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

俯冲带是一个构造板块俯冲到另一个板块之下的地方，是具有破坏性地质灾害的地方，包括爆炸性火山爆发和地球上最大的地震。这些危险是由含水矿物变质作用释放的水催化的，这些水必须沿着矿脉和韧性变形带等路径流向地震或融化的源头。这些路径太小，无法用现代俯冲带的遥感来解决，但它们的时空分布对俯冲带的危害起着一级控制作用。被挖掘出来并暴露在地球表面的俯冲岩石为研究水从微观尺度向板块尺度的输送途径提供了机会。然而，先前对这类岩石的研究集中在小尺度上，阻碍了对水在何时何地以及如何流向地质灾害源头的板块尺度的观察。该项目利用暴露在意大利阿尔卑斯山的俯冲板块的完整横截面，将水通道的实地测绘与与水流相互作用的岩石和矿物的化学和结构特征结合起来，构建水在俯冲板块中如何运输的地图。这张地图将为水如何引发俯冲带危险的概念模型提供信息，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吸引不同群体的本科生动手研究。四名本科生将有机会在野外绘制和收集样本，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，将支付学生的研究时间，并支付与实地工作，数据收集和会议相关的所有旅行费用。这项研究的收益将通过当地地质的公开讲座回馈给意大利野外地区周围的社区。含水流体调节了地震活动、弧岩浆产生和地球化学循环等关键俯冲带过程。这些流体是由俯冲板块的变质反应产生的，并沿着影响危险的位置、时间、程度和地球化学循环的路径网络输送到这些过程的地点。对潜没带岩石的地质研究是在单个通道尺度上解决流体输送网络的唯一途径，它们表明脉状带和剪切带都可以通过板块输送流体。然而，以前的研究主要集中在单个露头或次级岩性上，阻碍了对板级流体输送的整体看法，以及脉体与剪切带的相对重要性。拟议的项目将利用非常暴露的榴辉岩相Monviso蛇绿岩（W. Alps, Italy）来确定在俯冲板块规模上输送流体的主要途径和机制。Monviso蛇绿岩完整板截面上的脉体和剪切带的详细野外填图将用于约束榴辉岩相下洋壳的流体通道的结构和丰度。结合这种宏观尺度的测绘，对具有代表性的流体路径进行微观尺度的岩石学-地球化学-微观结构分析，将重建它们的流体-变形时间历史。主要和微量元素（EPMA和激光烧蚀ICPMS）和原位斜沸石氧同位素地球化学（SIMS）将用于追踪沿流体路径输送的流体来源。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原位测量将限制流体沿不同流体路径运移的相对时间，以及单一路径内流体源的时间分布。利用同位原位微结构（EBSD）和地球化学测量，流体输运和韧性变形之间的反馈将受到约束，这些测量将定义变形机制，并将其与流体渗透直接联系起来。最终成果将是一张榴辉岩相的临时解析板水文图，它将为通过俯冲板的流体输送及其与危险和地球化学循环的关系的概念模型提供信息，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, 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.