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Tracing subduction zone fluid-rock interactions using trace element and Mg-Sr-Nd isotopes
Wang, Shui-Jiong1,2; Teng, Fang-Zhen2; Li, Shu-Guang1; Zhang, Li-Fei3; Du, Jin-Xue1,3; He, Yong-Sheng1; Niu, Yaoling4,5
摘要Slab-derived fluids play a key role in mass transfer and elemental/isotopic exchanges in subduction zones. The exhumation of deeply subducted crust is achieved via a subduction channel where fluids from various sources are abundant, and thus the chemical/isotopic compositions of these rocks could have been modified by subduction-zone fluid-rock interactions. Here, we investigate the Mg isotopic systematics of eclogites from southwestern Tianshan, in conjunction with major/trace element and Sr-Nd isotopes, to characterize the source and nature of fluids and to decipher how fluid-rock interactions in subduction channel might influence the Mg isotopic systematics of exhumed eclogites. The eclogites have high LILEs (especially Ba) and Pb, high initial Sr-87/Sr-86 (up to 0.7117; higher than that of coeval seawater), and varying Ni and Co (mostly lower than those of oceanic basalts), suggesting that these eclogites have interacted with metamorphic fluids mainly released from subducted sediments, with minor contributions from altered oceanic crust or altered abyssal peridotites. The positive correlation between Sr-87/Sr-86 and Pb* (an index of Pb enrichment; Pb* = 2*PbN/[CeN + Pr14]), and the decoupling relationships and bidirectional patterns in Sr-87/Sr-86-Rb/Sr, Pb*-Rb/Sr and Pb*-Ba/Pb spaces imply the presence of two compositionally different components for the fluids: one enriched in LILEs, and the other enriched in Pb and Sr-87/Sr-86. The systematically heavier Mg isotopic compositions (delta Mg-26 = 0.37 to + 026) relative to oceanic basalts (-0.25 +/- 0.07) and the roughly negative correlation of delta Mg-26 with MgO for the southwestern Tianshan eclogites, cannot be explained by inheritance of Mg isotopic signatures from ancient seafloor alteration or prograde metamorphism. Instead, the signatures are most likely produced by fluid-rock interactions during the exhumation of eclogites. The high Rb/Sr and Ba/Pb but low Pb* eclogites generally have high bulk-rock delta Mg-26 values, whereas high Pb* and Sr-87/Sr-86 eclogites have mantle-like delta Mg-26 values, suggesting that the two fluid components have diverse influences on the Mg isotopic systematics of these eclogites. The LILErich fluid component, possibly derived from mica-group minerals, contains a considerable amount of isotopically heavy Mg that has shifted the Mg-delta 26 of the eclogites towards higher values. By contrast, the Sr-87/Sr-86- and Pb-rich fluid component, most likely released from epidote-group minerals in metasediments, has little Mg so as not to modify the Mg isotopic composition of the eclogites. In addition, the influence of talc-derived fluid might be evident in a very few eclogites that have low Rb/Sr and Ba/Pb but slightly heavier Mg isotopic compositions. These findings represent an important step toward a broad understanding of the Mg isotope geochemistry in subduction zones, and contributing to understanding why island arc basalts have averagely heavier Mg isotopic compositions than the normal mantle. (C) 2017 Elsevier B.V. All rights reserved.
关键词Mg Isotopes Subduction Channel Fluid-rock Interaction Eclogite Tianshan
作者单位1.China Univ Geosci, State Key Lab Geol Proc & Mineral Resources, Beijing 100083, Peoples R China
2.Univ Washington, Dept Earth & Space Sci, Isotope Lab, Seattle, WA 98195 USA
3.Peking Univ, Sch Earth Space Sci, Beijing, Peoples R China
4.Chinese Acad Sci, Inst Oceanol, Qingdao 266071, Peoples R China
5.Univ Durham, Dept Earth Sci, Durham DH1 3LE, England
GB/T 7714
Wang, Shui-Jiong,Teng, Fang-Zhen,Li, Shu-Guang,et al. Tracing subduction zone fluid-rock interactions using trace element and Mg-Sr-Nd isotopes[J]. LITHOS,2017,290:94-103.
APA Wang, Shui-Jiong.,Teng, Fang-Zhen.,Li, Shu-Guang.,Zhang, Li-Fei.,Du, Jin-Xue.,...&Niu, Yaoling.(2017).Tracing subduction zone fluid-rock interactions using trace element and Mg-Sr-Nd isotopes.LITHOS,290,94-103.
MLA Wang, Shui-Jiong,et al."Tracing subduction zone fluid-rock interactions using trace element and Mg-Sr-Nd isotopes".LITHOS 290(2017):94-103.
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