Unique composition and evolutionary histories of low velocity mantle domains

The two `large low velocity provinces' (LLVPs) are broad seismic wave speed anomalies in Earth's lower mantle beneath Africa and the Pacific Ocean. Recent research suggests they represent volumes that contain relatively dense subducted oceanic crust (SOC), but the distribution of recycled material within them is an open question. Using simulations of 3-D global-scale mantle circulation over the past 1 Gyr, we find that simulated large low velocity provinces (S-LLVPs), with similar properties and locations to LLVPs, develop as a consequence of the recycling of SOC. The Pacific S-LLVP is enriched in SOC by up to 53% compared to the African S-LLVP and is therefore more dense, potentially explaining topological differences between the two structures. Shear wave velocity reductions in the two domains are similar due to the dominating influence of temperature over composition. Differences in melting ages between the two S-LLVPs reveal distinct formation histories, where the Pacific S-LLVP is maintained by a steady replenishment of young SOC, while the African S-LLVP comprises older, well mixed material.