Water in garnet as a record of fluid flow during metamorphism

水是生命的重要成分，但它在地球深处的存在也具有重大影响。地球中的水影响岩石的变形和融化，并在地震、火山和具有经济价值的金属富集等对社会至关重要的现象中发挥着重要作用。地球深处数十公里处形成的变质岩提供了这些过程的快照，对这些岩石及其中的矿物质的研究可以更好地了解水如何影响这些重要现象。石榴石矿物常见于变质岩中，并且在同心生长带中生长一段时间。这些生长区就像树木的年轮，记录着矿物经历的历史信息，而不仅仅是时间的最终快照。对石榴石不同区域含水量的测量可以作为地壳深处水的释放和流动的记录器。这项工作将使用来自五个不同现场地点的石榴石含水量测量来寻找证据，以了解石榴石生长过程中发生了多少次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和富集具有经济价值的金属的过程之间建立联系。拟议的工作还将包括在马里兰日期间向当地社区宣传水在地球中的重要性，当时马里兰大学向公众敞开大门。该提案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 H2O 进行现场测量，以研究石榴石生长期间的流体释放过程和流体流动事件，并限制周期性以及各个流体流动事件的持续时间。该项目将调查来自五个不同地点的石榴石，每个地点都记录了流体释放或流体流动事件的证据。对各种地质环境下的变质岩的研究显示了一系列流体流动事件的持续时间，其中一些非常快，规模为数周至数百年。这些估计基于同位素和岩石示踪剂的大块岩石测量，因此代表了时间积分测量。变质岩中的许多特征表明流体流动是循环的并且可能会重新访问路径，在这种情况下，估计的持续时间可能高估了流体流动的持续时间并低估了流体流动事件的数量。拟议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。通过将石榴石中的 H 的测量与其他流体流动示踪剂相结合，将充分探索使用石榴石中的 H 作为流体流动和流体释放事件示踪剂的适用性。拟议的工作将限制流体传输的性质、周期性和持续时间，有助于理解变质环境中的流体流动过程。出于多种原因，了解流体流动的周期性和时间尺度非常重要——流体传输热量和质量、驱动变质反应、影响流变学并促进地震活动。这项工作还将提供对 H 在石榴石中的行为和掺入的更好理解。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力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.