Collaborative Research: Mantle to Crust Fluid Transfer in an Active Flat-slab Subduction Zone - Insights from Peruvian Thermal Spring Geochemistry

流体在与板块构造俯冲带相关的构造过程中起着重要作用，包括造山、火山活动、流体-断层相互作用和地震。关于流体来源和流动路径的大部分已知信息来自于位于沿着俯冲带弧的活火山附近的温泉和洞穴的地球化学表征（例如，环太平洋火山（Pacific Ring of Fire）相比之下，缺乏活火山作用的俯冲带沿着流体的来源和成分并不清楚。该项目采用一套创新的地球化学技术，对秘鲁安第斯山脉的温泉进行研究，调查这种环境中流体的来源和化学性质。安第斯山脉的这一部分成为袭击目标，因为它是当今地球上最好的例子。平板？俯冲作用，表现为火山活动沿着弧的一个突出缺口。在这种地质背景下对流体的研究对于从根本上理解它们在俯冲周期这一阶段的作用非常重要，这项工作可以移植到世界各地的其他古代和现代俯冲带。泉水可能含有高浓度的金属，这项研究将确定对秘鲁公众造成潜在水质问题的泉水系统。除了这些科学和社会影响外，该项目还有助于培养研究生和本科生以及STEM学科的劳动力发展。它促进美国和秘鲁科学家之间的科学合作，并为秘鲁学生提供实地和分析方法的培训机会。该项目还将支持在美国西部举办一个关于水质和地热问题的讲习班。主要研究目标是利用秘鲁温泉的同位素地球化学，绘制地幔和板块衍生挥发物在板块俯冲背景之上地壳中的分布图。这一提议建立在先前的结果的基础上，这些结果在科迪勒拉布兰卡温泉中确定了高达25%的地幔来源的氦，从科迪勒拉布兰卡（南约9°）的地球化学取样断面，向南包括科迪勒拉Huayhuash，一个拟议的“板状撕裂”，纳斯卡山脊，以及向Altiplano高原（南约15°）下方陡峭俯冲的过渡。这些数据测试的主要假设，地幔氦流体以上的秘鲁平板部分板块下沉的纳斯卡海岭和正在进行的板块脱水和流体转移到覆盖岩石圈北部的地球化学表现。第二个目标是通过比较春天的地球化学和古流体地球化学保存在断层岩石中的科迪勒拉布兰卡与科迪勒拉Huayhuash断层/剪切带的作用，将流体转移到表面。将使用多种分析工具来解决这些目标，包括温泉水和气体化学，氦同位素比（3氦/4氦），129碘，流体（碳，氮，氯，氧和氢）和断层岩（云母的氢同位素比）的稳定同位素分析，以及断层岩的流体包裹体分析。该项目解决了关于平板俯冲环境中流体性质的数据缺口，可能确定秘鲁海沟中有俯冲沉积物的部分，并为地球物理调查确定的纳斯卡海脊北部拟议的板状撕裂提供表面地球化学测试。这个项目的成果也可以作为一个现代模拟的作用，流体在古代平板俯冲系统，如法拉隆板在北美西部。

Fluids play a prominent role in tectonic processes associated with plate tectonic subduction zones including mountain building, volcanic activity, fluid-fault interaction, and earthquakes. Most of what is known about fluid sources and flow pathways is from geochemical characterization of hot springs and fumaroles near active volcanoes located along subduction zone arcs (e.g., the Pacific Ring of Fire). In contrast, the source and composition of fluids along subduction zones segments that lack active volcanism is not well known. This project investigates the source and chemistry of fluids in this type of setting using an innovative set of geochemical techniques applied to hot springs in the Peruvian Andes. This part of the Andes is targeted because it is the best example on Earth today of ?flat-slab? subduction, that manifests as a prominent gap in volcanic activity along an arc. Research on fluids in this geological setting is important for the fundamental understanding of their role in this stage of the subduction cycle, and this work is transportable to other ancient and modern subduction zones worldwide. Springs can contain high concentrations of metals, and this research will identify spring systems that pose potential water quality issues for the Peruvian public. Beyond these scientific and societal impacts, the project is contributing to training of graduate and undergraduate students and workforce development in a STEM discipline. It is facilitating scientific collaboration between U.S. and Peruvian scientists, and is providing training opportunities for Peruvian students in field and analytical methods. The project will also support a workshop on water quality and geothermal issues in the western U.S. The primary research goal is to map the distribution of mantle- and slab-derived volatiles in the crust above a flat-slab subduction setting using the isotope geochemistry of thermal springs in Peru. This proposal builds on the prior results that identified up to 25 percent of mantle-derived helium in Cordillera Blanca hot springs with a geochemical sampling transect from the Cordillera Blanca (~9° South), southward including the Cordillera Huayhuash, a proposed 'slab-tear', the Nazca ridge, and the transition to steep subduction beneath the Altiplano plateau (~15° South). These data test the primary hypothesis that mantle helium in fluids above the Peruvian flat slab segment are geochemical manifestations of slab foundering north of the Nazca Ridge and ongoing slab dehydration and fluid transfer to the overriding lithosphere. A secondary goal targets the role of fault/shear zones in transferring fluids to the surface by comparing spring geochemistry and the paleofluid geochemistry preserved in fault rocks in the Cordillera Blanca versus the Cordillera Huayhuash. Multiple analytical tools will be used to address these objectives, including thermal spring water and gas chemistry, helium isotope ratios (3Helium/4Helium), 129Iodine, stable isotope analysis of fluids (carbon, nitrogen, chlorine, oxygen, and hydrogen) and fault rocks (hydrogen isotope ratios of micas), and fluid inclusion analysis of fault rocks. This project addresses the data gap on the nature of fluids in a flat-slab subduction setting, potentially identifying segments of the Peruvian trench that have subducted sediments, and providing a surface geochemical test for a proposed slab tear north of the Nazca ridge identified by geophysical investigations. Outcomes of this project may also serve as a modern analog for the role of fluids in ancient flat-slab subduction systems such as the Farallon slab under western North America.