86 0.62

123 128

108.4 114.2

117.9 121.3

235

495

0.68

127

112.7

120.6

Notes. Pbellow is the hydrogen gas pressure with the MS sample bellow fully open; mVcup 8 is the current voltage of mass 2; corresponding to the gas pressure in the sample bellow; H2O equiv. is the amount of water recalculated on the basis of the voltage obtained for a known aliquot of NBS30_bt std.; ppm FTIR is the average water content of omphacite determined by FTIR; ppmIRMS is the average water content of omphacite calculated on mV basis by mass spectrometry. dDmeas is measured dD values. The values in ppm FTIR, ppm IRMS, %blank, dDmeas, dD_corr_FTIR, dD_corr_Pbellow are the average of the two measurements. % blank = (ppmIRMS  ppm FTIR)/ppmIRMS; dD_corr._FTIR = (106 * %blank) + (1  %blank) * dDmeas; dD_corr._ Pbellow = (106 * 0.3) + (1  0.7) * d Dmeas.

Y.-M. Sheng et al. / Geochimica et Cosmochimica Acta 71 (2007) 2079–2103

Table 3 Hydrogen isotope analysis of omphacites in UHP eclogites of eastern Dabie, China

Water in UHP minerals

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Fig. 2. Representative IR spectra after baseline correction of garnets in UHP eclogites of eastern Dabie, China. (a) Three spots from No. 2 grain in SH9 eclogite (SH9-2) showing heterogeneous water content between rim (R) and core (C); (b) two spots from No. 8 grain in SH6 eclogite (SH6-8) showing homogeneous water content.

coefficient variation) for garnet and probably up to 50% for omphacite including additional errors from absorption coefficient and unpolarized analysis. However, when comparing the different values within single grains, the uncertainty is only from baseline correction and thickness variation and estimated to <10%. All analyzed garnet spots were considered in the discussion because unpolarized analysis of the optically isotropic garnet can yield accurate water content. In contrast, only the average of multiple grains from the same thin section (with at least 3 spots per grain) was used to represent ‘‘true’’ water content of omphacite in that sample because of unpolarized analysis. Some omphacite grains exhibit core-rim variations, with higher water content at the core and lower content at the rim (see Section 5.2). Lower water contents at the rims were ascribed to hydroxyl exsolution upon the initial exhumation, and only core data in these cases were averaged. For grains with homogeneous water contents, the average of all analyzed spots was used to calculate an average water content of the mineral in that sample. Water content of rutile inclusions in garnets were not calculated because the thicknesses were difficult to evaluate. 4.2. EMP analysis Electron microprobe (EMP) analysis was conducted on five eclogite samples from Shuanghe and seven samples from Bixiling. For two samples from Shuanghe (SH5 and SH9), in which large water content variations were detected in both garnet and omphacite, EMP and FTIR analyses were conducted on the same spots to check for the possible chemical effects on water incorporation. EMP analyses were performed in the State Key Laboratory of Mineral Deposits at Nanjing University using a JEOL JXA 8800 electron probe with 15 kV accelerating potential and 15 nA Faraday cup current. The average values of all

analytical points from individual sample are shown in Tables 1 and 2. 4.3. D/H analysis Optically pure omphacite samples (BXL series) were chosen for D/H measurements; garnets were avoided due to the presence of tiny fluid inclusions. The omphacite grains were separated under the microscope and treated according to the procedure described in Bell et al. (1995). As described in detail by Bell and Ihinger (2000), the significance of dD data obtained for NAMs is controversial because of the uncertainties related to the procedure of water extraction. NAMs contain very low amounts of water and the possibility that the H-isotope composition of structural hydrogen may be significantly affected by instrumental blanks is real. In the present work, the following issues were addressed: (1) contribution of ambient water adsorbed on omphacite grain surface; (2) quantitative H2O extraction from omphacite aliquots; and (3) residual water in the vacuum line (blank). (1) Omphacite grains were degassed overnight at 150 C under dynamic (diffusion) vacuum in order to remove adsorbed water. After initial sample degassing, pressure in the vacuum line decreased and was less than 103 mbar for 12 h (monitored by two pirani gauges). The vials used for water collection were also degassed overnight at T = 150 C. The extraction line was wrapped into heating tape and constantly kept at 120 C. Between one sample extraction and the next, the initial vacuum conditions were restored. Water was extracted from omphacite by vacuum fusion of 250 and 500 mg sample aliquots, following the procedure of Venneman and O’Neil (1993). After extraction and water conversion to molecular hydrogen

Locality

Shuanghe

Sample

SHI

Thickness (cm) 0.033

Grain

1 2 3

0.041

1 2 3 4 5 6

SH5

0.026

1

2 3

4

5 6 7 8 9 10 11 12 13 14 15

16 17

1 2 1 1 1 1 1 1 1 2 1 1 2 3 4 5 1 1 2 3 1 2 3 1 1 1 1 1 2 1 1 1 1 1 1 2 3 1 1 2

Position

C R

C R C C C R R C R R C C R

R R

C R R R R

Group I (3620–3640 cm1)

Group II (3560–3580 cm1)

Group III (3400–3420 cm1)

Intensity

FWHH

Area

Intensity

Area

Intensity

FWHH

0.158 0.240 0.076 0.050

98 109 107 58

16.5 27.7 8.6 3.1

0.202 0.169 0.084 0.147

62 66 74 79

13.4 11.8 6.7 12.3

0.330 0.214 0.122 0.207

270 214 247 310

95.0 48.8 32.0 68.2

651 861 333 336

545

0.059 0.041 0.033 0.066 0.076

66 55 84 58 73

4.2 2.4 3.0 4.1 5.9

0.243 0.139 0.088 0.161 0.218 0.079 0.314

66 79 74 91 82 105 107

17.1 11.7 7.0 15.7 19.1 8.8 35.7

0.143 0.115 0.057 0.090 0.224 0.070 0.374

200 194 212 142 270 285 296

30.4 23.6 12.8 13.6 64.5 21.1 117.8

374 248 174 346 440 155 626

338

0.110 0.126 0.065 0.044 0.059 0.011 0.056 0.050 0.039 0.093 0.112 0.035 0.074 0.100 0.047 0.134 0.011 0.009 0.029 0.061 0.018 0.055 0.301 0.105 0.109 0.111 0.329 0.008 0.015

79 79 148 116 134 149 72 91 90 96 86 129 108 71 142 96 101 84 99 86 116 95 112 88 84 86 120 136 154

9.3 10.5 10.3 5.5 8.4 1.8 4.3 4.9 3.7 9.5 10.2 4.9 8.6 7.5 7.1 13.7 1.2 0.8 3.0 5.5 2.2 5.6 35.8 9.8 9.7 10.2 41.9 1.1 2.5

0.094 0.125 0.035 0.045 0.040

198 304 87 129 188

19.8 40.5 3.2 6.1 8.0

0.067 0.028 0.022 0.049 0.091 0.015 0.056 0.109 0.082 0.127

228 157 150 184 166 118 155 295 249 157

16.4 4.7 3.5 9.5 16.0 1.8 9.3 34.1 21.7 21.3

0.013 0.050 0.011 0.072 0.268 0.066 0.088 0.109 0.345 0.003 0.004

116 185 120 176 159 171 168 177 134 96 82

1.6 9.8 1.5 13.6 45.4 12.0 15.8 20.6 49.2 0.3 0.3

339 292 286 164 234 86 265 164 125 308 365 150 264 209 195 431 36 33 103 186 71 282 1145 299 321 341 1238 46 81

278

0.034

80

2.9

0.009

49

0.5

0.021 0.053 0.021 0.018 0.029 0.041 0.013 0.020

56 93 47 42 51 68 40 45

1.3 5.2 1.0 0.8 1.6 3.0 0.6 1.0

0.036 0.002 0.007 0.008 0.023 0.004 0.030 0.061 0.018 0.027 0.027 0.048 0.011 0.010

49 34 53 76 49 85 145 85 53 66 72 57 48 41

1.9 0.11 0.4 0.7 1.2 0.4 4.6 5.5 1.0 1.9 2.1 2.9 0.5 0.4

FWHH

Area

‘‘Water’’ bound in structural OH (H2O ppm wt)

Average water content (H2O ppm wt)

Y.-M. Sheng et al. / Geochimica et Cosmochimica Acta 71 (2007) 2079–2103

SH4

Spot

2086

Table 4 FTIR analysis of garnets in UHP eclogites of eastern Dabie, China

Locality

Sample

SH6

Thickness (cm) 0.040

Grain

1 2 3 4 5 6 7 8 9

10

0.027

1 2

3 4

5 6 7 8 9 10 11 12 13 14

1 2 1 1 1 1 1 1 2 1 2 1 2 3 4 5 1 2 3 4 5 1 1 2 3 1 1 2 3 4 5 1 1 2 1 1 1 1 1 1 2 1 1 2 3

Position

C R

C R C R C C C R R R R C C C C R R C C C R R C R

C C C R R

Group I (3620–3640 cm1)

Group II (3560–3580 cm1)

Group III (3400–3420 cm1)

Intensity

FWHH

Intensity

FWHH

Area

Intensity

FWHH

0.043 0.056 0.078 0.066 0.055 0.072 0.067 0.040 0.062 0.068 0.087 0.031 0.071 0.072 0.051 0.056 0.055 0.053 0.073 0.077 0.062

119 114 99 66 71 105 73 61 72 73 97 71 103 82 53 88 65 75 67 71 92

5.5 6.8 8.3 4.6 4.2 8.0 5.2 2.6 4.8 5.3 8.9 2.3 7.8 6.3 2.9 5.3 3.8 4.2 5.1 5.8 6.1

0.031 0.047 0.131 0.198 0.062 0.071 0.148 0.121 0.155 0.197 0.153 0.195 0.128 0.140 0.093 0.122 0.117 0.113 0.156 0.165 0.146

79 75 79 83 80 67 70 83 81 83 77 135 77 76 84 90 80 79 77 74 78

2.6 3.8 11.1 17.5 5.3 5.0 11.1 10.6 13.3 17.3 12.5 28.0 10.5 11.4 8.3 11.7 10.0 9.5 12.8 13.0 12.2

0.032 0.0561 0.145 0.186 0.078 0.082 0.156 0.109 0.150 0.183 0.171 0.173 0.147 0.148 0.084 0.104 0.118 0.112 0.152 0.161 0.139

158 219 187 205 269 195 206 177 186 173 199 132 236 231 202 149 216 210 223 228 192

5.3 14.2 28.9 40.6 22.3 17.1 34.3 20.6 29.7 33.8 36.2 24.3 37.1 36.5 18.0 16.5 27.2 25.0 36.0 39.0 28.4

145 190 348 398 170 234 294 238 325 407 386 546 329 319 201 306 249 246 323 338 328

0.138

81

11.9

0.092

84

8.3

0.134 0.133 0.088 0.069 0.102 0.181 0.154 0.119 0.127 0.065 0.211 0.128 0.056 0.072 0.082 0.090 0.228 0.140 0.112

107 104 64 62 55 57 66 61 63 59 66 63 53 56 55 61 103 74 66

15.2 14.8 5.9 4.5 6.0 11.0 10.8 7.7 8.5 4.1 14.9 8.6 3.2 4.3 4.7 5.8 25.0 11.1 7.8

0.279 0.343 0.217 0.217 0.351 0.219 0.228 0.320 0.179 0.300 0.602 0.400 0.339 0.115 0.184 0.636 0.118 0.196 0.225 0.321 0.289 0.411 0.246 0.273

73 119 74 119 118 81 82 94 73 70 77 79 80 73 89 81 73 83 71 71 81 75 82 82

21.6 43.4 17.0 27.6 44.2 18.8 19.9 32.0 14.0 22.5 49.1 33.9 29.0 8.9 17.5 55.0 9.1 17.3 17.0 24.1 25.0 33.0 21.5 23.7

0.419 0.462 0.266 0.242 0.419 0.390 0.390 0.410 0.179 0.324 0.555 0.399 0.335 0.139 0.212 0.692 0.141 0.182 0.353 0.315 0.275 0.563 0.266 0.272

264 243 245 222 214 234 226 171 73 228 200 200 208 236 226 211 235 210 284 227 188 221 206 193

117.9 119.6 69.3 57.2 95.6 97.3 93.8 74.6 14.0 78.5 118.3 85.1 74.2 35.0 51.0 155.7 35.1 40.7 106.7 76.2 55.0 132.6 58.3 55.9

892 1155 675 736 1177 905 924 1010 493 759 1603 1190 977 466 575 1863 472 545 568 770 821 1544 867 841

Area

Area

‘‘Water’’ bound in structural OH (H2O ppm wt)

Average water content (H2O ppm wt)

301

Water in UHP minerals

SH9

Spot

909 2087

(continued on next page)

2088

Table 4 (continued) Locality

Sample

SH14 WM1

WM6

Maowu

Xindian

0.024 0.031

0.023

Group I (3620–3640 cm1)

Group II (3560–3580 cm1)

Group III (3400–3420 cm1)

Intensity

FWHH

Intensity

Intensity

FWHH

0.022 0.020

72 108

1.7 2.4

0.046 0.039

87 75

4.2 3.1

0.070 0.028

273 288

20.4 8.6

179 164

172

1 2 3 4 5 6

0.043 0.011 0.017 0.033 0.013 0.015

53 30 96 39 34 42

2.4 0.4 0.9 1.4 0.5 0.7

0.169 0.042 0.005 0.147 0.033 0.051

105 110 197 110 127 119

18.9 4.9 1.0 17.1 4.5 6.4

0.139 0.021

207 159

30.5 3.6

0.110 0.030 0.033

207 202 148

24.1 6.5 5.2

496 123 45 430 115 164

229

1 2 3 4 5

0.140 0.018 0.005 0.009 0.024

67 72 32 33 62

10.0 0.7 0.2 0.3 1.6

0.434 0.007 0.045 0.015 0.086

95 81 151 114 104

43.7 0.6 7.3 1.8 9.5

0.455

245

118.7

0.036 0.003 0.055

207 76 184

7.9 0.3 10.7

1678 39 233 66 348

473

Grain

1 2

Spot

1 1

Position

Area

FWHH

Area

Area

‘‘Water’’ bound in structural OH (H2O ppm wt)

Average water content (H2O ppm wt)

WM2

0.033

1 2 3 4 5 6

0.074 0.104 0.203 0.248 0.225 0.187

58 57 60 50 72 58

4.6 6.3 13.0 13.3 9.1 11.6

0.094 0.249 0.347 0.451 0.207 0.162

102 97 98 105 104 98

10.3 25.6 36.01 50.5 22.9 16.9

0.015 0.143 0.185 0.274 0.082 0.164

201 216 182 201 201 224

3.1 32.9 36.0 58.6 17.5 11.2

324 695 1071 1391 546 621

775

WM5

0.032

1 2 3 4 5 6 7

0.044 0.014 0.036 0.012 0.032 0.044 0.212

54 30 54 30 60 39 96

2.5 0.5 2.1 0.4 2.1 1.8 21.7

0.155 0.027 0.164 0.044 0.141 0.281 0.317

101 103 104 111 107 120 93

16.7 2.9 18.1 5.1 16.1 36.0 31.3

0.140 0.010 0.114 0.020 0.106 0.203 0.365

214 189 208 170 194 193 233

31.9 2.1 25.3 3.6 21.9 41.8 90.6

433 76 453 124 407 852 1191

505

1 2 3 4 5 6 7 8 9

0.057 0.140 0.115 0.021 0.105 0.042 0.115 0.161 0.101

73 113 70 82 99 61 74 75 72

4.4 16.9 8.5 1.8 11.1 2.7 9.0 12.8 7.7

0.084

101

9.0

0.180 0.017

104 81

20.0 1.4

0.067 0.226 0.222 0.158

102 94 97 93

7.3 22.6 22.8 15.6

0.094 0.128 0.216 0.024 0.075 0.091 0.211 0.245 0.184

242 252 251 271 268 300 213 240 255

24.2 34.3 57.9 7.0 21.4 29.0 47.9 62.7 50.0

321 406 685 78 265 239 757 854 559

463

1

0.090

70

6.7

0.086

65

6.0

0.113

291

35.1

882

MW1

XD1

0.030

0.039

Y.-M. Sheng et al. / Geochimica et Cosmochimica Acta 71 (2007) 2079–2103

Wumiao

Thickness (cm)

Locality

Sample

Thickness (cm)

Grain

2 3 4 5 6 7 8 9 10 11 12 05XD1

Huangwei

HW1

0.016

0.041

Position

Group I (3620–3640 cm1)

Group II (3560–3580 cm1)

Group III (3400–3420 cm1)

Intensity

Intensity

FWHH

Area

Intensity

FWHH

0.179 0.117 0.290 0.200 0.178 0.061 0.151 0.067 0.071 0.118 0.139

105 150 124 112 131 60 144 55 82 115 138

20.0 18.7 38.3 23.8 24.7 3.9 23.1 3.9 6.2 14.5 20.3

0.094 0.084 0.270 0.162 0.205 0.124 0.123 0.158 0.031 0.120 0.087

286 169 210 244 205 224 182 301 360 226 158

0.014

208

3.03

0.010

201

2.07

0.081 0.049 0.100 0.179 0.016

260 220 331 181 250

22.49 11.49 35.08 34.50 4.35

0.110 0.010 0.004

314 167 107

36.85 1.78 0.43

FWHH

Area

29 106

0.8 14.1

0.149

73

11.5

0.029 0.028

37 33

1.2 1.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

0.033 0.022 0.018 0.020 0.020 0.020 0.023 0.020 0.082 0.068

103 123 142 161 110 124 134 134 101 121

3.59 2.92 2.70 3.47 2.34 2.60 3.25 2.90 8.88 8.79

0.034 0.015 0.019 0.054 0.026 0.021 0.019

93 125 129 68 129 136 84

3.34 2.00 2.68 3.91 3.63 3.10 1.73

1 2 3 4 5 6 7 8

0.032 0.002 0.050 0.050 0.055 0.062 0.029 0.037

78 41 79 88 86 107 107 133

2.68 0.08 4.23 4.74 5.03 7.05 3.34 5.17

1 2 3 4 5 6 7

0.010 0.126 0.012 0.015 0.008 0.090 0.228

149 107 154 208 75 112 91

0.8 14.5 2.0 1.8 0.6 10.8 22.2

0.003 0.004

88 110

114 135

8.87 34.49

0.003 0.002 0.067 0.005

102 62 74 68

0.27 0.14 5.26 0.34

0.004

105

0.42

0.070 0.007 0.086 0.060 0.085 0.094 0.055 0.041

90 164 85 66 84 72 81 72

6.69 1.30 7.76 4.21 7.60 7.14 4.72 3.17

0.005

54

0.3

59

28.5 15.1 60.5 42.1 44.6 29.6 23.9 50.6 12.1 28.9 14.7

896 624 1823 1214 1294 879 868 1218 337 820 663

960

148 120 111 143 96 118 154 119 365 361 365 1418 137 94 116 377 163 127 89

243

424 62 542 405 571 642 365 378

424

0.28 0.49

0.073 0.239

0.002

Average water content (H2O ppm wt)

0.063 0.003 0.089 0.112 0.090 0.131 0.049 0.053

185 116 213 230 213 253 179 194

12.36 0.33 20.20 27.40 20.47 35.45 9.23 11.01

0.133

304

43.1

0.103 0.187

321 327

35.3 65.2

0.1

19 254 36 31 14 190 389 (continued on next page)

2089

0.027 0.126

Area

‘‘Water’’ bound in structural OH (H2O ppm wt)

Water in UHP minerals

05XD2

0.018

Spot

Locality

BZY1

Thickness (cm)

0.031

Group I (3620–3640 cm1)

Group II (3560–3580 cm1)

Group III (3400–3420 cm1)

Intensity

FWHH

Area

Intensity

Intensity

FWHH

Area

‘‘Water’’ bound in structural OH (H2O ppm wt)

Average water content (H2O ppm wt)

8 9 10 11 12 13 14 15 16 17 18

0.202 0.096 0.153 0.149 0.289 0.091 0.106

93 95 104 115 109 108 107

19.9 9.7 16.8 18.2 33.6 10.5 12.1

0.025 0.041 0.038

100 89 100

2.7 3.8 4.0

0.165 0.082 0.123 0.180 0.301 0.117 0.095 0.021 0.025 0.037 0.021

334 337 373 383 358 334 309 250 250 232 230

58.5 29.3 48.9 73.4 114.8 41.7 31.2 5.5 6.5 9.2 5.1

349 170 295 319 589 184 212 31 47 67 71

182

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

0.089 0.043 0.083 0.057 0.043

70 64 58 60 60

6.7 2.9 5.2 3.6 2.7

0.016 0.018 0.024 0.063

38 52 58 112

0.6 1.0 1.5 7.5

0.108 0.062 0.113 0.126 0.113 0.119 0.085 0.071 0.055 0.050 0.123 0.178 0.165 0.207 0.177 0.118 0.111 0.091 0.047 0.069 0.110 0.012 0.019 0.053 0.161 0.147 0.047 0.212

211 207 220 212 218 196 191 196 198 239 222 222 210 183 179 222 208 207 185 228 207 131 210 194 204 206 186 218

24.2 13.6 26.5 28.4 26.1 24.9 17.3 14.8 11.7 12.7 29.1 42.0 36.8 40.3 33.7 27.8 24.6 20.1 9.2 16.8 24.3 1.6 4.3 10.9 35.0 32.3 9.3 49.2

423 232 380 456 444 537 344 269 230 174 388 581 565 834 682 380 428 390 240 217 554 111 85 248 640 566 276 831

Grain

Spot

Position

0.015

0.016 0.019

32 36

0.5 0.7

0.013

32

0.4

0.010

40

0.4

0.108 0.024 0.026 0.026 0.064 0.052 0.024 0.161

64 60 76 60 66 57 62 74

7.3 1.5 2.1 1.6 4.5 3.2 1.6 12.8

FWHH

110

Area

1.8

0.094 0.058 0.085 0.129 0.130 0.159 0.109 0.092 0.080

116 115 124 116 119 137 123 108 99

11.5 7.1 11.2 16.0 16.4 23.2 14.2 10.6 8.5

0.127 0.186 0.186 0.259 0.212 0.123 0.139 0.120 0.078 0.080 0.119 0.020 0.017 0.072 0.183 0.164 0.070 0.187

124 127 123 129 127 125 122 132 119 110 130 153 87 119 118 122 138 116

16.7 25.0 24.3 35.4 28.6 16.4 18.0 16.8 9.9 9.3 16.5 3.3 1.5 9.0 23.1 21.2 10.3 23.0

Y.-M. Sheng et al. / Geochimica et Cosmochimica Acta 71 (2007) 2079–2103

Baizhangyan

Sample

2090

Table 4 (continued)

12.1 5.7 85 52 0.134 0.104

29 30 31 32 33 34 01BXL1 01BXL-1 01BXL-4 01BXL-11 BXL-01 BXL-04 BXL-07 BXL-08 BXL-15 BXL-09 Bixiling

Notes. The average water contents of garnets in the eight Bixiling eclogites, which were reported in detail by Xia et al. (2005), were also shown. FWHH: full width at half-height; Intensity, FWHH and Area of each band were obtained by Gaussian fit after baseline correction. Structural water content was calculated by the Beer–Lambert law using the sum of absorption area of group h and group h bands; the group h bands likely from submicroscopic fluid inclusions (see text) were not included. C-core; R-rim; spots not labeled are all from core.

592 551 793 869 1133 547 562 563 601 667

580 1211 840 1117 1344 1780 1915 66.2 86.4 50.6 53.8 85.3 78.8 189 257 182 171 182 170 0.330 0.316 0.261 0.296 0.441 0.436 48.0 14.0 40.5 57.9 64.6 76.8 118 76 118 141 118 122 0.382 0.173 0.324 0.385 0.513 0.592 4.2 22.2 7.6 71 101 79

FWHH Area FWHH

Sample Locality

Thickness (cm)

Grain

Spot

Position

0.055 0.207 0.090

Area FWHH Intensity Intensity Intensity

Area

Group II (3560–3580 cm1) Group I (3620–3640 cm1)

Group III (3400–3420 cm1)

‘‘Water’’ bound in structural OH (H2O ppm wt)

Average water content (H2O ppm wt)

Water in UHP minerals

2091

D/H ratios were measured on a Thermo-Finnigan DELTA Plus IRMS. The dD values obtained for the NBS-30 biotite standard (dDaccepted = 67&) in the course of omphacite measurements were 62.8 ± 4& (n = 8). (2) Complete water released from samples was assumed after there was no observed pressure increase upon prolonged flame heating. Vacuum fusion of omphacite samples was completed in 40–45 min, in order to keep blanks as low as possible. Similar dD values obtained for different aliquots of the same samples suggest that the extracted water had a similar source. This water may represent a mixture of cpx structural water and blank water (water adsorbed on quartz vessels and/or the pyrex tubes of the vacuum line). (3) We attempted to determine both the magnitude and isotopic composition of the vacuum line blank by measuring the water collected from a flame-heated empty quartz vial. The gas pressure produced in this way and read in the MS bellow-pirani gauge was slightly above 1 mbar (sample bellow fully opened = 40 ml), resulting in an ionized gas voltage at cup 8 (mass 2) <150 mV. This amount of gas corresponded to a maximum of 20–30% of the gas extracted from the cpx, and was not sufficient for hydrogen isotope analysis. The gas extracted from a carousel (n = 6) of empty vials was enough to be measured and gave a dD = 106&. An indirect evaluation of the blank was then attempted by comparing the amounts of H2O measured by FTIR with those calculated on mass spectrometry basis. In principle these amounts should be comparable; in practice, they were significantly different. The amounts of water on IRMS basis were calculated comparing the voltage of hydrogen gas evolved from the water fraction of NBS30 Bt standard aliquots (3.5 wt%) with the voltage of hydrogen gas resulting from omphacite water extraction. Similar comparison can be done using the voltage of the Pbellow, P and mV being linearly proportional for pure gases. This method is only qualitative because of the uncertainties related to the (slightly) different volumes of the vials used for water reduction, and, eventually, to the daily stability of the MS. Indeed, the water extracted from a vial and passed through the line seems to be significantly less than the calculated blank from FTIR subtraction from IRMS measurements. This implies that: (1) the water was retained in the line regardless heating of the latter; (2) other plausible sources of water contamination like symplectitic structures at mineral rimes avoided in FTIR measurements could not be avoided in bulk mineral fusion. Nevertheless, the data corrected by an average of 30% contribution of blank water (Pblank  1 mbar) are slightly lower than, but in a similar range of the data of the dD calculated comparing FTIR and IRMS water contents (118 to 133, and 108 to 114, respectively). Moreover, they are similar to the few dD data of high pressure OH-bearing mineral phases from the Dabie eclogites (Zheng et al., 2003). The detailed results are reported in Table 3.

2092

Sample

Thickness (cm)

Group I (3450–3465 cm1)

Group II (3520–3535 cm1)

GroupIII (3620–3635 cm1)

Intensity

FWHH

Area

Intensity

FWHH

Area

Intensity

FWHH

Area

Water content (ppm H2O)

SH1 SH4 SH5 SH6 SH9 SH14 WM2 05XD1 05XD2 01BXL 01BXL-1 BXL-07 BXL-01 BXL-04 BXL-08 BXL-15 01BXL-04

0.033 0.041 0.026 0.040 0.027 0.024 0.033 0.018 0.016 0.031 0.038 0.043 0.029 0.035 0.031 0.032 0.042

0.273 0.136 0.120 0.215 0.161 0.205 0.075 0.046 0.095 0.097 0.199 0.055 0.070 0.078 0.128 0.078 0.127

139 135 144 145 167 128 71 126 138 103 134 105 123 130 212 137 129

40.2 19.6 17.9 33.3 27.4 28.1 5.7 6.66 14.51 10.5 27.5 6.1 9.4 15.2 25.3 11.4 17.2

0.112 0.095 0.084 0.079 0.066 0.090 0.056 0.013 0.015 0.033 0.034 0.024 0.020 0.027 0.033 0.07813 0.030

29 57 45 27 28 32 61 34 35 32 41 49 27 54 159 49 27

3.7 7.5 3.8 2.4 2.0 3.1 3.7 0.48 0.060 1.1 1.6 1.5 0.6 1.7 5.0 1.3 1.0

0.049 0.078 0.056 0.067 0.049 0.028 0.018 0.011 0.027 0.016 0.065 0.035 0.009 0.237 0.341 0.048 0.026

81 87 70 82 76 75 69 117 101 110 107 79 71 84 69 101 89

4.5 7.9 4.5 5.7 4.0 2.5 1.5 1.39 2.98 2.0 7.4 3.0 0.7 16.3 20.7 5.3 2.7

630 360 455 430 560 500 140 205 480 210 380 115 180 395 695 235 225

Notes. All analyzed grains from individual samples were averaged. For zoned grains (higher water content at the core and lower content at the rim), only core point was used for average, because the lower water content at the rim is interpreted to hydroxyl exsolution during the initial exhumation. Structural water content was calculated by the Beer–Lambert law using the sum of absorption area of all three bands.

Y.-M. Sheng et al. / Geochimica et Cosmochimica Acta 71 (2007) 2079–2103

Table 5 FTIR analysis of omphacites in UHP eclogites of eastern Dabie, China

Table 6 Chemical compositions of representative garnet and omphacite grains with heterogeneous water content Mineral

Grain

Spot

Position

Na2O

MgO

Al2O3

SiO2

P2O5

K2O

CaO

TiO2

Cr2O3

MnO

FeO

NiO

Total

H2O/ppm

SH9

gt

2

omp

3

gt

1

1 2 3 1 2 3 1 2 3 4 5 1 2 3 1 2 3 1 2 3 4 5

C R R C R R C C C R R C C R C R R R R R C C

0.01 0.02 0.04 6.80 6.89 6.92 0.04 0.06 0.02 0.07 0.04 0.01 0.03 0.09 5.26 5.04 4.76 5.52 5.34 5.21 5.52 5.22

9.07 9.36 8.76 8.14 8.28 8.27 5.44 6.17 5.65 6.46 6.77 6.33 5.46 5.60 10.22 10.43 10.32 10.43 10.16 9.93 10.08 10.02

22.90 21.76 21.93 9.40 8.95 9.05 21.07 21.25 21.09 22.16 21.42 22.15 20.95 22.04 5.61 6.03 5.68 5.92 5.72 5.56 6.22 5.13

38.02 38.95 37.9 54.26 55.95 56.03 37.80 36.76 38.16 36.20 38.64 36.96 38.28 36.22 53.58 53.43 55.43 54.28 54.2 53.67 52.92 55.21

0.04 0.02 0.07 0.04 0.03 0.01 0.04 0.01 0.01 0.04 0.02 0.01 0.04 0.02 0.01 0.03 0.01 0.01 0.04 0.04 0.05 0.02

— 0.00 0.01 0.02 — 0.01 0.01 0.01 — — 0.01 0.02 — — — 0.02 0.01 0.02 0.01 — 0.00 0.02

7.69 6.43 6.78 13.24 12.86 13.01 12.16 10.00 9.95 9.70 8.78 9.12 11.78 11.09 15.89 16.23 15.78 15.76 15.95 16.09 15.78 15.75

0.03 0.05 0.03 0.08 0.011 0.05 0.13 0.10 0.13 0.21 0.08 0.11 0.06 0.09 0.01 0.03 0.02 0.11 0.10 0.12 0.10 0.08

0.05 0.02 0.12 0.02 0.06 0.03 0.03 0.03 0.06 0.02 0.01 0.01 0.03 0.01 0.68 0.40 0.05 0.02 0.02 0.04 0.06 0.05

0.04 0.12 0.06 0.02 0.02 0.03 0.07 0.02 0.05 0.02 0.04 0.01 0.03 0.01 0.02 0.06 0.05 0.02 0.04 0.02 0.03 0.03

21.84 23.05 23.40 7.64 7.02 6.86 23.17 25.44 24.21 24.64 23.99 24.82 23.34 24.03 8.37 8.05 7.71 7.83 7.91 8.22 8.65 8.48

0.01 0.03 0.07 0.06 0.01 0.03 0.01 0.01 0.05 0.04 0.03 0.04 0.01 0.01 0.02 0.04 0.03 0.01 0.02 0.02 0.05 0.03

99.69 99.82 99.26 99.72 100.18 100.30 99.95 99.85 99.37 99.55 99.83 99.58 100.00 99.20 99.67 99.79 99.84 99.94 99.53 98.91 99.47 100.06

1155 675 736 733 429 318 339 292 286 164 234 308 365 150 607 496 416 257 228 263 513 305

SH5

4

omp

1

2

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Sample

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Fig. 3. Comparison of representative IR spectra after baseline correction of the Dabie garnets (exampled as No. 11 grain in SH5 eclogite) with a natural hydrogrossular (GRR1358, Rossman and Aines, 1991), a natural hydrous andradite (GRR1669, Amthauer and Rossman, 1998) and a synthetic hydroandradite (GAIV1/4, Amthauer and Rossman, 1998).

Fig. 4. The relationship between intensity of structural OH (sum of integrated area of groups I and II band) and intensity of water in submicroscopic inclusions (integrated area of group III band) in garnets from UHP eclogites of eastern Dabieshan. All 275 analyzed garnet spots are included.

5. RESULTS The chemical composition of garnet and omphacite from Shuanghe and Bixiling is homogeneous within single grains and among the different grains in individual samples, but quite variable between different samples. Garnet compositions range from pyrope-rich (Pyr: 64, Alm + Sps: 26; Grs + And + Uva: 10) to almandine-rich (Pyr: 19, Alm + Sps: 57; Grs + And + Uva: 23). These different compositions, observed in samples from different outcrops

of the Dabie-Sulu orogen, are interpreted as primary features reflecting differences in the bulk compositions of the protoliths of these eclogites (e.g. Ferrando et al., 2005a). The jadeite content (XJd) of omphacite is generally P50; only samples SH5 and BXL04 are Jd-poor ferroan omphacites (XJd = 36 and 25, respectively) likely recrystallized at lower pressures. The garnet-clinopyroxene geothermometer of Powell (1985) for an assumed pressure of 30 kb yields temperatures in the 630–820 C range for Shuanghe and Bixiling eclogites (only sample BXL01 yields an estimated

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Fig. 5. Representative IR spectra after baseline correction of rutile in UHP eclogites of eastern Dabie, China.

Fig. 6. Representative IR spectra after baseline correction of omphacites in UHP eclogites of the Dabie orogen, eastern China. (a) Three spots from No. 3 grain in SH9 eclogite (SH9-3) showing heterogeneous water content between rim (R) and core (C); (b) two spots from No. 2 grain in SH5 eclogite (SH5-2) showing homogeneous water content. Note that these values are not real water content because of unpolarized light on unorientated grain; they are just used to show relative difference between core and rim within individual grains.

temperature of 480 C), in agreement with estimates reported in previous works (e.g. Cong et al., 1995; Zhang et al., 1995). 5.1. Hydrogen speciation 5.1.1. Garnet The IR spectra of the 275 analyzed spots on the Dabie garnet grains show several absorption bands in the typical OH stretching vibration region (3000–3800 cm1); these are superimposed on the low energy flank of a strong

absorption likely due to Fe2+ in the dodecahedral sites. Some spectra display a weak band at 3710 cm1, which is probably created by subtraction of a background spectrum slightly contaminated by the presence of water-vapor. The OH absorption bands can be divided into three groups: (I) 3610–3630 cm1; (II) 3560–3580 cm1; (III) 3400– 3420 cm1. Representative IR spectra are shown in Fig. 2. Details on intensity, full width at half-height (FWHH), and integrated absorbance of the bands fitted using the Gaussian method after baseline correction are given in Table 4. Note that the Bixiling samples (64 spots) are not

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Fig. 7. The relationship between water content and M2 vacancy in omphacites from UHP eclogites of the Dabie orogen, eastern China.

Fig. 8. Histogram of water content in garnets, omphacites and whole rocks of UHP eclogites from eastern Dabie, China. For garnet (a) all analyzed spots were shown, except the rim spots with lower water content than core. Data from Xia et al. (2005) for Bixiling samples were also included. For omphacite (b) average value of all analyzed spots from the same eclogite, except the rim spots with lower water content than core, was used to represent water content in that sample. Water content of whole rocks (c) was estimated from garnet and omphacite without considering minor minerals (quartze, rutile, etc.).

included because they have been reported by Xia et al. (2005). The group III bands lie out of the energy range (generally 3500–3700 cm1) of structural OH in natural and synthesized garnets (Aines and Rossman, 1984a,b; Rossman et al., 1989; Rossman and Aines, 1991; Bell and Rossman, 1992b; Geiger et al., 1991, 2000; Beran et al., 1993; Langer et al., 1993; Snyder et al., 1995; Lu and Keppler, 1997; Amthauer and Rossman, 1998; Matsyuk et al., 1998; Withers et al., 1998; Ingrin and Skogby, 2000; Xia et al., 2000, 2005; Zhang et al., 2001; Su et al., 2002). Group III bands are generally much broader (FWHH of >200 cm1) than groups I and II bands (<100 cm1)

(Table 3). They are typical of the stretching vibrations (t3 + t1) of molecular water, which may occur in eventually submicroscopic fluid inclusions in garnets. Fluid inclusion investigations (Xiao et al., 2000; Fu et al., 2001, 2003; Qiu et al., 2004; Ferrando et al., 2005b) have shown that garnets commonly contain very tiny (<1–2 lm) fluid inclusions. In agreement with previous studies on natural garnets (Rossman et al., 1989; Rossman and Aines, 1991; Bell and Rossman, 1992b; Beran et al., 1993; Langer et al., 1993; Amthauer and Rossman, 1998; Matsyuk et al., 1998; Zhang et al., 2001), we ascribe group III bands to submicroscopic fluid inclusions rather than structural OH.

Water in UHP minerals

The shape and position of groups I and II bands are similar to the spectra of the hydrogrossular (Rossman and Aines, 1991) and hydroandradites (Amthauer and Rossman, 1998) (Fig. 3), which were ascribed to structural OH in the form of the SiO4 4 $ ðO4 H4 Þ4 substitution. The similarities suggest that groups I and II bands in the Dabie garnets are caused by structural OH, and that the SiO4 4 $ ðO4 H4 Þ4 substitution is important. The lower energies of the two absorptions in the Dabie garnets compared to hydrogrossular (Rossman and Aines, 1991) may be related to greater average tetrahedral cation–anion distances. Groups I and II bands are also the main structural OH bands in garnets of diamondiferous UHP metamorphic rocks from Kazakhstan (Langer et al., 1993; Katayama et al., 2006). Fig. 4 shows the amount of structural OH (sum of the integrated absorption area of groups I and II bands) and H2O contents of submicroscopic fluid inclusions (integrated absorption area of group III band) are positively correlated. This suggests that the water in submicroscopic fluid inclusions and structural OH in the host garnet have a common source, with the implication that (1) they were incorporated at the same time during subduction, or (2) the water trapped in inclusions exsolved from the host garnet upon an abrupt decrease in pressure during the initial exhumation, or (3) the fluids incorporated within garnet leaked from the inclusions upon an abrupt decrease in pressure during the initial exhumation. All three possible interpretations imply that the water in submicroscopic fluid inclusions came from the subducted slab itself rather than from ‘‘external infiltration.’’ Further investigation is needed to assess the relationship between the water in fluid inclusions and structural water in garnets, particularly where multiphase peak-metamorphic inclusions have been recovered (Ferrando et al., 2005b). Rutile inclusions commonly occur within garnet; Fig. 5 shows the spectrum of a rutile grain in garnet of 01BXL1. A sharp band occurs at 3280 cm1, which is typical OH absorption of rutile (Hammer and Beran, 1991; Vlassopoulos et al., 1993; Maldener et al., 2001). Additional bands at 3620, 3570 and 3410 cm1 are obviously produced by the host garnet. 5.1.2. Omphacite Omphacite grains from seventeen eclogites were analyzed. The absorption bands of omphacite in the typical OH vibration region (3000–3800 cm1) can be divided into three groups: (I) 3450–3465 cm1; (II) 3520–3535 cm1; (III) 3620–3635 cm1. Groups I and II bands are prominent, whereas group III always appears as a shoulder band (Fig. 6). The weak band at 3710 cm1 is ascribed to the subtraction of the background spectrum slightly contaminated by the presence of water-vapor. Because omphacite is optically inhomogeneous mineral and unpolarized light was used in this study, more than 10 grains from the same section (at least 3 spots per grain) were analyzed. The average value of all analyzed spots was used to calculate the value of grains with homogeneous water contents. Other grains show core-rim variations (higher water contents at the core and lower contents at the

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rim, see Section 5.2), which we ascribe to hydroxyl exsolution from grain rims during the initial exhumation and subsequent loss of water from the garnet rim. For these samples only analytical data on mineral cores were averaged. Detailed information about intensity, FWHH and integrated absorbance of three group bands using Gaussian fit method after baseline correction is shown in Table 5. The positions and relative intensities of OH absorption bands in the Dabie omphacites are identical to those in omphacites from Kazakhstan eclogites (Katayama and Nakashima, 2003). OH absorption bands in omphacites from eclogite xenoliths in South Africa and Yakutia kimberlites also appear in these positions (Smyth et al., 1991; Beran et al., 1993). Skogby et al. (1990) examined 51 pyroxenes from a wide range of geological environments and found that OH absorption bands in clinopyroxenes usually appear in four positions: 3350–3355; 3450–3465; 3520–3535 and 3620–3640 cm1. The latter three correspond to the bands of the Dabie omphacites, which we ascribe to structural OH absorption. Fig. 7 shows the relationship between water content (Table 5) and M2 site vacancies (Table 2). Even discarding BXL04, which may be considered anomalous for its low Jd (25) and high Wef (75) components, no correlation can be seen. In contrast, a clear positive correlation was reported for clinopyroxenes in South African eclogite xenoliths (Smyth et al., 1991) and omphacites of UHP metamorphic eclogites from Kazakhstan (Katayama and Nakashima, 2003). Note that the inferred peak metamorphic conditions are different for the three eclogite occurrences, since P and T are lower for the Dabie Shan eclogites (Eide, 1995; Cong, 1996; Liou et al., 1996; Li et al., 1999, 2000; Zheng et al., 2003) and the clinopyroxene chemical compositions are quite different. Evidence from experimental studies indicates that (1) water solubility in pure jadeite reaches its maximum at 2 GPa and slowly decreases with increasing pressure, (2) substitutions of low valency cations in the M1 site strongly influence the capacity of omphacite to incorporate OH, and (3) bulk composition is more important than pressure and temperature in determining the capacity of omphacite to store water (Bromiley and Keppler, 2004). The observed difference in the H2O contentM2 site vacancy correlation may thus arise from both chemical difference and differences in the pressure–temperature conditions of formation among the three eclogite outcrops. A larger omphacite dataset will hopefully provide further insight into this. 5.2. Water content 5.2.1. Garnet The water contents measured in 275 spots range from 14 to 1915 ppm, with most being between 200 and 800 ppm (Table 4). The detailed information of 64 spots from Bixiling garnets have been reported in Xia et al. (2005) and the average water content of individual samples were shown in Table 3. Water contents vary among different samples and within individual samples (e.g. three grains in sample SH1 range from 330 to 860 ppm; six grains in sample SH4 vary from 155 to 620 ppm; 17 grains in SH9 vary from 470 to

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1860 ppm). The water content distribution is homogeneous within each garnet grain (Fig. 2b and Table 4), and only a few grains (e.g. sample SH9) show water contents at the core higher than those at the rim (SH9-2 core = 1155 ppm; SH9-2 rim = 675–736 ppm; [Fig. 2a]; SH9-4 core = 900– 1000 ppm, SH9-4 rims = 500–750 ppm), irrespective of the chemical composition (Table 6). Lower water contents likely result from hydrogen exsolution (see Section 6).

The water content histogram represents the variability in water distribution within the studied samples (Fig. 8). 5.2.2. Omphacite The water contents of omphacite in the investigated eclogites range from 105 to 695 ppm (Table 5; Fig. 9). A heterogeneous distribution of water was found among different samples from the same locality: omphacite water

Fig. 9. Comparison of H2O contents in the Dabie garnets (Xia et al., 2005; this study) with garnets from Kazakhstan UHP metamorphic rocks (Langer et al., 1993; Katayama et al., 2006) and garnets in eclogite xenoliths from kimberlites (Aines and Rossman, 1984b; Bell and Rossman, 1992b; Matsyuk et al., 1998; Snyder et al., 1995).

Table 7 Water content in whole rocks of UHP ecologites of eastern Dabie, China Sample

H2O in gnt (ppm wt)

H2O in omp (ppm wt)

Vgnt  Vomp

H2O in eclogite

H2Oomp/H2Ognt

SH1 SH4 SH5 SH6 SH9 SH14 WM2 05XD1 05XD2 01BXL 01BXL1 01BXL04 BXL01 BXL07 BXL08 BXL15 BXL04

545 340 270 300 910 170 780 240 425 340 330 550 930 300 520 550 470

620 360 425 440 520 595 280 205 480 180 405 210 160 105 695 240 400

35–60 65–30 75–20 40–55 65–30 45–45 75–20 65–30 70–25 65–30 60–35 55–35 60–35 80–20 45–50 70–25 65–30

565 330 290 360 750 340 640 220 420 280 340 380 610 260 580 445 420

1.1 1.1 1.6 1.5 0.6 3.5 0.4 0.9 1.1 0.5 1.2 0.4 0.2 0.4 1.3 0.4 0.9

Notes. Water content of omphacite from 17 eclogites is directly from Table 2; water content of garnet is the average value of all analyzed grains from the same eclogite. For zoned grains (higher water content at the core and lower content at the rim), only core point was used for average, because the lower water content at the rim is interpreted to hydroxyl exsolution during the initial exhumation. Vgnt and Vomp mean volume percent of garnet and omphacite, respectively.

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Fig. 10. Comparison of H2O contents in the Dabie omphacites (this study) with omphacites from Kazakhstan UHP metamorphic rocks (Katayama et al., 2006) and omphacites in eclogite xenoliths from kimberlites (Smyth et al., 1991; Beran et al., 1993; Koch-Muller et al., 2004).

contents range from 105 to 695 ppm in the Bixiling (n = 8) eclogites and from 360 to 620 ppm in the Shuanghe eclogites (n = 6). Most of the investigated omphacite grains have homogeneous water contents at the scale of the single grain (Fig. 5b), and only a few have a variable distribution of water within the same grain (e.g. SH9-3 core = 733 ppm; SH9-3 rim = 318–429 ppm; Fig. 5a; SH9-1 core = 572– 641 ppm; SH9-1 rim = 301–341 ppm). Although the use of unpolarized light on unorientated grains hinders the correct estimation of water contents, the relative difference within a single grain is significant and, similar to garnet, independent of the chemical composition of the measured grains (Table 6). Table 5 summarizes FTIR data on omphacites in the Dabie UHP eclogites. Due to the use of unpolarized light on unoriented minerals, the omphacite water content was calculated for each sample by averaging the water contents of more than 10 grains per sample (with at least three spots per grain). All spots measured in grains with homogeneous water contents were averaged. In minerals with a heterogeneous distribution of water, only grain-core data were used to calculate water contents, because rims with low water contents were interpreted as the domains where hydroxyl exsolution occurred (see Section 6). 5.2.3. Whole rock Using the volume proportions of omphacite and garnet, the whole-rock water contents of the 17 Dabie eclogites were estimated to be 260–750 ppm (Table 7, Fig. 10). Because the water content in rutile inclusion and fluid inclusions in some garnets were difficult to be determined precisely, they were not included in calculating whole rock water content although they could contain a significant amount of ‘‘internal’’ water (see 5.1.1). Therefore, these

estimated whole rock water contents represent a lower limit. 6. DISCUSSION Hydrous minerals (amphibole, mica, epidote, lawsonite, etc.) are inferred to be out of their stability field at the UHP metamorphic conditions estimated for the UHP metamorphic rocks of the Dabie-Sulu orogen (Liou et al., 1996; Poli and Schmidt, 1997; Okamoto and Maruyama, 1999; Li et al., 2000; Forneris and Holloway, 2003). However, FTIR studies of NAMs (garnet, omphacite, rutile) from Dabie eclogites (Xia et al., 2000, 2005; Zhang et al., 2001, 2004; Su et al., 2002, 2004) revealed that significant amounts of water (in excess of 1000 ppm) were transported down to upper mantle conditions. Katayama et al. (2006) studied six eclogites from the Kokchetav UHP massif by FTIR and SIMS, and obtained a partition coefficient of 5–10 for water between omphacite and garnet. They therefore suggested that water in deeply subducted crust is preferentially hosted in omphacite rather than garnet. In the case of the Dabie eclogites, if the average water content of all analyzed garnets and omphacites from the same sample (except rim spots with lower water contents) is considered representative of garnet and omphacite water contents, the partition coefficient for water between garnet and omphacite ranges from 0.2 to 3.5 (Table 7). This indicates that garnet may be as important as clinopyroxene in incorporating and transporting water into the mantle upon subduction. Langer et al. (1993) found that water contents in garnet from diamond-bearing UHP rocks from Kazakhstan were generally higher than 200 ppm and up to 2500 ppm. Based on amphibole exsolutions, Song et al. (2005) estimated the initial water content in garnet

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from unltra-deep (>200 km) subducted garnet–peridotite of North Qaidam UHPM belt, NW China was up to 1000 ppm. With increasing pressure, the abundance of omphacite in the eclogite may decrease due to dissolution into majoritic garnet, but the total water content in the eclogite probably remains stable due to the large storage capacity of garnet. The whole-rock water contents of the Dabie eclogites were estimated to range from 260 to 750 ppm (Table 7, Fig. 10). These values are higher than water contents of mantle peridotite (<300 ppm, Bell and Rossman, 1992a), and suggest that pods of eclogite situated in the peridotitic mantle may locally form domains with higher water contents. Variable water contents and hydrogen isotope compositions may also occur at the microscale as the result of minerals with zoned OH contents. Most Dabie garnets and omphacites have retained homogeneous water contents, but zoning was observed in some samples (e.g. SH1, SH4 and SH9). Jadeite grains with variable amounts of water and correlated variations in Na and Ca contents, as well as M2-site vacancy, have been reported for jadeite grains in Dabie UHP jadeitebearing quartzites (Su et al., 2004). These characteristics indicate that bulk mineral composition plays an important role in determining the incorporation of hydrogen (Bromiley and Keppler, 2004). In addition, experimental studies show that hydroxyl solubility in garnet and clinopyroxene increases with increasing pressure (Lu and Keppler, 1997; Bromiley et al., 2004). Accordingly, the heterogeneity of water in some of the Dabie garnets and omphacites can be interpreted in terms of hydroxyl exsolution during pressure release (exhumation). It seems plausible that omphacite crystals incorporated progressively more water at high pressures, i.e. during the phase(s) of increasing ‘‘storage’’ capacity (subduction). The solubility of OH in garnet and clinopyroxene decreased upon isothermal decompression of the rocks. During this stage, OH groups are expected to diffuse out from garnet and omphacite crystal defects. If so, significant water release from the decompressing UHP rocks is expected during the very early stages of exhumation. This implies that some of the retrograde fluids are internally-derived, as also indicated by stable isotope data on the Dabie-Sulu UHP metamorphic rocks (Zheng et al., 1999, 2003; Rumble et al., 2002). The dD data obtained for Bixiling omphacites are influenced by method limitations discussed in Section 4.3. However, it is worth noting that dD values are between 108.4 and 114.2& (or 117.9& and 133.8&, Table 3), in the range of primary UHP phengites form Qilongshan (99& to 109&; Zheng et al., 2003). The dD values of UHP minerals as low as 100& argue for the involvement of meteoric water in the formation of the protoliths of the metamorphic rocks. In turn, the preservation of premetamorphic dD and d18O values coupled with the observed heterogeneity in mineral water contents at the hand-specimen scale suggest very limited external fluid influx during metamorphic or post-metamorphic processes (Zheng et al., 2003; Xia et al., 2005; this work) and substantial preservation of primary hydrogen isotope signature in nominally anhydrous omphacite.

7. CONCLUSIONS The combined FTIR and hydrogen isotope investigation of omphacite grains from UHP eclogites of the Dabie orogen, eastern China revealed water contents of 105–695 ppm (H2O wt) and dD values of 108.4 and 114.2& (or 117.9& and 133.8&, Table 3), irrespective of water contents. Coexisting garnet grains have H2O contents of 14–1915 ppm, indicating that they have a similar potential for carrying water to mantle depths. Water contents are either homogeneous at the grain scale or zoned (higher at the core-lower at the rim), the lower water content of crystal rims likely resulting from hydroxyl exsolution during the decrease in pressure. The.distribution of water in the omphacite and garnet of the Dabie UHP eclogites indicates that the metamorphic fluids were internally-derived. Moreover, water contents in garnet from UHP metamorphic rocks (Langer et al., 1993; Katayama et al., 2006; Xia et al., 2005; this study) are significantly higher than those in garnet from eclogite xenoliths hosted by kimberlites (Aines and Rossman, 1984b; Bell and Rossman, 1992b; Snyder et al., 1995; Matsyuk et al., 1998) (Fig. 8). Water contents in omphacite from UHP metamorphic rocks (Beran et al., 1993; Katayama et al., 2006) are similar to those in eclogite xenoliths (Smyth et al., 1991; Beran et al., 1993; KochMuller et al., 2004) (Fig. 9). The protracted residence at mantle conditions of eclogite xenoliths might result in significant release of water to the surrounding peridotitic mantle (average H2O content <300 ppm; Bell and Rossman, 1992a), therefore lower OH contents in garnets. Due to the short duration of the subduction–exhumation process which affected the UHP eclogites, these rocks have preserved the water-rich character of the protolith. ACKNOWLEDGMENTS We thank Prof. Zhou Cun-Ting for help with collecting samples, Prof. Liu Jin-Ling for assistance with FTIR analysis in Shanghai and Prof. Wang Rucheng for assistance with EMP analysis in Nanjing. This study was funded by the Natural Science Foundation of China (40172027), the Chinese Ministry of Education within the framework of the Program for New Century Excellent Talents in University (NCET), and the CAS-CNR cooperation project. Comments by two anonymous reviewers and thorough reviews by T. Chacko greatly improved the manuscript. REFERENCES Asimow P. D., and Langmuir C. H. (2003) The important of water to oceanic mantle melting regimes. Nature 421, 815–820. Aines R. D., and Rossman G. R. (1984a) The hydrous component in garnets: Pyraspites. Am. Mineral. 69, 1116–1126. Aines R. D., and Rossman G. R. (1984b) Water content of mantle garnets. Geology 12, 720–723. Amthauer G., and Rossman G. R. (1998) The hydrous component in andradite garnet. Am. Mineral. 83, 835–840. Bell D. R., and Rossman G. R. (1992a) Water in the earth’s mantle: the role of nominally anhydrous minerals. Science 255, 1391–1397.

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