Molybdenum mobility and isotopic fractionation during subduction at the Mariana arc
Molybdenum mobility and isotopic fractionation during subduction at the Mariana arc
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DOI：
10.1016/j.epsl.2015.10.006
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发表时间：
2015-12-15
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;Vils, Flurin;Elliott, Tim
通过板块构造循环进入对流地幔的地壳物质的命运对于理解地球的化学和物理演化非常重要。钼在地球表面的显著同位素变化提供了提供这种回收材料的独特签名的希望。然而，表征的行为，钼在俯冲过程中需要评估潜在的钼同位素比值示踪剂的全球地球化学循环。在这里，我们提出了钼同位素数据的输入和输出组件的原型马里亚纳弧：马里亚纳弧熔岩，沉积物ODP网站800，801和802马里亚纳海沟附近的蚀变镁铁质，大洋地壳（AOC），从ODP网站801，连同样品的大洋地壳更深的ODP网站1256。我们还报告了新的高精度的马里亚纳弧熔岩和数据集的Pb同位素比值从ODP网站800，801和802沉积物的Pb同位素数据。马里亚纳弧熔岩富集钼相比，类似的不相容元素在上地幔熔融，并具有独特的，同位素重钼（高Mo-98/Mo-95）相对于上地幔，高达0.3千分之一。与此相反，各种俯冲沉积岩性主要主机同位素轻钼。耦合铅和钼富集在马里亚纳弧熔岩表明这些元素的共同来源，我们进一步使用铅同位素来确定同位素重钼的起源。我们推断，高[Mo]，[Pb]，高Mo-98/Mo-95和非放射成因铅的水流体成分来自俯冲，镁铁质洋壳。虽然俯冲，镁铁质地壳的顶部几百米有一个高的Mo-98/Mo-95，作为海水蚀变的结果，严格定义的马里亚纳弧熔岩的铅同位素阵列外推到一个流体成分类似于新鲜的太平洋洋中脊玄武岩。这与主要来自高度蚀变的最上部镁铁质地壳或类似印度地幔楔的通量相反。因此，我们推断，铅和钼预算的流体成分占主导地位的贡献，从更深的，较少改变（冷却器）部分的俯冲太平洋地壳。该通量的高Mo-98/Mo-95可能是由脱水过程中的同位素分馏和板中的流体流动引起的。因此，残留的镁铁质地壳变得比它所来自的上地幔更轻。我们的研究结果表明，弧岩浆活动产生的大陆地壳应该有同位素重钼组成的地幔相比，而深循环洋壳的贡献，一些洋岛玄武岩的来源可能是显而易见的同位素轻钼签名。(C)2015 Elsevier B. V.版权所有。
The fate of crustal material recycled into the convecting mantle by plate tectonics is important for understanding the chemical and physical evolution of the planet. Marked isotopic variability of Mo at the Earth's surface offers the promise of providing distinctive signatures of such recycled material. However, characterisation of the behaviour of Mo during subduction is needed to assess the potential of Mo isotope ratios as tracers for global geochemical cycles. Here we present Mo isotope data for input and output components of the archetypical Mariana arc: Mariana arc lavas, sediments from ODP Sites 800, 801 and 802 near the Mariana trench and the altered mafic, oceanic crust (AOC), from ODP Site 801, together with samples of the deeper oceanic crust from ODP Site 1256. We also report new high precision Pb isotope data for the Mariana arc lavas and a dataset of Pb isotope ratios from sediments from ODP Sites 800, 801 and 802. The Mariana arc lavas are enriched in Mo compared to elements of similar incompatibility during upper mantle melting, and have distinct, isotopically heavy Mo (high Mo-98/Mo-95) relative to the upper mantle, by up to 0.3 parts per thousand. In contrast, the various subducting sediment lithologies dominantly host isotopically light Mo. Coupled Pb and Mo enrichment in the Mariana arc lavas suggests a common source for these elements and we further use Pb isotopes to identify the origin of the isotopically heavy Mo. We infer that an aqueous fluid component with elevated [Mo], [Pb], high Mo-98/Mo-95 and unradiogenic Pb is derived from the subducting, mafic oceanic crust. Although the top few hundred metres of the subducting, mafic crust have a high Mo-98/Mo-95, as a result of seawater alteration, tightly defined Pb isotope arrays of the Mariana arc lavas extrapolate to a fluid component akin to fresh Pacific mid-ocean ridge basalts. This argues against a flux dominantly derived from the highly altered, uppermost mafic crust or indeed from an Indian-like mantle wedge. Thus we infer that the Pb and Mo budgets of the fluid component are dominated by contributions from the deeper, less altered (cooler) portion of the subducting Pacific crust. The high Mo-98/Mo-95 of this flux is likely caused by isotopic fractionation during dehydration and fluid flow in the slab. As a result, the residual mafic crust becomes isotopically lighter than the upper mantle from which it was derived. Our results suggest that the continental crust produced by arc magmatism should have an isotopically heavy Mo composition compared to the mantle, whilst a contribution of deep recycled oceanic crust to the sources of some ocean island basalts might be evident from an isotopically light Mo signature. (C) 2015 Elsevier B.V. All rights reserved.