Using Barium Isotopes to Investigate the Origin of Fluids in Subduction Zones

俯冲带是地球上的海洋地壳和沉积物（俯冲板块）被引入地球深部的位置，导致与大量火山和地震危险有关的表面突出的弧火山。俯冲带也是地球表面和深部物质转移的主要场所，它控制着长期气候，并在地球热量收支的演变中发挥着关键作用。众所周知，从俯冲板释放的物质赋予弧火山作用独特的化学特征，地球化学证据表明，俯冲沉积物和热液蚀变洋壳（AOC）在弧熔岩生成中起着重要作用。然而，负责运输板材料进入电弧的确切物理过程是最近重大辩论的主题。从本质上讲，一种模式认为，所有俯冲成分在板片顶部混合，形成一个“混杂岩”层，随后成为弧熔岩的主要源区。第二个模型调用的俯冲板片的一般结构是完整的，通过大部分的俯冲过程，只有当沉积物变得足够热融化和AOC脱水这些组件释放到地幔楔和诱导熔融。在这个项目中，研究人员使用一种新的同位素工具来研究这两种模型中哪一种是正确的。这个团队已经确定了元素钡（Ba）及其同位素作为一种地球化学示踪剂，可以用来研究从俯冲板释放的物质的起源，从而，区分认为沉积物熔融和洋壳脱水与混杂岩熔融是岛弧板块物质主要来源的模型熔岩在物理上，两个端元模型的板状物质的运输是非常不同的，有不同的后果的热结构，火山的分布，和化学预算的地壳再循环在俯冲带。然而，这两种模型同样预测了弧熔岩的大多数独特的化学和同位素特征。因此，开发能够区分这两种不同模型的科学测试至关重要。在这两种模型中，流体是板料输运的重要载体。然而，这些流体的最终来源是不同的，在一个模型中，流体来自混杂岩层，而在另一个模型中，流体主要来自AOC。钡是一种流动性很强的移动的元素，在电弧熔岩中比镧和钍等类似的不相容元素表现出特征性富集。这些研究人员认为，Ba同位素可能在混杂岩、沉积岩和AOC来源的流体中显示出不同的值。因此，过量Ba的同位素组成可能会限制携带Ba的流体的最终来源。他们假设，基于Ba同位素的证据可以为他们选择研究的俯冲带中的板状物质运输机制提供新的约束。在更广泛的影响方面，该项目将支持研究生将这项工作纳入他们的论文，并将培训他们使用尖端的地球化学工具。这项研究的结果也将对其他研究控制俯冲带岩浆活动的物理参数的领域产生广泛的兴趣，如地震学家和大地电磁学家，他们使用他们的工具来定位俯冲带中的流体流动。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Subduction zones are locations on Earth where oceanic crust and sediments (the subducted slab) are introduced back into the deep Earth resulting in the prominent arc volcanoes at the surface that are associated with substantial volcanic and earthquake hazards. Subduction zones are also the main places of mass transfer between the surface and deep Earth, which controls long-term climate and plays a critical role in the evolution of Earth's heat budget. It is well-known that material released from the subducted slab imparts distinct chemical signatures to arc volcanism and geochemical evidence suggests that both subducted sediment and hydrothermally altered oceanic crust (AOC) play significant roles in arc lava generation. However, the exact physical processes responsible for transporting slab material into the arc is the subject of significant recent debate. In essence, one model poses that all the subducted components are mixed at the top of the slab, forming a 'melange' layer, which subsequently is the main source region for arc lavas. The second model invokes that the general structure of the subducted slab is intact through a large portion of the subduction process and only when sediments become hot enough to melt and AOC dehydrates are these components released to the mantle wedge and induce melting. In this project, researchers use a novel isotopic tool to investigate which of these two models is correct.This team has identified the element barium (Ba) and its isotopes as a geochemical tracer that can be used to investigate the origin of material released from the subducting slab and, thereby, distinguish between models that invoke sediment melting and ocean crust dehydration versus melange melting as the primary source of slab material in arc lavas. Physically, the two end-member models of slab material transport are very different and have different consequences for the thermal structure, distribution of volcanoes, and chemical budgets of crustal recycling in subduction zones. However, both models equally predict most of the unique chemical and isotopic characteristica of arc lavas. It is, therefore, critical to develop scientific tests that are capable of distinguishing the two different models. In both models, fluids are important vectors of slab material transport. However, the ultimate source of these fluids are different in that fluids are sourced from melange layers in one model and primarily extracted from AOC in the other. Barium is a highly fluid mobile element that displays characteristic enrichment over similarly incompatible elements like lanthanum and thorium in arc lavas. These researchers argue that Ba isotopes likely displays different values in melange, sedimentary and AOC sources of fluids. The isotope composition of the excess Ba is, therefore, likely to constrain the ultimate source of the fluids that carry the Ba. They hypothesize that evidence based on Ba isotopes can provide new constraints on the slab material transport mechanism in the subduction zones they have selected for study. In terms of Broader Impacts, this project will support graduate students who will incorporate this work into their theses, and will train them in cutting-edge geochemical tools. The results of this research will also be of broad interest to other fields that investigate the physical parameters that govern subduction zone magmatism, like seismologists and magnetotelluricists who use their tools to locate fluid flow in subduction zones.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.