Effects of low-viscosity post-perovskite on the stability and structure of primordial reservoirs in the lower mantle

We performed numerical experiments of thermochemical convection in 3-D spherical geometry to investigate the effects of a low viscosity of post-perovskite (pPv) on the stability and structure of primordial reservoirs of dense material in the lower mantle of the Earth. Our results show that weak pPv (1000x viscosity reduction in regions containing pPv) strongly increases the core-mantle boundary (CMB) heat flux. The volume-averaged mantle temperature with weak pPv is slightly higher than that with regular pPv, except in the lowermost mantle. This is because weak pPv weakens the base of the cold downwellings, allowing cold slabs to spread more easily and broadly along the CMB. The stability and size of the dense reservoirs is not substantially altered by weak pPv. In the weak pPv case, slabs spreading along the CMB slightly decreases the stability of dense reservoirs, i.e., the amount of dense material entrained upward is slightly larger than in the regular pPv case (i.e., viscosity of pPv identical to that of perovskite). Furthermore, the topography and steepness of these reservoirs slightly increase. However, as in the regular pPv case, the dense reservoirs are maintained over periods of time comparable to the age of the Earth.