Structural and Chemical Flexibility of Hydrous Phases D and H

Abstract:

Two hydrous phases, phase D and the newly described phase H, are stable to the pressures and temperatures of the Earth’s lower mantle, suggesting that the Earth’s lower mantle may participate in the cycling of water. We present the results of ab initio calculations on the stability of hydrous phases D and H, explaining the large pressure- and temperature stability field of these mineral phases. Calculations in the SiO2-δAlOOH-Mg(OH)2 system show that substitutions in octahedral sites are locally charge balanced by hydrogen. This stabilizes the structure relative to the anhydrous assemblage of the Earth’s lower mantle and explains the high-temperature stability of these minerals. Molecular dynamic calculations of these minerals demonstrate the mobility of H in these hydrous solid solutions, suggesting a geophysical means to detect the presence of water in the Earth’s lower mantle.