Tracing serpentinite dehydration in a subduction channel: Chromium element and isotope evidence from subducted oceanic crust
Tracing serpentinite dehydration in a subduction channel: Chromium element and isotope evidence from subducted oceanic crust
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追踪俯冲通道中的蛇纹岩脱水：俯冲洋壳中的铬元素和同位素证据
DOI：
10.1016/j.gca.2021.06.030
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发表时间：
2021-06
影响因子：
5
通讯作者：
Yu Huimin
中科院分区：
文献类型：
--
作者：
Shen Ji;Wang Shui-Jiong;Qin Liping;Ni Huaiwei;Li Shuguang;Du Jinxue;Shen Tingting;Zhang Lifei;Yu Huimin
蛇纹岩是俯冲带挥发物和流体可动元素的重要来源之一。人们曾尝试使用各种地球化学工具将蛇纹岩流体与其他来源（如沉积物、蚀变海洋地壳）区分开来，但事实证明，这种区分是多变的，因为来自不同岩性的流体在俯冲通道中动态混合。蛇纹岩的主要特征是高Cr含量和不同程度的53cr过量。基于实验测定的富cl变质流体中Cr的高迁移率，Cr元素和同位素组成可能是蛇纹岩流体介导的传质的潜在标志。本文通过分析天山造山带西南部变质玄武岩、乡村云母片岩、大理岩和蛇纹岩的Cr浓度和同位素组成来验证这一假设。变质玄武岩中Cr含量变化大（30.0 ~ 625 ppm）， δ53Cr变化可分辨（- 0.25‰~ - 0.05‰），云母片岩和大理岩同位素组成相似，Cr含量低（云母片岩δ53Cr为- 0.15‰~ - 0.14‰，Cr含量为89.5 ~ 110 ppm，大理岩δ53Cr为- 0.21‰~ - 0.17‰，Cr含量为4.1 ~ 8.73 ppm）。大部份变质玄武岩为碳酸盐岩。炭化变质玄武岩的燃失量（LOI）、CO2、Sb和Sb/Ce与Cr含量及δ53Cr呈正相关，表明变质流体至少部分来源于蛇纹岩和沉积碳酸盐岩。高压蛇纹岩δ53Cr变化范围为- 0.19‰~ - 0.02‰，但Cr含量较高，为934 ~ 4920 ppm。这些蛇纹岩的铬同位素变化不可能是由蛇纹岩化引起的，因为Cr同位素和蛇纹岩化指数的变化趋势与以往的观测结果相反。相反，在俯冲/挖掘过程中，蛇纹岩的脱水导致了Cr同位素的变化，产生了高Cr浓度和相对较轻的同位素组成的流体。根据Rayleigh脱水模型，估算出流体与残留蛇纹岩之间的Cr同位素分馏因子（α -流-残）约为0.99995 ~ 0.99975，表明俯冲蛇纹岩衍生流体中的Cr可能以Cr3+- cl−单键dh2o （OH -）的配合物形式存在。混合模式表明，所研究的碳酸化变质玄武岩来源于最初演化的海洋玄武岩，这些玄武岩是由蛇纹岩脱水流体变质而形成的，并有沉积碳酸盐岩的贡献。本文强调，耦合的Cr元素和同位素系统具有识别俯冲带复杂流体来源的潜力，特别是蛇纹岩。
Serpentinite is one of the most important sources of volatiles and fluid-mobile elements in subduction zones. Attempts have been made to discriminate serpentinite-derived fluid from other sources (e.g., sediments, altered oceanic crusts) using a variety of geochemical tools, but such differentiation has proven to be changeling because fluids from different lithologies are dynamically mixed in the subduction channel. Serpentinites are essentially distinguished by high Cr contents and variable degrees of53Cr excess. Given high Cr mobility in Cl-rich metamorphic fluids based on experimental determination, Cr elemental and isotope compositions could be potential markers for fluid-mediated mass transfer from serpentinites. Here, we test this hypothesis by analyzing Cr concentrations and isotope compositions of meta-basalts, as well as country mica schists, marbles and serpentinites from the southwestern Tianshan Orogen. The meta-basalts have highly variable Cr concentrations (30.0–625 ppm) and resolvable δ53Cr variations (−0.25‰ to −0.05‰), while mica schists and marbles have similar isotope compositions and low Cr contents (δ53Cr of −0.15‰ to −0.14‰ with Cr contents of 89.5–110 ppm for mica schists, δ53Cr of −0.21‰ to −0.17‰ with Cr contents of 4.1–8.73 ppm for marbles, respectively). Most of the investigated meta-basalts were carbonated. The positive correlations of loss on ignition (LOI), CO2, Sb and Sb/Ce with Cr contents, as well as with δ53Cr for these carbonated meta-basalts indicate that the metamorphic fluids are at least partially sourced from serpentinites and sedimentary carbonates. High pressure serpentinites display similar δ53Cr variations from −0.19‰ to −0.02‰, but high Cr contents ranging from 934 to 4920 ppm. Chromium isotope variations of these serpentinites could not be generated by serpentinization, due to opposite trends of Cr isotopes and serpentinization indexes compared with previous observations. Instead, dehydration of serpentinites during subduction/exhumation accounts for Cr isotope variations, generating fluids with high Cr concentrations and relatively lighter isotope compositions. According to a Rayleigh dehydration model, the estimated Cr isotope fractionation factor (αfluid-residue) between fluids and residual serpentinites is approximately 0.99995 –0.99975, implying that Cr species in the subducted serpentine derived fluid is probably as complexes of Cr3+-Cl−single bondH2O(OH–). The mixing model reveals that investigated carbonated meta-basalts were sourced from initial evolved oceanic basalts metamorphosed by fluids from serpentinite dehydrations, with contributions from sedimentary carbonates. This work emphasizes that coupled Cr element and isotope systems present the potential ability to discriminate the complex fluid sources in subduction zones, particularly serpentinites.