Study of water solubility in bridgmanite at the lower mantle conditions

几乎所有的矿物质在其结构中都能吸收至少少量的水，尽管吸收水分的能力因矿物而异。由于地幔优势矿物的巨大质量，即使水在矿物中的溶解度很小，也可能在地球内部形成一个巨大的水库。例如，水在过渡带主要矿物（瓦德斯莱石和环伍德石）中的溶解度为2- 3%，这导致地球内部这部分地区的储水量相当于地球表面两个半海洋的储水量。了解不同矿物中水的溶解度，有助于人们了解通过俯冲作用进入地球内部的水的命运、深水循环及其对相应区域地震波传播的影响。虽然对上地幔和过渡带矿物的水溶性研究较多，但对下地幔优势矿物桥辉石的相关研究，特别是对压力和水逸性的影响研究甚少。该项目将系统地研究桥菱石晶体结构中吸水能力与压力和水逸度的关系。研究活动将主要在佛罗里达国际大学，少数民族服务机构（MSI）和美国大陆最大的西班牙裔服务机构（HSI）进行。PI将以“地球内部有多少海洋”为主题，通过授课向本科生和研究生介绍研究进展，以引起少数民族学生对科学、技术、工程、数学（STEM）的兴趣。事实证明，这种自然联系对于PI吸引学生参与本科生研究经历和/或独立研究非常有效。本工程将确定桥菱石含水浓度计算通式中的参数。样品将由水含量饱和的氧化物和氢氧化物（SiO2, MgO, FeO， Al2O3和Mg(OH)2）的混合物合成。多砧压力机与硬质碳化钨和烧结金刚石砧将被用来生产样品具有大晶体尺寸的水浓度表征。傅里叶变换红外光谱（FTIR）和次级离子质谱（SIMS）将用于测定合成样品中的含水量，最大限度地减少由于可能包含H2O流体或含羟基相而产生的不确定性。该项目将描述深部地幔的储水能力，为地球动力学建模提供关键的矿物物理数据，从而提高我们对地球深部内部地球物理和地球化学的认识和理解。该项目利用了多砧装置的新发展，利用烧结金刚石砧达到远高于下地幔顶部的压力。金刚石砧细胞（DAC）实验中较大的样本量将确保回收样品中水浓度分析表征的质量。利用FTIR和SMIS进行对比分析将有助于区分晶体包裹体中的结构结合羟基和分子水。

Nearly all minerals can take at least small amount of water in their structure although the capability of water uptake varies from mineral to mineral. Due to the huge mass of mantle dominant minerals, even very small solubility of water in the minerals may produce a massive water reservoir in Earth's interior. For example, 2-3 weight % solubility of water in the transition zone major minerals (wadsleyite and ringwoodite) results in a capability of water storage in this part of Earth's interior equivalent to two and a half total oceans on Earth's surface. Knowledge about the water solubility in difference minerals helps people understand fate of water brought into Earth's interior by subduction processes, deep water cycling and its influence on seismic wave propagation in the corresponding area. While water solubility in the upper mantle and transition zone minerals are well investigated, related studies on the lower mantle dominant mineral, bridgmanite, especially the influence of pressure and water fugacity, are very limited. This project will systematically study the capacity for water uptake in the bridgmanite crystal structure as a function of pressure and water fugacity. The research activities will be carried out mainly at Florida International University, a Minority Serving Institution (MSI) and the largest Hispanic Serving Institution (HSI) in mainland US. Progress of the research will be introduced to the undergraduate and graduate students through classes taught by the PI under the topic of how many oceans are there inside the Earth to draw the interest in science, technology, engineering, and mathematics (STEM) among students of underrepresented minorities. Such a natural connection has been proved very effective for the PI to attract students to become involved in Research Experiences for Undergraduates and/or Independent Research.The project will determine the parameters in the general formula in water concentration calculation for bridgmanite. The samples will be synthesized from a mixture of oxides and hydroxide (SiO2, MgO, FeO, Al2O3 and Mg(OH)2) with saturated water content. A multi-anvil press with hard tungsten carbide and sintered diamond anvils will be used to produce samples with large crystal size for water concentration characterizations. Both Fourier transform infrared spectroscopy (FTIR) and secondary ion mass spectrometry (SIMS) will be used for determining the water contents in the synthesized specimens, minimizing the uncertainty due to possible inclusions of H2O fluid or hydroxyl bearing phases. The project will characterize the water storage capability in the deep mantle, provide critical mineral physics data for geodynamic modeling, and therefore advance our knowledge and understanding in geophysics and geochemistry of Earth's deep interior. The project takes the advantage of new developments in the multi-anvil apparatus to reach pressures far higher than that at the top of the lower mantle using sintered diamond anvils. The larger sample size with respect to diamond anvil cell (DAC) experiments will ensure the quality of analytical characterization of water concentration in the recovered specimens. Comparative analyses using FTIR and SMIS will help for distinguishing the structural bound hydroxyls from molecular water in crystal inclusions.