Comparative Simulation Study of Vacancy Diffusion in MgO and MgSiO3 in Very High Pressures

MD simulations with pairwise potential on MgO and MgSiO3 vacancies pair were conducted to obtain the Newtonian viscosity of lower mantle. The results of MgO are such that the viscosity of the upper half portion in the lower mantle shows uniform structure but in the lower half it rapidly decreases to the core mantle boundary by two order of magnitude. On the other hand, the viscosity of MgSiO3 perovskite decreases gradually with increasing pressure in the whole lower mantle conditions. To clarify mineral controlling the rheology of the deep lower mantle, we compare the diffusion coefficients of slowest atoms of MgO and MgSiO3 in the lower mantle conditions. The results indicate that the viscosity of the whole lower mantle is governed by lattice diffusion of MgO. Thus, the actual lower mantle rheology should also show the viscosity lowering in the deepest lower mantle.