Influences of dissipation and rheology on mantle plumes coming from the D″-layer

We have investigated the possibilities for thermochemical convective instability of a low viscous, dense D″-layer heated from below, in either the extended Boussinesq approximation (i.e. with dissipation) or the Boussinesq approximation only. We find, that the compositional excess density in the D″-layer, required for its stratification at the core-mantle boundary is greatly overestimated in the Boussinesq approximation (Di=0, R_ρ≈0.8). In the extended Boussinesq approximation the stabilizing excess density is far less (Di=0.3, R_ρ=0.3). Therefore, in a dissipative mantle, a strongly stratified D″-density profile is unlikely, because it would stabilize the D″-layer and inhibit the development of large mantle plumes. A too low viscosity drop, greater than 10², of the D″-layer would lead to stratification of the layer and small-scale convection taking place inside it.