Growth kinetics of MgSiO3 perovskite reaction rim between stishovite and periclase up to 50 GPa and its implication for grain boundary diffusivity in the lower mantle

The growth rate of MgSiO3 perovskite reaction rims between periclase and stishovite was investigated at 24–50 GPa and 1650–2150 K using a Kawai-type high-pressure apparatus. The textural observations of the recovered samples and rim growth kinetic data revealed that the reaction is controlled by coupled grain boundary diffusion of MgO and grain coarsening in the perovskite reaction layer. Assuming a high diffusivity of O compared with Mg, the grain boundary diffusivity of Mg in the perovskite was determined to be δ D g b Mg [ m 3 / s ] = 10 − 15.1 exp { − [ 176 , 000 + ( P − 24 ) × 3.8 × 10 3 ] / RT } , which is ∼3–5 orders of magnitude faster than that of Si. We found that the bulk diffusivity of Mg in polycrystalline perovskite is affected by the grain boundary when we consider the possible grain sizes and temperatures in the lower mantle. Accordingly, grain boundary diffusion in perovskite may be an effective mechanism for chemical transportation of divalent cations in the lower mantle.