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沿俯冲带弧（例如，太平洋火环）的活火山附近的温泉和喷气孔的地球化学特征。相反，在缺乏活火山作用的俯冲带段，流体的来源和组成尚不清楚。该项目利用一套创新的地球化学技术，在秘鲁安第斯山脉的温泉中研究了这类环境中流体的来源和化学成分。安第斯山脉的这一部分之所以成为目标，是因为它是当今地球上“平板”的最佳例证。俯冲作用，表现为沿弧的火山活动出现明显的间隙。在这一地质背景下对流体的研究对于了解它们在这一阶段俯冲旋回中的作用具有重要意义，并且这项工作可推广到世界上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构成潜在水质问题的泉水系统。除了这些科学和社会影响之外，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。它正在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方面的培训机会。该项目还将支持一个关于美国西部水质和地热问题的研讨会。主要研究目标是利用秘鲁温泉的同位素地球化学，绘制出平板俯冲环境上方地壳中地幔和板块衍生挥发物的分布图。这一建议建立在先前的结果的基础上，通过对科迪勒拉布兰卡（南纬~9°）的地球化学采样样例，在科迪勒拉布兰卡温泉中发现了高达25%的地幔衍生氦，向南包括科迪勒拉华华，一个被提议的“板裂”，纳斯卡山脊，以及在Altiplano高原（南纬~15°）下向陡峭俯冲的过渡。这些数据验证了秘鲁平板段上方流体中的地幔氦是纳斯卡山脊以北板块沉降和板块脱水以及流体向上覆岩石圈转移的地球化学表现的初步假设。第二个目的是通过对比白垩纪科迪勒拉和华华纪科迪勒拉断裂岩中保存的泉水地球化学和古流体地球化学，研究断裂/剪切带在流体向地表输送中的作用。将使用多种分析工具来实现这些目标，包括温泉水和气体化学、氦同位素比率（3Helium/4Helium）、129碘、流体（碳、氮、氯、氧和氢）和断层岩石（云母的氢同位素比率）的稳定同位素分析，以及断层岩石的流体包裹体分析。该项目解决了平板俯冲环境中流体性质的数据缺口，有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山脊以北的拟议板块撕裂提供地表地球化学测试。该项目的结果也可以作为古代平板俯冲系统中流体作用的现代模拟，例如北美西部的法拉龙板块。

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.