Study of water solubility in bridgmanite at the lower mantle conditions

几乎所有的矿物都可以在其结构中至少吸收少量的水，尽管不同矿物的吸水能力不同。由于地幔主导矿物的巨大质量，即使矿物中非常小的水溶解度也可能在地球内部产生大量的水水库。例如，在过渡区主要矿物（硅镁石和林伍德石）中2-3重量%的水溶解度导致地球内部的这一部分中的水储存能力相当于地球表面上的两个半海洋。了解不同矿物的水溶性有助于人们了解俯冲过程、深水循环过程中带入地球内部的水的去向及其对相应地区地震波传播的影响。上地幔矿物和过渡带矿物的水溶性研究比较深入，而下地幔优势矿物--桥镁石的水溶性研究，特别是压力和水逸度的影响研究非常有限。该项目将系统地研究硼镁石晶体结构中的吸水能力作为压力和水逸度的函数。研究活动将主要在佛罗里达国际大学，一个少数民族服务机构（MSI）和美国大陆最大的西班牙裔服务机构（HSI）进行。研究的进展将通过PI教授的课程向本科生和研究生介绍，主题是地球内部有多少海洋，以吸引少数民族学生对科学，技术，工程和数学（STEM）的兴趣。这种自然的联系已经被证明是非常有效的PI吸引学生成为参与本科生和/或独立研究的研究经验。该项目将确定在水的浓度计算的一般公式中的参数硼镁石。样品将由氧化物和氢氧化物（SiO2、MgO、FeO、Al 2 O3和Mg（OH）2）的混合物合成，含水量饱和。将使用具有硬质碳化钨和烧结金刚石砧的多砧压机来生产用于水浓度表征的具有大晶体尺寸的样品。傅里叶变换红外光谱法（FTIR）和二次离子质谱法（西姆斯）将用于确定合成样品中的水含量，最大限度地减少由于可能包含H2O流体或羟基轴承相的不确定性。该项目将确定地幔深部水储存能力的特征，为地球动力学建模提供关键的矿物物理数据，从而增进我们对地球深部内部地球物理和地球化学的认识和理解。该项目利用多砧装置的新发展，使用烧结金刚石砧达到远高于下地幔顶部的压力。相对于金刚石对顶砧单元（DAC）实验的较大样品尺寸将确保回收样品中水浓度的分析表征的质量。FTIR和SMIS的对比分析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.