Some mineral physics constraints on the rheology and geothermal structure of Earth's lower mantle

We explore the implications of recent mineral physics measurements of diffu sion coefficients and melting temperatures of lower mantle materials on the rheological and geothermal structure of Earth's lower mantle. We show that MgSiO3 perovskite is significantly stronger than MgO periclase and therefo re the rheology of the lower mantle depends strongly on the geometry of a w eaker phase, periclase. We calculate viscosities of the lower mantle for tw o cases: (1) where periclase occurs as isolated grains and (2) where pericl ase occurs as continuous films, using mineral physics data and models of tw o-phase rheology. We find that the effective viscosity for the former is ab out similar to 10-1000 times higher than the latter. We therefore suggest t hat the rheology of the lower mantle is structure- and hence strain-depende nt. leading to weakening at large strains due to the formation of continuou s films of periclase. Overall depth variation of viscosity depends not only on the pressure dependence of creep but also on the geothermal gradient. B oth MgSiO3 perovskite and periclase have relatively small activation energi es (E-. = gRT(m) with g = 10-14, where R is the gas constant and T-m is mel ting temperature), and therefore the depth variation of viscosity is rather small, even for a nearly adiabatic temperature gradient. However, the geot hermal gradients consistent with the geodynamical inference of nearly depth -independent viscosity are sensitive to the pressure dependence of viscosit y which is only poorly understood. A superadiabatic gradient of up to simil ar to0.6 K/km is also consistent with mineral physics and geodynamical obse rvations.