Ab Initio Evaluation of Point Defects in Bridgmanite Under Lower Mantle Conditions

Abstract

High-pressure experiments and theoretical calculations have indicated that bridgmanite hosts abundant point defects under lower mantle conditions potentially exerting a significant influence on mantle viscosity and conductivity. Vacancies may also act as a sink for impurity elements and a potential reservoir for the noble gases and other volatiles, so understanding of how defect type and abundance change with depth in the mantle is important for the storage and transport of such elements. Here, we performed ab initio calculations to investigate various types of point defects in bridgmanite (MgSiO₃) at conditions of the lower mantle. The defect formation free energies and concentrations of Schottky defect, oxygen vacancy and cation (Mg²⁺) vacancy in bridgmanite were calculated under both MgO-saturated and SiO₂-saturated conditions. Among the three types of defects studied, the Schottky defect has the lowest concentration and the oxygen vacancy is the most abundant with a concentration up to 10⁻². In agreement with recent experiments and previous theoretical studies, the concentrations of these defects decrease strongly with pressure, but the oxygen vacancy concentration exhibits a non-monotonic behavior with depth. Nevertheless, a small population still exists even at deep lower mantle conditions. In addition, the strong temperature dependence of these vacancies means that their concentrations in early forming mantle minerals would be much higher, which could still exist in any primordial domain if diffusion may not be fast enough to remove them. This might have important implications for both the rheology and the storage of noble gases in these regions.