Hydration State of the Transition Zone and Lowermost Mantle

过渡带和最低地幔的水合状态

• 批准号: 1452344
• 负责人: Steven Jacobsen
• 金额: $ 34.35万
• 依托单位: Northwestern University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-01-01 至 2018-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1452344&HistoricalAwards=false
• 关键词: Hydration; State; Transition; Zone; Lowermost

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.

全球水循环通过板块构造与地球内部深处相连。海洋地壳内的含水矿物将水（H2O）的成分带入会聚板块边界的地幔，对熔体生成和火山作用产生影响，作为在地质时期反复将H2O返回地表的循环的一部分。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，某些地幔矿物，如在称为过渡带（410-660公里深）的地层中发现的林伍德石，可能含有地球上重要的-如果不是最大的-H2O地球化学水库。过渡带矿物中结合的水的重量百分比仅为几个百分点，就构成了比海洋中更多的水。确定地幔中H2O的规模和分布对于理解地球化学水循环、深部熔体的产生以及确定地球的组成和地球水的起源具有重要意义。高压和高温下的水合地幔材料，以及NSF Earthscope（USAray）新的和即将进行的区域地震研究限制了地球地幔过渡带水的规模和分布。在深地幔条件下，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）形式结合。含水熔体也可能存在于矿物如硼镁石的H2O储存能力相对较低的深度。实验技术，包括GHz的超声波干涉测量在西北大学和布里渊散射在先进的光子源，阿贡国家实验室，将被用来测量水化作用的地幔矿物，如wadsleyite，ringwoodite和majoritic石榴石的弹性性能的影响，以及硅酸盐玻璃。这些结果将用于建立一个公开的热弹性数据库，该数据库提供了弹性模量及其压力-温度导数等输入，这些输入是对地幔中预期地震速度作为深度、温度和含水量的函数进行正演模拟所需的。结合区域尺度地幔地震研究的观测结果，其结果将被用来推断北美以及随着高分辨率地震数据的出现而在全球范围内的地幔水合状态。