H2O contents and D/H ratios of nominally anhydrous minerals from ultrahigh-pressure eclogites of the Dabie orogen, eastern China

Sheng, Yue; Xia, Qunke; Dallai, Luigi; Yang, Xiaozhi; Hao, Yan-Tao

doi:10.1016/j.gca.2007.01.018

Cited by 81 publications

(90 citation statements)

References 73 publications

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“…While some of these hydrous minerals may be present in small amounts in HP to UHP eclogites, some UHP eclogites may contain neither of these hydrous minerals. Instead, they contain hundreds to thousands of parts per million (ppm) H 2 O within NAMs such as garnet and pyroxene, not only as structural OH in point defects (Katayama and Nakashima 2003;Xia et al 2005;Katayama et al 2006;Chen et al 2007;Sheng et al 2007;Zhao et al 2007a) but also as molecular H 2 O in surface and volume defects (Su et al 2002;Xiao et al 2002;Fu et al 2003;Gao et al 2007;Ni et al, 2008;Zhang et al 2008;Meng et al 2009;Mukherjee and Sachan 2009). Analyses of the total water in NAMs yield the maximum water contents of about 2,500 ppm and about 3,500 ppm, respectively, in garnet and omphacite at UHP conditions (Chen et al 2011;Gong et al 2013).…”

Section: Water In Subduction-zone Rocksmentioning

confidence: 99%

Geochemistry of continental subduction-zone fluids

2014

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The composition of continental subduction-zone fluids varies dramatically from dilute aqueous solutions at subsolidus conditions to hydrous silicate melts at supersolidus conditions, with variable concentrations of fluid-mobile incompatible trace elements. At ultrahigh-pressure (UHP) metamorphic conditions, supercritical fluids may occur with variable compositions. The water component of these fluids primarily derives from structural hydroxyl and molecular water in hydrous and nominally anhydrous minerals at UHP conditions. While the breakdown of hydrous minerals is the predominant water source for fluid activity in the subduction factory, water released from nominally anhydrous minerals provides an additional water source. These different sources of water may accumulate to induce partial melting of UHP metamorphic rocks on and above their wet solidii. Silica is the dominant solute in the deep fluids, followed by aluminum and alkalis. Trace element abundances are low in metamorphic fluids at subsolidus conditions, but become significantly elevated in anatectic melts at supersolidus conditions. The compositions of dissolved and residual minerals are a function of pressure-temperature and whole-rock composition, which exert a strong control on the trace element signature of liberated fluids. The trace element patterns of migmatic leucosomes in UHP rocks and multiphase solid inclusions in UHP minerals exhibit strong enrichment of large ion lithophile elements (LILE) and moderate enrichment of light rare earth elements (LREE) but depletion of high field strength elements (HFSE) and heavy rare earth elements (HREE), demonstrating their crystallization from anatectic melts of crustal protoliths. Interaction of the anatectic melts with the mantle wedge peridotite leads to modal metasomatism with the generation of new mineral phases as well as cryptic metasomatism that is only manifested by the enrichment of fluid-mobile incompatible trace elements in orogenic peridotites. Partial melting of the metasomatic mantle domains gives rise to a variety of mafic igneous rocks in collisional orogens and their adjacent active continental margins. The study of such metasomatic processes and products is of great importance to understanding of the mass transfer at the slab-mantle interface in subduction channels. Therefore, the property and behavior of subduction-zone fluids are a key for understanding of the crust-mantle interaction at convergent plate margins.

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Section: Water In Subduction-zone Rocksmentioning

confidence: 99%

Geochemistry of continental subduction-zone fluids

2014

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“…Many studies have been devoted to petrology, stable isotopes, fluid inclusions and mineral water contents in the Dabie-Sulu UHP metamorphic rocks [12,22,23,54,90,91,[126][127][128][129][130][131][132][133][134][135][136][137] . The results demonstrate that fluid activity is very small during the UHP metamorphism of supracrustal rocks at mantle depths [12,126,127] , but it became significantly large during the initial exhumation because the decomposition of hydrous minerals and the exsolution of structural hydroxyl have been the source of retrograde fluid [12,23,54,91,128,129,[134][135][136] . Inspection of the relationship between the distance, petrography and δ 18 O values of adjacent samples from the CCSD main hole reveals O isotope heterogeneities between the different and same lithologies on scales of 20 to 50 cm [91] , corresponding to the maximum scales of fluid mobility during the continental collision.…”

Section: Fluid Activity In the Continental Deep-subduction Zonementioning

confidence: 99%

A perspective view on ultrahigh-pressure metamorphism and continental collision in the Dabie-Sulu orogenic belt

2008

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“…dD values of À114 to À108% were obtained by the conventional extraction method for omphacite from eclogite at the same locality. 34 In this regard, omphacite is depleted in D relative to garnet. Such a fractionation relationship is also observed from eclogite at Taohang in the Sulu orogen.…”

Section: Nominally Anhydrous Mineralmentioning

confidence: 99%

An online method combining a thermal conversion elemental analyzer with isotope ratio mass spectrometry for the determination of hydrogen isotope composition and water concentration in geological samples

Gong

Zheng

Chen

2007

Rapid Comm Mass Spectrometry

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An online continuous-flow method, combining a thermal conversion elemental analyzer (TC/EA) with isotope ratio mass spectrometry (MS), is evaluated for the determination of both the hydrogen isotope composition and the water concentration of hydrous and nominally anhydrous minerals. The technique involves reduction of hydrous minerals or nominally anhydrous minerals by reaction with glassy carbon at 1450 degrees C in a helium stream. The product gases, H2 and CO, are separated on a gas chromatographic column prior to analysis in the mass spectrometer. Calibration curves for the H concentration analysis were generated from a standard of benzoic acid (C7H6O2) that has an H concentration of 5.0 wt%; the analytical uncertainties were better than +/-0.05% in our runs. Two standards of material with given D values, polyethylene IAEA-CH-7 and biotite NBS-30, were tested for the purpose of calibrating a natural garnet 04BXL02 representing nominally anhydrous minerals. Preheating at 90 degrees C for 12 h was found to be suitable for removing adsorption water on the sample surface. This results in constant D values and total H2O contents for the garnet, with weighted means of -94 +/- 1 and 522 +/- 11 ppm (wt), respectively. The TC/EA-MS technique allows routine analysis of sample sizes as small as 0.01 microL H2O. For natural minerals, absolute reproducibilities for D values are +/-0.5 to +/-2 (1) and relative uncertainties for total H2O concentrations are at levels of +/-1% to +/-3% (1). Therefore, this online method can be used for the quantitative determination of H isotope composition and H2O concentration of either hydrous or anhydrous minerals.

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H2O contents and D/H ratios of nominally anhydrous minerals from ultrahigh-pressure eclogites of the Dabie orogen, eastern China

Cited by 81 publications

References 73 publications

Geochemistry of continental subduction-zone fluids

Geochemistry of continental subduction-zone fluids

A perspective view on ultrahigh-pressure metamorphism and continental collision in the Dabie-Sulu orogenic belt

An online method combining a thermal conversion elemental analyzer with isotope ratio mass spectrometry for the determination of hydrogen isotope composition and water concentration in geological samples

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