Dry (Mg,Fe)SiO3 perovskite in the Earth's lower mantle

Abstract

AbstractCombined synthesis experiments and first‐principles calculations show that MgSiO3‐perovskite with minor Al or Fe does not incorporate significant OH under lower mantle conditions. Perovskite, stishovite, and residual melt were synthesized from natural Bamble enstatite samples (Mg/(Fe + Mg) = 0.89 and 0.93; Al2O3 < 0.1 wt % with 35 and 2065 ppm weight H2O, respectively) in the laser‐heated diamond anvil cell at 1600–2000 K and 25–65 GPa. Combined Fourier transform infrared spectroscopy, X‐ray diffraction, and ex situ transmission electron microscopy analysis demonstrates little difference in the resulting perovskite as a function of initial water content. Four distinct OH vibrational stretching bands are evident upon cooling below 100 K (3576, 3378, 3274, and 3078 cm−1), suggesting four potential bonding sites for OH in perovskite with a maximum water content of 220 ppm weight H2O, and likely no more than 10 ppm weight H2O. Complementary, Fe‐free, first‐principles calculations predict multiple potential bonding sites for hydrogen in perovskite, each with significant solution enthalpy (0.2 eV/defect). We calculate that perovskite can dissolve less than 37 ppm weight H2O (400 ppm H/Si) at the top of the lower mantle, decreasing to 31 ppm weight H2O (340 ppm H/Si) at 125 GPa and 3000 K in the absence of a melt or fluid phase. We propose that these results resolve a long‐standing debate of the perovskite melting curve and explain the order‐of‐magnitude increase in viscosity from upper to lower mantle.