Effect of the mid-mantle viscosity and phase-transition structure on 3D mantle convection

Recent geophysical evidence shows some sort of layering in the lower part of the mantle transition zone down to a depth of 1000 km. Seismic observations have revealed a sharp reflector surface at around 900 or 1000 km depth for which a possible explanation can be given in terms of a new phase transition of the lower-mantle constituent minerals. Furthermore, new results from the inversion of the oceanic geoid show the existence of a second low viscosity zone (LVZ) somewhere between 660 and 1000 km depth. The existence of the second LVZ may be linked to the mid-mantle phase transitions. The phase and viscosity stratification of the transition zone have been included in a series of 3D convection simulations in a 4 X 4 X 1 rectangular box with a surface Rayleigh number of 2 X 10⁷. Beneath the well-known 400 and 660 km phase changes, we assumed a hypothetical weak endothermic transition at 1000 km in some of our models. The principal controlling factor of the style of mantle convection is still the 660 km endothermic transition, which sets up a partial or full barrier to flow, causing stratified circulation. We used various viscosity profiles with emphasis on the model containing the second LVZ. The main consequence of this zone is to enhance flow layering. Many plumes can emanate from the transition zone and small-scale instabilities develop in the second LVZ. When the 1000 km endothermic phase transition is included, these instabilities can grow only at a few places but then they form strong downwellings. Two distinct types of penetrative, deep downwellings can be present at the same time: one which crosses the whole transition zone, and another one which crosses only the 660 km discontinuity and stops at 1000 km at least temporarily. This can explain seismological observations which suggest that subducted slabs can be retarded not only by the 660 km boundary but also by some deeper obstacle near 1000 km depth.