Formation of Large Low Shear Wave Velocity Provinces (LLSVP) by Entrainment and Segregation of Iron and Silicates at the Core-Mantle Boundary.

Global seismology observes massive lower-mantle plume conduits known as LLSVPs which are up to 800 km in diameter and may extend vertically more than 2000 km from the core-mantle boundary, however the origin of these structures is unknown. Geochemical studies suggest these large structures may have a primordial signature ~4.5 Gyr old. Here we propose that LLSVPs may be formed as a product of planetary accretion events. We suggest that molten silicates and light elements formed within a magma ocean during meteorite impacts, are entrained by liquid iron descending to the core, and later migrate slowly out of the core. Here we perform precision laboratory fluid experiments to measure a thin film layer of glucose solution surrounding liquid metal gallium drops. Our experiments suggest that metal drops may entrain higher volumes of silicates with increasing viscosity magmas and drop size. We find that liquid metal drops entrain 1.0% less than solid metal spheres. To better understand the physical thin film entrainment, we consider boundary layer theoretical analysis. Preliminary analysis predicts that the velocity profile controls the volume of material entrained but is variable within the thin film layer, depending directly on drop radius but inversely on fluid viscosity. Recent studies show that upward migration of light elements entrained in the core is slow, occurring over 0.5 - 5 Gyr (Fleck et al., 2018) and may still be releasing from the core today. Our preliminary results suggest that this core formation model for LLSVP growth could entrain 5 - 40% of the volume of a descending iron drop into the core. If we assume 20% of the core volume entrained light elements within film layers surrounding emulsified metal diapirs, our model suggests the volume of silicates migrating out of the core today may be at least 4.0% of the total mantle volume which is consistent with the volume of LLSVPs observed by global seismic tomography in the mantle today.