Collaborative Research: Developing crystal clocks in metamorphic rocks: Using lithium in subduction zone garnets to decipher fluid release timescales

俯冲带是地球的一个板块潜入另一个板块之下的地方。它们是许多自然灾害的发生地，包括圣海伦火山（1980 年）和皮纳图博山（1991 年）等火山喷发，以及日本东北地区（2011 年）等地震和海啸。在俯冲带，水和其他流体的释放在产生致命危险方面发挥着重要作用，但流体流动过程和持续时间尚不清楚。在古代俯冲带深处形成并被带到地表的变质岩可能含有记录此类流体的释放和运动的矿物质。 这些矿物提供了一种直接对俯冲带形成的物质进行采样的方法。在许多变质岩中发现的矿物石榴石在同心生长区中生长一段时间。生长区就像树木的年轮，记录着矿物的地质历史信息。一种称为锂的元素的不同质量（或同位素）的比率在石榴石内的生长区域中发生变化，这些比率的变化可以充当流体流动的“秒表”。这项工作将测量从六个不同地点收集的石榴石中的锂同位素，并使用实验室实验来确定流体流动时间尺度。这项工作的结果将把岩石中记录的流体流动过程与活动俯冲系统中发生的过程（包括不同类型的地震事件）联系起来。拟议的工作包括开发地球科学入门课程的在线迷你课程。通过本课程，学生将探索不同地质环境中的晶体钟，以更好地了解地质过程运行的时间尺度。本提案旨在使用双管齐下的方法确定流体流动时间尺度。第一种方法是调查来自六个不同的俯冲相关地点的自然样本，其中包含流体流动事件的证据。对这些样品中石榴石中锂同位素的测量将解决以下关键问题：1）与俯冲相关的流体是否会重新访问以前的路径以及现有的时间尺度是否反映了流体的单个脉冲或多个脉冲？ 2) 如果流体流动发生在多个脉冲中，这些流体流动事件的周期性程度是多少？ 3) 单个流体脉冲的持续时间是多少？第二种方法是使用旨在确定石榴石中锂扩散率的实验。实验将解决以下关键问题：4）石榴石中锂的扩散率是多少？ 5）氧逸度的作用是什么？将使用两种类型的实验：密封石英管实验，以解决温度和石榴石成分对扩散的影响；气体混合实验，以解决氧逸度对扩散的影响。将扩散实验的结果应用于天然石榴石内的拟议测量，将使我们能够量化问题 3 中描述的流体脉冲的持续时间。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力优点和更广泛的影响审查标准进行评估，被认为值得支持。

Subduction zones are places where one of Earth's tectonic plates dives beneath another. They are the location of many natural hazards, including volcanic eruptions such as Mt St Helens (1980) and Mt Pinatubo (1991), and earthquakes and tsunamis, like Tohoku, Japan (2011). In subduction zones, the release of water and other fluids plays an important role in generating deadly hazards, but fluid flow processes and duration are not well understood. Metamorphic rocks that formed deep in ancient subduction zones and have been brought to the surface can contain minerals that record the release and movement of such fluids. These minerals provide a way to directly sample materials formed in subductions zones. The mineral garnet, found in many of these metamorphic rocks, grows over a period of time in concentric growth zones. Growth zones are like tree rings, recording information about the geologic history of the mineral. Ratios of different masses (or isotopes) of an element called lithium change across growth zones within a garnet and variations in these ratios can act as a “stopwatch” for fluid flow. This work will measure lithium isotopes in garnets collected from six different sites and use laboratory experiments to determine fluid flow timescales. The results of this work will link fluid flow processes recorded in the rocks with processes occurring in active subduction systems, including different types of seismic events. The proposed work includes development of an online mini-lesson for introductory geoscience classes. Through this lesson, students will explore crystal clocks in different geologic settings to better understand the timescales over which geologic processes operate.This proposal aims to determine fluid flow timescales using a two-pronged approach. The first approach is to investigate natural samples from six different subduction-related localities that contain evidence for fluid flow events. Measurements of lithium isotopes across garnets from these samples will address the following key questions: 1) Do subduction-related fluids revisit former pathways and do existing timescales reflect a single pulse or multiple pulses of fluid? 2) If fluid flow occurs in multiple pulses, what is the degree of cyclicity of those fluid flow events? 3) What are the durations of individual fluid pulses? The second approach is to use experiments designed to determine the diffusivity of Li in garnets. The experiments will address the following key questions: 4) What is the diffusivity of Li in garnet? 5) What is the role of oxygen fugacity? Two types of experiments will be used: sealed silica tube experiments to address the effects of temperature and garnet composition on diffusion and gas mixing experiments to address the role of oxygen fugacity on diffusion. The combination of the results from the diffusion experiments applied to the proposed measurements within natural garnet will allow us to quantify the duration of fluid pulses described in question 3.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.