Mineral hydrogen isotopes and water contents in ultrahigh-pressure metabasite and metagranite: Constraints on fluid flow during continental subduction-zone metamorphism
Mineral hydrogen isotopes and water contents in ultrahigh-pressure metabasite and metagranite: Constraints on fluid flow during continental subduction-zone metamorphism
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DOI：
10.1016/j.chemgeo.2010.12.002
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发表时间：
2011-02
期刊：
影响因子：
3.9
通讯作者：
Ren‐Xu Chen;Yong‐Fei Zheng;B. Gong
中科院分区：
文献类型：
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作者：
Ren‐Xu Chen;Yong‐Fei Zheng;B. Gong
对苏鲁造山带中国大陆科学钻探工程主孔连续岩芯段超高压榴辉岩、角闪岩、片岩和花岗片麻岩中的变质矿物进行了氢同位素和总含水量的系统测定。结合之前对露头和钻井样品的研究，这些结果产生了一个最广泛的数据集，其中包括来自超高压变质岩(特别是超高压变质岩)的含水和名义上无水的矿物。这为大陆俯冲带变质过程中流体流动的性质和规模提供了洞察。含水矿物和名义上无水矿物的δD值在−15 5~−2 0‰范围内变化很大，这与高温热液蚀变使大气水进入超高压变质岩原岩相一致。共存的含水矿物和名义上无水矿物之间存在平衡和不平衡氢同位素分馏，表明在深俯冲陆壳折返过程中，退变质作用对超高压矿物氢同位素体系的不同影响。除了结构羟基以外，名义上的无水矿物(NAM)也含有大量的水分子形式的水，相对于结构羟基，水分子中的D是贫化的。榴辉岩中石榴石、绿辉石和金红石的总含水率与δD值呈负相关，表明在减压折返过程中，NAMS中的贫D分子水优先通过扩散损失，而不是结构OH。因此，镁铁质矿物之后的共晶石相对于它们的前驱矿物来说，D是亏损的。角闪岩中的角闪石和斜长石的δD值与相邻榴辉岩中的石榴石和绿辉石相近或更大。这表明逆流流体相对于榴辉岩的NAMS来说是富D的，因此来源于富D的水合矿物的分解。在任何一种情况下，逆行流体都以稳定的同位素成分在内部进行缓冲。然而，从NAMS中释放出的分子水和结构羟基是贫D流体的原因，而水合矿物的分解是富D流体的原因。对不同超高压岩性矿物的系统含水率测量表明，榴辉岩和片麻岩的总含水率存在显著差异。这可能是它们在折返过程中流体活动行为不同的根本原因。
Systematic measurements of hydrogen isotopes and total water contents were carried out for metamorphic minerals from ultrahigh-pressure (UHP) eclogite, amphibolite, schist and granitic gneiss in continuous core segments from the main hole of the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen. Together with previous studies of both outcrop and drilling samples, the results yield a dataset that is the most extensive for hydrous and nominally anhydrous minerals from UHP metamorphic rocks (especially for UHP metagranite). This provides insights into the property and scale of fluid flow during continental subduction-zone metamorphism. Hydrous and nominally anhydrous minerals show large variations in δD values from −155 to −20‰, consistent with the incorporation of meteoric water into protoliths of UHP metaigneous rocks by high-T hydrothermal alteration. Both equilibrium and disequilibrium H isotope fractionations occur between coexisting hydrous and nominally anhydrous minerals, indicating the differential effects of retrograde metamorphism on the H isotope systems of UHP minerals during the exhumation of deeply subducted continental crust. Besides the structural OH, nominally anhydrous minerals (NAMs) also contain large amounts of water in the form of molecular H2O that is depleted in D relative to the structural OH. There are negative correlations between total water contents and δD values for garnet, omphacite and rutile in the eclogite, indicating a preferential loss of the D-poor molecular water from the NAMs by diffusion relative to the structural OH during the decompression exhumation. As a consequence, symplectites after the mafic minerals are depleted in D relative to their precursors. On the other hand, amphibole and plagioclase in the amphibolite have δD values similar to, or greater than, garnet and omphacite from the adjacent eclogite. This suggests that the retrograde fluid is enriched in D relative to the NAMs of eclogite and thus derived from the decomposition of D-rich hydrous minerals. In either case, the retrograde fluid is internally buffered in stable isotope compositions. Nevertheless, the exsolution of molecular water and structural hydroxyl from the NAMs is responsible for D-poor fluid, whereas the decomposition of hydrous minerals is responsible for the D-rich fluid. The systematic measurements of water contents for various minerals from the different UHP lithologies demonstrate significant differences in total water contents between eclogite and gneiss. This may be a basic cause for their different behaviors of fluid activity during the exhumation.