Effects of thermal, compositional and rheological properties on the long-term evolution of large thermochemical piles of primordial material in the deep mantle

Seismic tomography observations have shown that there are two large low shear velocity provinces (LLSVPs) above the core-mantle boundary beneath Africa and the Pacific. The thermal and compositional properties of these two LLSVPs may differ from those of the ambient mantle, and they are suggested to be thermochemical piles of primordial material in the lower mantle. Their evolution is of great importance to our understanding of mantle dynamics. In this study, we systematically conducted numerical experiments to investigate the effects of the buoyancy ratio (B), compositional viscosity ratio (Delta(eta c)), and heat-producing ratio (Lambda) of the primordial material on the long-term evolution of thermochemical piles. Our results show that the buoyancy ratio plays the most important role in the stability of these piles. When the buoyancy ratio is small, and the primordial material is enriched in heat-producing elements (Lambda>1), the stability of these piles decreases with increasing compositional viscosity ratio or heat-producing ratio. For cases with homogeneous heat production (Lambda=1), the stability of these piles increases with increasing Delta(eta c). We further compare constant internal heating with radioactive decay internal heating, and find that the long-term stability of thermochemical piles slightly decreases with radioactive decay heating, but the overall differences between these two internal heating modes are relatively small.