Uranium isotope fractionation during slab dehydration beneath the Izu arc
Uranium isotope fractionation during slab dehydration beneath the Izu arc
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DOI：
10.1016/j.epsl.2019.07.006
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发表时间：
2019-09-15
影响因子：
5.3
通讯作者：
Elliott, Tim
中科院分区：
文献类型：
--
作者：
Freymuth, Heye;Andersen, Morten B.;Elliott, Tim
俯冲板片释放的流体赋予火山弧岩浆和残留板片特征的地球化学特征，这些岩浆和残留板片被输送到更深的地幔。然而，从板释放的微量元素的来源和传输机制是推测性的。从太平洋伊豆火山弧熔岩中U-238/U-235和放射成因Pb同位素比值的角度研究了俯冲板片释放的流体。伊豆弧熔岩是流体为主的端元型岩浆，允许板状流体的密切表征。伊豆岛弧熔岩的U-238/U-235比值低于地球和大洋中脊玄武岩（MORB）。低U-238/U-235（Δ U-238 = -0.46至-0.33ppm，其中Δ U-238 = U-238/U-235（样品）/U-238/U-235（CRM 145）- 1）与加入岩浆源的低Th/U的板片衍生流体有关。放射性成因的Pb同位素比值的熔岩形成一个阵列之间的“印度”型MORB和俯冲沉积物，是不一致的流体来自蚀变镁铁质洋壳（AMOC）。我们推断，“流体流动”的元素，包括U和Pb的动员基本上不变，更深的部分的镁铁质地壳的迁移流体是来自下面的蛇纹岩脱水。铀仅以铀-六价铀的形式在流体中活动，需要从未蚀变的岩浆岩中的主要铀-四价铀氧化，才能被流体活动。在此过程中，需要在三角洲U-238中进行类似于千分之0.2的铀同位素分馏，以在流体中产生低U-238/U-235。我们认为，通道化的流体流经变质席状岩墙和镁铁质地壳的辉长岩部分局部氧化和活化铀。我们认为，U同位素分馏发生在流体通道内，并与平衡同位素分馏过程中的U氧化和U-IV纳入第二相，如绿帘石，磷灰石和锆石的通道内生长。预计这些阶段将把同位素重铀带入俯冲带以外的更深地幔。因此，三角洲U-238正在追踪俯冲板块的脱水过程。在不同的弧和俯冲相关的变质岩中，对其他“稳定同位素”系统也进行了类似的观察，从而突出了它们在俯冲过程中研究板片内发生的过程的潜力。这些信息对于理解俯冲板片和地幔楔之间的元素分配和限制俯冲带在全球地球化学循环中的作用至关重要。(C)2019爱思唯尔B. V.保留所有权利。
Fluids released from subducted slabs impart characteristic geochemical signatures on volcanic arc magmas and residual slabs transported into the deeper mantle. Yet, the sources and transport mechanisms of trace elements released from the slab are speculative. We investigate fluids released from subducted slabs from the perspective of U-238/U-235 and radiogenic Pb isotope ratios in lavas from the Izu volcanic arc in the Pacific ocean. Izu arc lavas are fluid-dominated end-member type magmas that allow a close characterisation of slab fluids. The Izu arc lavas have low U-238/U-235 ratios compared to the bulk Earth and mid-ocean ridge basalt (MORB). The low U-238/U-235 (delta U-238 = -0.46 to -0.33 parts per thousand, where delta U-238 = U-238/U-235(sample)/U-238/U-235(CRM145) - 1) is associated with slab-derived fluids low in Th/U that are added to the magma sources. The radiogenic Pb isotope ratios of the lavas form an array between 'Indian' type MORB and subducting sediments that is inconsistent with fluids derived from the altered mafic oceanic crust (AMOC). We infer that 'fluid-mobile' elements, including U and Pb are mobilised from largely unaltered, deeper sections of the mafic crust by migrating fluids that are derived from the dehydration of underlying serpentinites. Uranium is only fluid-mobile as U-VI and needs to be oxidised from predominant U-IV in unaltered magmatic rocks in order to be mobilised by fluids. Uranium isotope fractionation of similar to 0.2 parts per thousand in delta U-238 during this process is required to generate the low U-238/U-235 in the fluids. We propose that channelised fluid flow through the metamorphosed sheeted dyke and gabbroic sections of the mafic crust locally oxidises and mobilises U. We suggest that U isotope fractionation occurs within the fluid channels and is related to equilibrium isotope fractionation during the oxidation of U and the incorporation of U-IV into secondary phases such as epidote, apatite and zircon that grow within the channels. These phases are predicted to carry isotopically heavy U into the deeper mantle beyond subduction zones. The delta U-238 is thus tracing the dehydration process of subducting slabs. Similar observations have been made for other, 'stable isotope' systems in different arcs and subduction-related metamorphic rocks, thus highlighting their potential for studying processes occurring within the slabs during subduction. This information is essential for understanding and the partitioning of elements between subducted slabs and the mantle wedge and constraining the role of subduction zones in global geochemical cycles. (C) 2019 Elsevier B.V. All rights reserved.