Growth kinetics of forsterite reaction rims at high-pressure

•Growth kinetics of forsterite reaction rims under deep upper mantle conditions.•Mg and O are the diffusing species.•Mg grain boundary diffusion coefficient in forsterite was determined.•Significance of Mg grain boundary diffusion increases with depth in the mantle. Growth kinetics of forsterite (Fo) reaction rims between periclase (Per) and enstatite (En) were studied experimentally at pressure (P) and temperature (T) conditions of 3.0–11.1GPa and 1473–1873K, respectively. Pt markers originally placed at the Per-En interface were always observed at the Per-Fo interface, which indicates that Mg and O are the diffusing species in Fo rim growth (Mg-O coupled diffusion). The presence of some En inclusions in Fo grains and the growth rate of the Fo rim suggests that grain boundary diffusion is dominant rather than lattice diffusion. Considering the very fast grain boundary diffusion of O in olivine, the Mg-O coupled grain boundary diffusion in Fo is deduced to be rate-limited by the diffusivity of Mg. Based on an analysis of data collected under dry conditions, the product of the Mg grain boundary diffusion coefficient (Dgb) and the effective grain boundary width (δ) was determined to be δDgb=δDgb,0exp[−(E∗+PV∗)/RT] with δDgb,0=10−9.68±1.51m3/s, E∗=379±44kJ/mol and V∗=−1.9±1.4cm3/mol. Our results, combined with previously reported data on Mg lattice diffusion in Fo, suggest that for Mg, the significance of grain boundary diffusion increases with depth in the Earth’s upper mantle, although lattice diffusion is still dominant for typical mantle grain sizes of 1–10mm.