Water in garnet as a record of fluid flow during metamorphism

水是生命的基本成分，但它在地球深处的存在也有重大影响。地球上的水影响着岩石的变形和融化，它在地震、火山和具有经济价值的金属的集中等对社会至关重要的现象中起着重要作用。在地球深处10公里处形成的变质岩提供了这些过程的快照，对这些岩石和其中的矿物质的研究可以更好地理解水是如何影响这些重要现象的。矿物石榴石通常在变质岩中发现，它在同心生长带中生长一段时间。这些生长区域就像树木的年轮，记录了矿物经历的历史信息，而不仅仅是最后的快照。测量石榴石不同区域的含水量可以作为地壳深处水的释放和流动的记录。这项工作将使用测量五个不同地区石榴石的含水量来寻找证据，证明在石榴石生长过程中发生了多少流动事件，以及它们可能持续了多长时间。这项工作的结果将允许在岩石中记录的水流与产生地震活动的过程和具有经济价值的金属集中之间建立联系。拟议的工作还将包括在马里兰日期间向当地社区展示水在地球上的重要性，这一天马里兰大学向公众敞开大门。本研究旨在结合其他流体流动示踪剂（O同位素、Li同位素、石榴石中的微量元素浓度）对石榴石中的水进行原位测量，以研究石榴石生长过程中流体释放和流体流动事件的过程，并限制单个流体流动事件的循环性和持续时间。该项目将调查五个不同地点的石榴石，每个地点都有流体释放或流体流动事件的记录证据。对来自不同地质环境的变质岩的研究表明，流体流动事件的持续时间范围不同，其中一些非常迅速，从几周到几百年不等。这些估计是基于同位素和岩石学示踪剂的大块岩石测量，因此代表了时间集成测量。变质岩的许多特征表明，流体流动是循环的，可能会重游路径，在这种情况下，估计的持续时间可能会高估流体流动的持续时间，而低估流体流动事件的数量。提出的工作解决了变质过程中流体释放和流体流动的基本问题。将石榴石中的氢与其他流体流动示踪剂的耦合测量，充分探索将石榴石中的氢作为流体流动和流体释放事件示踪剂的适用性。所提出的工作将限制流体输送的性质、循环性和持续时间，有助于理解变质环境中的流体流动过程。了解流体流动的循环性和时间尺度是很重要的，原因有很多——流体传递热量和质量，驱动变质反应，影响流变学，并有助于地震活动性。这项工作也将更好地理解H在石榴石中的行为和结合。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。

Water is an essential ingredient for life, but its presence in the deep Earth also has significant impacts. Water in the Earth affects how rocks deform and melt and it plays a role in phenomena critical to society including earthquakes, volcanoes, and the concentration of metals of economic value. Metamorphic rocks that form 10s of km deep in the Earth provide a snapshot of these processes, and study of these rocks and the minerals in them provides a better understanding of how water affects these important phenomena. The mineral garnet is commonly found in metamorphic rocks, and it grows over a period of time in concentric growth zones. These growth zones are like tree rings, recording information about the history experienced by the mineral rather than just a final snapshot in time. Measurements of water contents in different zones of a garnet can act as a recorder of the release and flow of water deep in the crust. This work will use measurements of the water content of garnets from five different field localities to look for evidence for how many flow events occurred during garnet growth and how long they may have lasted. The results of this work will allow for connections to be made between the flow of water recorded in the rocks with processes generating seismicity and concentrating metals of economic value. The proposed work will also include a component of outreach to the local community showing the importance of water in the Earth during Maryland Day, when the University of Maryland opens its doors to the general public.This proposal seeks to use in situ measurements of H2O in garnet in conjunction with other tracers of fluid flow (O isotopes, Li isotopes, trace element concentrations in garnet) to investigate processes of fluid release and fluid flow events during the growth of garnet and to constrain the cyclicity and duration of individual fluid flow events. This project will investigate garnets from five different localities, each of which has documented evidence for either fluid release or fluid flow events. Studies of metamorphic rocks from a variety of geologic settings show a range of durations of fluid flow events, some of which are quite rapid, on the scale of weeks to hundreds of years. These estimates are based on bulk-rock measurements of isotopic and petrologic tracers, and therefore represent a time-integrated measurement. Many features in metamorphic rocks suggest that fluid flow is cyclic and may revisit pathways, in which case the estimated durations may overrepresent the duration of fluid flow and underestimate the number of fluid flow events. The proposed work addresses fundamental questions about fluid release and fluid flow during metamorphism. The applicability of using H in garnet as a tracer of fluid flow and fluid release events will be fully explored by coupling measurements of H in garnet with other tracers of fluid flow. The proposed work will constrain the nature, cyclicity, and duration of fluid transport, aiding in the understanding of fluid flow processes in metamorphic settings. Understanding the cyclicity and timescales of fluid flow is important for a variety of reasons – fluids transport heat and mass, drive metamorphic reactions, impact rheology, and contribute to seismicity. This work will also provide a better understanding of the behavior and incorporation of H in garnet.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.