致密含水镁硅酸盐（DHMS）在俯冲带和转换带中的弹性波速以及地球深部水赋存和循环的研究

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.

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。

由于水对地球深部的岩石、矿物、岩浆等物质的物理化学性质起着重要的作用，近年来深部水循环主题的研究工作在高温高压地学界开展得如火如荼，发表了大量的文章，尤其是关于普通含水矿物及名义上不含水矿物的稳定性、状态方程、弹性性质等。然而，对于可以将水带到更深的地幔过渡带甚至下地幔的致密含水硅酸镁硅酸盐（DHMS）则由于缺少天然矿物、合成温压条件较高、很难合成纯的物相等实验困难而研究较少。有些工作是用金刚石压腔结合同步辐射X射线衍射得到状态方程及体积模量，较少高温高压下原位的波速测量，尤其缺乏横波波速的数据。本项目基于以上考虑想要利用超声干涉法在大压机上测量高压DHMS相中的相A、D和E的波速，但是由于项目中期变更项目负责人、疫情影响国际合作与交流等种种原因本项目最终研究了以下几个方面内容：（1）超声干涉法测量了23Å相、含氟及钛的斜硅镁石和含氟黄玉0-14 GPa压强下的声速，高温下X射线粉末衍射法得到了斜硅镁石和黄玉的热膨胀系数；根据以上矿物（相）的数据结合前人的其他矿物的数据探讨了其对俯冲板块地震速度的影响；（2）开展了碳酸盐化泥质岩在5.5–15.5 GPa下的熔融实验研究，揭示了鸡蛋相能够作为一个可能的含水相随着板片俯冲到地球深部而并不发生分解，在地球深部水循环过程中起着较为重要的作用，同时讨论了其对富碱碳酸岩起源以及深部水和碳循环的影响；（3）研究了文石和方解石族碳酸盐的高温相变和热膨胀性质；（4）归纳总结了硅酸盐熔体状态方程的理论模型和实验测定方法并写了一篇综述文章分别以中、英文发表于《中国科学：地球科学》；（5）进行了富碱含水硅酸盐玻璃的超声波速研究，探讨了弹性性质、密度、水的偏摩尔体积和水溶解机理。以上研究均围绕着地球深部水循环、碳循环及氟循环，了解以上矿物（相）的性质对于理解如何将H2O、CO2、F等挥发分从俯冲带输送回地球深部的过程提供重要启示。