Water in the Mantle: Effects of Hydration on Physical Properties of Mantel Minerals

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Intellectual merit. Hydrogen is perhaps the most poorly constrained compositional variable in the bulk Earth, and its geochemical behavior at pressure poorly understood. At low pressures (5 GPa) it is highly incompatible, but its compatibility in mantle minerals increases with pressure (depth) so that its solubility in nominally anhydrous silicates increases to percent levels in deeper upper mantle and transition zone. At such concentrations it has a significant effect on elastic properties of these minerals so that it may be possible to detect the presence of hydration in these regions from seismic studies.This project aims to synthesize and measure physical properties of single-crystal samples of hydrous variants of nominally anhydrous high pressure silicates and to measure properties that can be used to constrain the role of hydrogen in the Earth's mantle. A collection of over 50 samples from previous multi-anvil experiments is available for study. Also, collaborative synthesis experiments on omphacite, wadsleyite II, and perovskite will be conducted using 1200 and 5000 ton presses in at Bayerisches Geoinstitut in Bayreuth, Germany. The effects of hydration on the crystal structure and compressibility of forsterite, wadsleyite and ringwoodite will be measured by single crystal X-ray diffraction and oriented, characterized samples will be provided for measurement of elastic properties using Brillouin spectroscopy. The effect of hydration on thermal expansion of olivine, wadsleyite, ringwoodite, and enstatite will be measured at room pressure and temperatures of 100 to 600 K by single crystal X-ray diffraction. Exploratory experiments to measure compressibility at high temperature by powder or single-crystal synchrotron diffraction will be conducted in collaboration with the group at University of Hawaii. Proton positions in these materials will be investigated by single-crystal neutron diffraction at ISIS (Oxford, UK) or SNS (Oak Ridge). The major objective is to provide constraints from mineral physics on the amount and role of H in the Earth's mantle.Broader impacts. This research will promote extensive inter-institutional and international collaboration and result in exchange of ideas and technology across a broad scientific spectrum. This project also will involve undergraduate and graduate in research as an integral part of the PI's teaching program.