Thermochemical piles in the lowermost mantle and their evolution

Two Large Low Shear-wave Velocity Provinces (LLSVPs) lie beneath Southern Africa and Southern Pacific Ocean. These LLSVPs may be thermochemical in origin. Studying the dynamic evolution of these thermochemical piles will help us understand the earth’s present structure and history. We investigate the entrainment and evolution of an initially isolated high viscosity thermochemical pile in detail. The entrainment rate of the dense pile increases with time until it being totally entrained. The pile’s survival time does not monotonically varies with its viscosity. The chemical pile obstructs the horizontal flow along CMB and turns it into upwelling flow. This may help explain the observation of plume generation zones. The morphology and location of the dense pile oscillate with time. A high viscosity pile can keep its location and morphology roughly unchanged for hundreds of millions years if the convective structure in the surrounding mantle keeps steady.