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 o f the mantle transition zone down to a depth of 1000 km, Seismic observatio ns have revealed a sharp reflector surface at around 900 or 1000 km depth f or which a possible explanation can be given in terms of a new phase transi tion of the lower-mantle constituent minerals, Furthermore, new results fro m the inversion of the oceanic geoid show the existence of a second low vis cosity zone (LVZ) somewhere between 660 and 1000 Bm depth. The existence of the second LVZ may be linked to the mid-mantle phase transitions. The phas e 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 10(7). 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 styl e of mantle convection is still the 660 km endothermic transition, which se ts 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 instabilit ies develop in the second LVZ. When the 1000 km endothermic phase transitio n is included, these instabilities can grow only at a few places but then t hey form strong downwellings. Two distinct types of penetrative, deep downw ellings can be present at the same time: one which crosses the whole transi tion zone, and another one which crosses only the 660 km discontinuity and stops at 1000 km at least temporarily. This can explain seismological obser vations 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. (C) 20 00 Elsevier Science B.V. All rights reserved.