Hydration State of the Transition Zone and Lowermost Mantle

全球水循环通过板块构造与地球深处的内部联系在一起。海洋地壳中的含水矿物将水的成分（H2O）带到汇聚的板块边界处的地幔中，对熔体的产生和火山活动产生影响，作为一个循环的一部分，该循环在地质时期内反复将H2O返回地表。水循环延伸到地幔的深度尚不清楚。然而，越来越多的证据表明，在过渡带（410-660公里深）中发现的某些地幔矿物，如环woodite，可能含有地球上一个重要的——如果不是最大的——地球化学储层。在过渡带的矿物质中，只要有几个重量百分比的水，就会构成比海洋中更多的水。确定地幔中水的规模和分布对认识地球化学水循环、深部熔体的产生以及确定地球水的组成和起源具有重要意义。本研究结合了高压高温下实验室生长的水合地幔物质的矿物物理实验，以及来自美国国家科学基金会地球望远镜（USArray）的新的和即将进行的区域地震研究，以限制地球地幔过渡带中水的规模和分布。在深部地幔条件下，水不再以熟悉的液体形式存在，而是通过高压硅酸盐晶体结构中的电荷耦合化学取代以羟基（OH）结合。含水熔体也可能存在于诸如桥菱石之类的矿物的水储存能力相对较低的深度。实验技术包括西北大学的ghz -超声波干涉测量和阿贡国家实验室先进光子源的布里渊散射，将被用来测量水化对地幔矿物（如瓦德利石、环伍德石和多数石榴石）弹性特性的影响，以及对硅酸盐玻璃的影响。这些结果将用于建立一个公开可用的热弹性数据库，该数据库提供弹性模量及其压力-温度导数等输入，用于正演模拟地幔中作为深度、温度和含水量函数的预期地震速度。结合区域尺度地幔地震研究的观测结果，随着高分辨率地震数据的出现，这些结果将用于推断北美和全球范围内的地幔水化状态。

The global water cycle is connected to the Earth's deep interior through plate tectonics. Hydrous minerals within oceanic crust carry the components of water (H2O) into the mantle at convergent plate boundaries, having influence on melt generation and volcanism as part of a cycle that returns H2O to the surface repeatedly over geologic time. How deeply the water cycle extends into the Earth's mantle is not known. However, evidence is mounting that certain mantle minerals such as ringwoodite, found in a layer called the transition zone (410-660 km depth), may contain a significant -if not the largest- geochemical reservoir of H2O in the planet. The presence of just a few weight percent H2O bound in minerals of the transition zone would constitute more water than is present in the oceans. Determining the scale and distribution of H2O in the mantle has implications for understanding the geochemical water cycle, deep melt generation, and determining the Earth's composition and origin of Earth's water.This study combines mineral physics experiments on laboratory-grown, hydrated mantle materials at high pressures and high temperatures with new and forthcoming regional seismic studies emanating from NSF's Earthscope (USArray) to constrain the scale and distribution of water in the Earth's mantle transition zone. At deep mantle conditions, water is no longer found in the familiar liquid form, but rather bound as hydroxyl (OH) through charge-coupled chemical substitutions in the crystal structure of high-pressure silicates. Hydrous melts may also be present at depths where the H2O storage capacity of minerals such as bridgmanite is relatively low. Experimental techniques including GHz-ultrasonic interferometry at Northwestern University and Brillouin scattering at the Advanced Photon Source, Argonne National Laboratory, will be employed to measure the influence of hydration on the elastic properties of mantle minerals such as wadsleyite, ringwoodite, and majoritic garnet, as well as on silicate glasses. Those results will be used to build a publically-available thermoelastic database, which provides input such as elastic moduli and their pressure-temperature derivatives needed to forward model expected seismic velocities in the mantle as a function of depth, temperature, and water content. Combined with observations from regional-scale seismic studies of the mantle, the results will be used to infer the mantle hydration state beneath North America and more globally as high-resolution seismic data become available.