How does water move through the subducting slab? Slab-scale fluid pathways and deformation-fluid flow feedbacks at eclogite facies

俯冲带，一个板块俯冲到另一个板块之下，是破坏性地质灾害的位置，包括火山爆发和地球上最大的地震。这些灾害是由含水矿物变质作用释放的水催化的，必须沿着矿脉和韧性变形带等路径传播到地震的源头或熔化。这些路径太小，不能用现代俯冲带的遥感来分辨，但它们的时空分布对俯冲带的危害实行了一级控制。在地球表面挖出并暴露出来的俯冲岩石为研究从微尺度到板块尺度的水输送路径提供了机会。然而，以前对这类岩石的研究集中在小范围内，阻碍了对水在哪里、何时以及如何移动到地质灾害源头的板块尺度的看法。利用暴露在意大利阿尔卑斯山的俯冲板块的完整横截面，该项目将把野外绘制的水路地图与岩石和矿物的化学和结构特征相结合，这些岩石和矿物与经过的水相互作用，以构建水在俯冲板块中如何输送的地图。这张地图将为概念模型提供信息，说明水如何引发俯冲带灾害，并用于解释对现代俯冲带的远程观测。这项工作的更广泛的影响集中在让不同的本科生群体参与动手研究。四名本科生将有机会绘制和收集实地样本，操作科学仪器，收集数据，并在会议上展示他们的发现。为了减少低收入学生参与研究的障碍，将向学生支付研究时间，并支付与实地工作、数据收集和会议相关的所有旅费。这项研究的好处将通过关于当地地质的公开讲座回馈给意大利田野地区周围的社区。水流体调节了关键的俯冲带过程，如地震活动和弧岩浆的产生，以及地球化学循环。这些流体是由俯冲板块中的变质反应产生的，并沿着影响灾害和地球化学循环的位置、时间和规模的路径网络输送到这些过程的位置。对折返俯冲带岩石的地质研究是在单个通道尺度上解决流体输运网络的唯一途径，它们表明脉状和剪切带都可以通过板片输送流体。然而，以前的研究主要集中在单一露头或从属岩性上，阻碍了对板片尺度流体运移以及脉状与剪切带的相对重要性的一致看法。拟议的项目将使用非常暴露的榴辉岩相Monviso蛇绿岩(意大利西阿尔卑斯山)来确定俯冲板块规模的流体输送的主要路径和机制。将使用Monviso蛇绿岩整个板岩横截面上的矿脉和剪切带的详细野外测绘，以约束榴辉岩相下洋壳中流体通道的结构和丰度。结合这一宏观填图，对具有代表性的流体通道进行微观岩石学-地球化学-微结构分析，将重建其流体变形时间历史。将使用常量元素和微量元素(EPMA和激光消融ICPMS)以及原位斜长石氧同位素地球化学(SIMS)来追踪沿流体路径输送的流体的来源。这些流体源将通过热力学模型与整个板坯的脱水反应联系起来。矿物分带的结构分析和现场测量将限制流体沿不同流体路径的相对运移时间，以及单一路径内流体来源的时间分布。流体输送和韧性变形之间的反馈将通过共址原位微结构(EBSD)和地球化学测量来限制，这些测量将定义变形机制并将它们与流体渗透直接联系起来。最终产品将是榴辉岩相板岩水文的时间分辨地图，它将为俯冲板块中流体运输的概念模型及其与灾害和地球化学循环的关系提供信息，并为解释现代俯冲带的地球物理观测提供关键输入。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Subduction zones, where one tectonic plate dives beneath another, are the location of damaging geologic hazards including explosive volcanic eruptions and the largest earthquakes on Earth. These hazards are catalyzed by water that is released by metamorphism of hydrous minerals and must travel to the source of earthquakes or melting along pathways such as veins and zones of ductile deformation. These pathways are too small to resolve with remote sensing of modern subduction zones, but their spatial and temporal distribution exercises a first-order control on subduction zone hazards. Subducted rocks that have been exhumed and exposed at Earth’s surface provide an opportunity to study the pathways transporting water from the micro- to plate-scale. However, previous studies of such rocks have focused at the small scale hindering a plate-scale view of where, when and how water moves to the source of geologic hazards. Using a complete cross-section of a subducting plate exposed in the Italian Alps, this project will combine field mapping of water pathways with chemical and structural characterization of rocks and minerals that interacted with passing water to construct a map of how water is transported in the subducting plate. This map will inform conceptual models of how water can initiate subduction zone hazards and be used to interpret remote observations of modern subduction zones. The broader impacts of this work focus on engaging a diverse group of undergraduate students in hands-on research. Four undergraduate students will have the opportunity to map and collect samples in the field, operate scientific instruments, collect data, and present their findings at conferences. To decrease barriers to participation in research among low-income students, students will be paid for their research time, and all travel expenses associated with field work, data collection and conferences will be covered. Benefits from this research will be returned to the communities around the field area in Italy through public lectures on the local geology. Aqueous fluids modulate key subduction zone processes like seismicity and arc magma production, and geochemical cycling. These fluids are produced by metamorphic reactions in the subducting slab and transported to the locations of these processes along a network of pathways that influences the location, timing, and magnitude of hazards and geochemical cycling. Geologic studies of exhumed subduction zone rocks are the only way to resolve fluid transport networks at the scale of individual pathways, and they suggest that both veins and shear zones can transport fluid through the slab. However, previous studies have largely focused on single outcrops or subordinate lithologies, impeding a cohesive view of slab-scale fluid transport and the relative importance of veins versus shear zones. The proposed project will use the exceptionally well-exposed eclogite-facies Monviso Ophiolite (W. Alps, Italy) to determine the dominant pathways and mechanisms for transporting fluid at the scale of the subducting slab. Detailed field mapping of veins and shear zones in the complete slab cross-section of the Monviso Ophiolite will be used to constrain the structure and abundance of fluid pathways in the eclogite-facies lower oceanic crust. Integrated with this macro-scale mapping, micro-scale petrological-geochemical-microstructural analysis of representative fluid pathways will reconstruct their fluid-deformation-time histories. Major and trace element (EPMA and laser ablation ICPMS), and in situ clinozoisite oxygen isotope geochemistry (SIMS) will be used to track the source of fluids transported along fluid pathways. These fluid sources will be linked to dehydration reactions throughout the slab using thermodynamic modeling. Textural analysis and in situ measurements of mineral zoning will constrain the relative timing of fluid transport along different fluid pathways, and the temporal distribution of fluid sources within a single pathway. Feedbacks between fluid transport and ductile deformation will be constrained using co-located in situ microstructural (EBSD) and geochemical measurements that will define deformation mechanisms and directly link them to fluid infiltration. The final product will be a temporally-resolved map of slab hydrology at eclogite facies that will inform conceptual models of fluid transport through the subducting slab and its relationship to hazards and geochemical cycling, and provide key inputs for interpreting geophysical observations of modern 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.