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构造板块俯冲到另一个构造板块之下的地方。它们是许多自然灾害的所在地，包括火山爆发，如圣海伦山（1980年）和皮纳图博山（1991年），以及地震和海啸，如日本东北部（2011年）。在俯冲带，水和其他流体的释放在产生致命的危险方面起着重要的作用，但流体流动过程和持续时间还没有得到很好的理解。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这种流体释放和运动的矿物。 这些矿物提供了一种直接对俯冲带中形成的物质进行取样的方法。在许多这些变质岩中发现的矿物石榴石，在一段时间内在同心生长带中生长。生长带就像树木的年轮，记录着矿物的地质历史。石榴石中一种叫做锂的元素的不同质量（或同位素）的比率在生长带之间发生变化，这些比率的变化可以作为流体流动的“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的时间尺度。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联系起来，包括不同类型的地震事件。拟议的工作包括为地球科学入门课程开发一个在线微型课程。通过本课，学生将探索不同地质环境中的水晶钟，以更好地理解地质过程的时间尺度。本提案旨在使用双管齐下的方法确定流体流动时间尺度。第一种方法是调查自然样本从六个不同的俯冲相关的地方，包含流体流动事件的证据。从这些样品中测量石榴石的锂同位素将解决以下关键问题：1）与俯冲有关的流体是否会重新访问以前的路径，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，那么这些流体流动事件的周期性程度是什么？3)单个液体脉冲的持续时间是多少？第二种方法是使用设计的实验来确定锂在石榴石中的扩散率。实验将解决以下关键问题：（4）锂在石榴石中的扩散系数是多少？5)氧逸度的作用是什么？将使用两种类型的实验：密封的石英管实验，以解决温度和石榴石组合物对扩散和气体混合实验的影响，以解决氧逸度对扩散的作用。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，并被认为值得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来支持。

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.