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

俯冲带是地球上海洋地壳和沉积物（俯冲板块）被引入地球深处的位置，导致地表出现突出的弧状火山，这与大量的火山和地震危险有关。俯冲带也是地球表层和深层物质传递的主要场所，它控制着长期气候，对地球热收支的演变起着关键作用。众所周知，俯冲板块释放的物质为弧火山活动赋予了独特的化学特征，地球化学证据表明，俯冲沉积物和热液蚀变洋壳（AOC）在弧熔岩的形成中发挥了重要作用。然而，准确的物理过程负责输送板材料进入弧是重要的最近辩论的主题。从本质上讲，一种模式认为所有俯冲成分在板块顶部混合，形成“混杂岩”层，随后成为弧熔岩的主要源区。第二种模型认为，在俯冲过程的大部分时间里，俯冲板块的总体结构是完整的，只有当沉积物变得足够热而熔化，AOC脱水时，这些成分才会释放到地幔楔中并引起融化。在这个项目中，研究人员使用一种新的同位素工具来调查这两种模型中哪一种是正确的。该团队已经确定元素钡（Ba）及其同位素是一种地球化学示踪剂，可以用来研究俯冲板块释放的物质的来源，从而区分沉积物融化和海洋地壳脱水的模型，以及混合岩融化作为弧熔岩板块物质的主要来源的模型。在物理上，两种板块物质输运的端元模式非常不同，对俯冲带的热结构、火山分布和地壳循环的化学收支有不同的影响。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式的科学测试是至关重要的。在这两种模型中，流体都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，因为在一个模型中流体来源于混杂岩层，而在另一个模型中主要是从AOC中提取的。钡是一种高度流动的可移动元素，在弧熔岩中表现出比类似的不相容元素如镧和钍富集的特征。这些研究人员认为，Ba同位素可能在混杂岩、沉积和AOC流体来源中显示出不同的值。因此，过量钡元素的同位素组成可能限制了携带钡元素的流体的最终来源。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的板状物质输运机制提供新的约束。就更广泛的影响而言，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，比如地震学家和大地电磁学家使用他们的工具来定位俯冲带的流体流动。该奖项反映了美国国家科学基金会的法定使命，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.