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

Abstract

Abstract Combined synthesis experiments and first‐principles calculations show that MgSiO 3 ‐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; Al 2 O 3 < 0.1 wt % with 35 and 2065 ppm weight H 2 O, 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 H 2 O, and likely no more than 10 ppm weight H 2 O. 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 H 2 O (400 ppm H/Si) at the top of the lower mantle, decreasing to 31 ppm weight H 2 O (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.