地球深部水赋存和循环不仅对研究地球演化具有重大意义，对于探索矿产资源和矿床成因从而对国民经济和社会发展也有重大影响. 现有模型认为地幔转换带深度以下的水和俯冲带上含水矿物在不同深度脱水、致水反应后生成的一系列致密含水镁硅酸盐（DHMS）（如Phase A,D,E,和H）有密切联系，其中Phase D有赋存高达20wt%水的潜能。不同于地幔中无水矿物，对于DHMS，目前尚无任何在俯冲带上转换带温压条件下弹性波速（尤其横波）的测量，这一不确定性对通过地震学数据推断水的赋存和循环带来很大难度。本课题的目标是运用先进的超声波和X光衍射技术对三个致密含水镁硅酸盐矿相（Phase A, D和E）在俯冲带深达700公里处温压条件下进行弹性波速和密度原位准确测量，并根据实验测得的波速特征变化和不同俯冲带地震观测结果相比较，从而对地球深部水赋存做定量研究。本研究预计将对揭示地球系统水循环提供非常重要的信息。

Understanding the circulation and storage of water in the deep mantle is of critical importance because it not only proves fundamentally further our knowledge about the evolution and dynamics of the Earth, but also helps discover the reserves and formations of mineral deposits that are of economical and societal values. Based on laboratory studies, water is suggested to be transported into the mantle transition zone and below in subduction zones through a series of mineral phases (e.g., Phase A, D, E, and H), named dense hydrous magnesium silicates (DHMS), whose water capacity can be as high as 20wt%. Unlike anhydrous mantle minerals, the elastic properties of DHMS under simultaneous pressure and temperature conditions of subduction zones have not been experimentally assessed, especially for the shear wave velocities. As a result, the characteristics of the compressional and shear waves for these DHMS phases during their hydration and dehydration are still obscure, which makes it difficult to infer their presence and thus the water storage and circulation through comparisons with seismic velocities in subduction zones. In this project, we propose to conduct simultaneous sound velocities and density measurements by using state-of-the-art ultrasonic and X-ray diffraction measurements in multi-anvil high pressure apparatus. Our main objective is to obtain compressional and shear wave velocities and densities for three DHMS phases, namely, Phase A, D, and E, up to the pressure and temperature of ~700 km depth in subduction zone. These results will allow us for the first time to perform reliable comparisons with seismic data from different regions to provide quantitative constraints on the water content at various depths, which in turn can provide new insights into large scale water cycle in the Earth system.