Study of water solubility in bridgmanite at the lower mantle conditions

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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.