Formation of Lower Mantle Heterogeneity due to Grain Size-Dependent Viscosity

Geochemical observations indicate that the Earth's mantle is highly heterogeneous. In the absence of convective layering, alternative models for survival of mantle heterogeneity are necessary. Previous studies pointed out that high viscosity regions are mixed very little by convection. In this study, we investigate the role of grain size-dependent viscosity in generation of high viscosity regions in the lower mantle. We develop convection models in which viscosity is a power law function of grain size. This corresponds to the diffusion creep regime which is supported by the lack of seismic anisotropy in most of the lower mantle. Grain size is controlled by phase transitions and grain growth. Phase transitions result in grain size reduction while grain growth increases grain size during circulation of material between phase boundaries and is described with a power law function of time. We show that high viscosity, poorly mixed regions form around centers of convective circulation below the spinel-perovskite phase transition where material never crosses a phase boundary and only undergoes grain growth. The effects of initial conditions and convective vigor on formation and preservation of mantle heterogeneity are explored.