Implications of post-perovskite transport properties for core-mantle dynamics

Recent evidence on perovskite to post-perovskite phase change in the lowermost mantle suggests that post-perovskite piles or lens should be present in the relatively cold downwelling areas, while the roots of the hot upwelling plumes consist of perovskite. Post-perovskite is often believed to be deformed predominantly by the dislocation creep and there are some hints that the activation parameters of the dislocation creep in post-perovskite induce lower viscosity than that of perovskite at the same pressure and temperature conditions. That can even result in a viscosity paradox in the lowermost mantle-viscosity in cold downwellings transformed to post-perovskite might be lower than the viscosity of warm perovskite plumes. Such a viscosity structure was indeed recently suggested by the geodynamical inversions of the geoid. Rheologically weak areas at the base of the subducted slabs may have important consequences for the slab deformation in the D ''. We investigate the dynamics of the cold slabs transformed to post-perovskite in the lowermost mantle by means of the simulations of thermal convection in a 2D Cartesian model of the lower mantle with composite rheology including diffusion creep and dislocation creep. Different creep parameters are used for the perovskite lower mantle and for the post-perovskite lens (or layer) respectively. While perovskite is considered to be deformed via linear diffusion creep, post-perovskite is modeled either as a constant viscosity material (with viscosity ranging from 10(20) Pa s to 10(23) Pa s) or it is deformed purely by the dislocation creep. The presence of the rheologically distinct post-perovskite not only strongly influences the slab deformation above the core-mantle boundary (CMB) but also results in different dynamic regimes of the CMB region (characterised by, e.g., the different length scales of the upwelling plumes) depending on the creep parameters of post-perovskite. Further, the presence of very low viscosity lens or a layer above the CMB strongly enhances the CMB heat flux in the cold slab areas. According to the amount of post-perovskite present in the D '' and the rheology of post-perovskite the heat flux across the CMB can vary considerably with time, which may have important implications for the core-mantle coupling and the character of the Earth's geomagnetic field. (C) 2009 Elsevier B.V. All rights reserved.