Variations in Grain Size and Viscosity Based on Vacancy Diffusion in Minerals, Seismic Tomography and Geodynamically Inferred Mantle Rheology

Abstract:

The effect of grain size on mantle viscosity is comparable to that of temperature and pressure. The current 3-D distribution of grain size in the mantle is, however, unknown. To explore the possible variability of grain size, we use the following: geodynamic inferences of effective viscosity, vacancy diffusion rates in upper mantle minerals and perovskite in the lower mantle, lateral variations in temperature derived from seismic images, and different geotherms. An important outcome of this modelling is a new mapping of lateral viscosity variations throughout the mantle. We find a correlation between grain size variability in the mantle and absolute changes with depth. We also find that lateral grain size variations give rise to reduced lateral viscosity variations. The corresponding 3-D variations in grain size are characterized by 2 orders of magnitude changes. Our results suggest that traditional models of 3-D variability in mantle viscosity based only on temperature-dependent vacancy diffusion, ignoring grain size variations, provides an incomplete description of the microphysics that determines its effective bulk viscosity.