A molecular dynamics study of grain boundary diffusion in MgO

Abstract

Molecular dynamics simulations are carried out on polycystalline periclase (MgO) to determine the structure and diffusivity at grain boundaries for pressures and temperatures relevant to Earth’s mantle. As temperature increases, the grain boundary structure becomes more disordered, with more ions having incomplete coordination and the system occupying regions of the energy landscape with shallower energy mimima. In contrast, as pressure increases the grain boundary structure becomes more ordered. The grain boundary diffusivity as a function of temperature and pressure can be understood in terms of these structural changes. At atmospheric pressure, the grain boundary diffusion coefficients for Mg and O extrapolate with increasing temperature to the values for the melt, indicating that the dynamics in the grain boundary are similar to those of a supercooled liquid. Just as in a supercooled liquid, diffusion in the grain boundary slows down with decreasing temperature for two reasons: there is less energy to surmount energy barriers, and the barriers are larger due to the more ordered structure. As pressure increases from zero pressure, the diffusivities first decrease sharply, due to the increase in energy barriers associated with the more ordered system, and then more gradually as pressure increases beyond ∼4 GPa. At conditions relevant to Earth’s core-mantle boundary region (135 GPa, 3500 °C), no diffusion is observed on the timescale of ∼50 ns, and the diffusion coefficients are thus constrained to be <2 × 10−13 m2/s. This upper limit is considerably smaller than values obtained from experimental measurements at lower pressures and temperatures, suggesting that grain boundary transport at the core-mantle boundary is less efficient than has been inferred previously.