Investigating ULVZ morphology associated with increased mineralogical grain-size of thermochemical piles resulting in higher intrinsic diffusion-creep viscosity

Earth's lowermost mantle contains two large regions of lower than average shear wave velocity beneath Africa and the central Pacific, known as the Large Low Shear Velocity Provinces (LLSVPs). Seismic evidence infers that LLSVPs may differ in density and composition from the background mantle, resulting in large-scale thermochemical mantle convection. Among several competing hypotheses as to the cause of LLSVPs, one model proposes the existence of dynamically-stable and long-lived thermochemical piles at the mantle's base. Although seismic tomography alone is unable to fully distinguish thermochemical piles from other models, Ultra-Low Velocity Zones (ULVZs)—tiny, seismically-detected patches of decreased seismic velocities residing in the lowermost 10km of the mantle, may provide constraints. Earlier geodynamical calculations (McNamara et al., 2010) found that if ULVZs are caused by a small volume of ultra-dense material, they are expected to accumulate along the margins of thermochemical piles, with an asymmetrical shape (thicker outboard and thinner inboard of the piles). However, these initial calculations assumed that piles have the same temperature-dependent rheology as background mantle. If piles have a larger mineral grain size than the background mantle, the diffusion creep viscosity of the piles would increase. Earlier work (McNamara and Zhong., 2004) has shown that this leads to the development of a viscous rind along the interior pile surface. Here, we explore whether such an increase in intrinsic pile-viscosity changes the ULVZ locations (with respect to piles), shape, thickness, and interaction with the pile material as compared to previous work that simply utilized a uniform temperature-dependent rheology.