Effects of H2O on structural transitions, and thermoelastic and electronic properties of olivine (Mg2SiO4) phases: Implications for deep-Earth seismic discontinuities

Abstract

Olivine (Mg2SiO4), the most abundant silicate phase in Earth's mantle, exhibits distinctive mechanical and electronic properties for various important industrial applications. Geoscientists have predicted pressure-dependent structural phase transformations of olivine (orthorhombic, Pbnm) → wadsleyite (Imma) → ringwoodite (cubic, Fd-3m) to interpret the seismic discontinuities at depths of 410 and 520 km in the mantle transition zone (MTZ). However, the response of the Mg2SiO4 system under extremely hydrous conditions remains largely unexplored. Using ab initio calculations, this study systematically investigates the behavior of the Mg2SiO4 system at elevated pressure (p) and temperature (T) as a function of H2O content. The findings suggest that 1.65 wt. % H2O results in a direct structural phase transition of orthorhombic olivine to cubic ringwoodite at pT = 11.7 GPa and T = 0 K. A further increase in the H2O content lowers the pT value. The present article also examines the thermo-elastic properties of both anhydrous and hydrous olivine phases, showing variations in elastic moduli, seismic wave velocities, and associated impedance during the direct olivine-to-ringwoodite transition. Based on the available data of shear wave velocity, this analysis constrains an upper limit of 3.3 wt. % for water storage in the MTZ. The dynamical stability tests of both anhydrous and hydrous olivine phases predict that all of them are dynamically stable within their respective thermodynamic stability ranges. Another major direction of this article investigates the electronic properties of the aforementioned phases, suggesting that the addition of water up to 3.3 wt. % H2O in them has little influence on their electronic bandgaps.