CAREER: Developing noble gases as tracers of metamorphic dehydration

地球的宜居性是由表面（大气+水圈）和内部储集层（地幔+地核）之间挥发性元素（如水、二氧化碳）的分布所控制的。埋藏期间的变质作用和大洋板块在俯冲带的加热调节了进入地幔的水供应，严重影响了地幔的流动方式。然而，水被输送到弧火山之外的具体途径和通量是不确定的。虽然由于俯冲水的同位素数量少而难以直接追踪，但稀有气体元素和同位素丰富，可通过物理过程分馏，是变质流体运动的理想示踪剂。稀有气体作为深水水循环示踪剂的应用，要求在俯冲过程中发展出稀有气体组成与失水量之间的联系。该项目将系统地描述俯冲海洋地壳古样品中稀有气体同位素的分布和组成，为用作变质流体示踪剂的稀有气体建立框架。本文提出的综合研究和教育计划将导致：1)量化控制挥发物俯冲过程的关键物理过程；2)为早期职业科学家和研究生提供职业发展机会；3)通过一系列针对少数民族服务机构申请人的形成性研究机构，扩大岩石学研究的参与度和多样性。地球的上地幔含有一种不可磨灭的重惰性气体（Ar, Kr, Xe）元素丰度模式，与海水惊人地相似。因为这是太阳系中一种独特的成分，唯一可信的解释是，一种未被破坏的、类似海水的稀有气体特征在俯冲过程中幸存下来，通常被认为排除了95%以上的输入挥发性物质。鉴于惰性气体在流体中的溶解性是地壳矿物的100到10万倍，在变质流体的释放和运输过程中，即使是少量的相分离也会使俯冲岩石中的惰性气体成分发生分异。那么，稀有气体是如何在俯冲过程中逃脱移除和显著分馏的呢？稀有气体可以用来定量地追踪水的俯冲吗？该项目将通过整合稀有气体成分、矿物学结合的H2O含量以及从挖掘出的与俯冲有关的岩石中对压力和温度的热力学估计来解决这些问题。本文提出的研究将系统地验证稀有气体组成主要受俯冲过程中变质脱水程度控制的总体假设。这个职业计划的核心教育部分是开发和实施一系列致力于加强岩石学研究多样性的暑期学院。该项目将培养一批本科生，其中许多人将来自少数民族院校。作为拟议研究计划的一部分，收集的岩石学和地球化学数据集将用于培养来自不同机构和背景的本科生对变质岩石学的兴趣，并增加他们对变质岩石学的理解。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。

The habitability of Earth is controlled by the distribution of volatile elements (e.g. water, carbon dioxide) between surface (atmosphere + hydrosphere) and interior reservoirs (mantle + core). Metamorphism during burial and heating of oceanic plates at subduction zones regulates the supply of water into the Earth’s mantle, critically affecting how the mantle flows. However, the specific pathways and fluxes by which water is transported beyond arc volcanoes are uncertain. While direct tracing of subducted water is made difficult by its small number of isotopes, the noble gases are elementally- and isotopically-rich, are fractionated by physical processes and are ideal tracers for the movement of metamorphic fluids. Application of the noble gases as tracers of the deep water cycle requires development of a linkage between noble gas composition and the magnitudes of water loss during subduction. This project will result in a systematic characterisation of the distribution and composition of noble gas isotopes in ancient samples of subducted oceanic crust, establishing the framework for noble gases to be used as tracers of metamorphic fluids. The integrated research and education program proposed here will result in: 1) quantification of the key physical processes that control subduction-processing of volatiles; 2) career-enhancement for an early career scientist and a graduate student, and 3) broadened participation and enhanced diversity in petrological research via a series of formative research institutes targeting applicants from minority-serving institutions.Earth's upper mantle contains an indelible elemental abundance pattern of heavy noble gases (Ar, Kr, Xe) that is strikingly similar to seawater. Because this is a unique composition in the solar system, the only credible explanation is that a non-disrupted, seawater-like noble gas signature survives the subduction process, generally thought to exclude more than 95% of input volatiles. Given that the noble gases are between 100 and 100,000 times more soluble in fluids than crustal minerals, even minor amounts of phase separation during release and transport of metamorphic fluids will fractionate the noble gas composition of subducting rock. How, then, do noble gases escape removal and significant fractionation during subduction? And, can the noble gases be used to quantitatively trace the subduction of water? This project will address these questions through the integration of noble gas compositions, mineralogically-bound H2O contents and thermodynamic estimates of pressure and temperature from exhumed subduction-related rocks. The proposed research will systematically test the overarching hypothesis that noble gas composition is principally controlled by the extent of metamorphic dehydration during subduction. At the core of the educational component of this CAREER program is the development and implementation of a series of summer institutes dedicated to enhancing diversity in petrological research. This program will result in the petrological training of a cohort of undergraduates, many of whom will come from minority-serving institutions. Petrological and geochemical datasets collected as part of the proposed research program will be used to nurture interest, and increase comprehension of undergraduate students, from a diverse array of institutions and backgrounds, in metamorphic petrology.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.