Post-perovskite MgSiO(3) is believed to be present in the D'' region of the Earth's lowermost mantle. Its existence has been used to explain a number of seismic observations, such as the D'' reflector and the high degree of seismic anisotropy within the D'' layer. Ionic diffusion in post-perovskite controls its viscosity, which in turn controls the thermal and chemical coupling between the core and the mantle, the development of plumes and the stability of deep chemical reservoirs. Here we report the use of first-principles methods to calculate absolute diffusion rates in post-perovskite under the conditions found in the Earth's lower mantle. We find that the diffusion of Mg(2+) and Si(4+) in post-perovskite is extremely anisotropic, with almost eight orders of magnitude difference between the fast and slow directions. If post-perovskite in the D'' layer shows significant lattice-preferred orientation, the fast diffusion direction will render post-perovskite up to four orders of magnitude weaker than perovskite. The presence of weak post-perovskite strongly increases the heat flux across the core-mantle boundary and alters the geotherm. It also provides an explanation for laterally varying viscosity in the lowermost mantle, as required by long-period geoid models. Moreover, the behaviour of very weak post-perovskite can reconcile seismic observation of a D'' reflector with recent experiments showing that the width of the perovskite-to-post-perovskite transition is too wide to cause sharp reflectors. We suggest that the observed sharp D'' reflector is caused by a rapid change in seismic anisotropy. Once sufficient perovskite has transformed into post-perovskite, post-perovskite becomes interconnected and strain is partitioned into this weaker phase. At this point, the weaker post-perovskite will start to deform rapidly, thereby developing a strong crystallographic texture. We show that the expected seismic contrast between the deformed perovskite-plus-post-perovskite assemblage and the overlying isotropic perovskite-plus-post-perovskite assemblage is consistent with seismic observations.

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14The discovery of extra-solar "super-Earth" planets with sizes up to twice that of Earth has 15 prompted interest in their possible lithosphere and mantle dynamics and evolution. Simple 16 scalings suggest that super-Earths are more likely than an equivalent Earth-sized planet to be 17 undergoing plate tectonics. Generally, viscosity and thermal conductivity increase with 18 pressure while thermal expansivity decreases, resulting in lower convective vigor in the deep 19 mantle, which, if extralopated to the largest super-Earths might, according to conventional 20 thinking, result in no convection in their deep mantles due to the very low effective Rayleigh 21 number. Here we evaluate this. First, as the mantle of a super-Earth is made mostly of post-22 perovskite we here extend the density functional theory (DFT) calculations of post-perovskite 23 activation enthalpy of to a pressure of 1 TPa, for both slowest diffusion (upper-bound 24 rheology) and fastest diffusion (lower-bound rheology) directions. Along a 1600 K adiabat 25 the upper-bound rheology would lead to a post-perovskite layer of a very high (~10 30 Pa s) but 26 relatively uniform viscosity, whereas the lower-bound rheology leads to a viscosity increase 27 of ~7 orders of magnitude with depth; in both cases the deep mantle viscosity would be too 28 high for convection. Second, we use these DFT-calculated values in numerical simulations of 29 mantle convection and lithosphere dynamics of planets with up to ten Earth masses. The 30 models assume a compressible mantle including depth-dependence of material properties and 31 plastic yielding induced plate-like lithospheric behavior. Results confirm the likelihood of 32 plate tectonics for planets with Earth-like surface conditions (temperature and water) and 33 show a novel self-regulation of deep mantle temperature. The deep mantle is not adiabatic; 34 instead feedback between internal heating, temperature and viscosity regulates the 35temperature such that the viscosity has the value needed to facilitate convective loss of the 36 radiogenic heat, which results in a very hot perovskite layer for the upper-bound rheology, a 37super-adiabatic perovskite layer for the lower-bound rheology, and an azimuthally-averaged 38 viscosity of no more than 10 26 Pa s. Convection in large super-Earths is characterised by large 39 upwellings (even with zero basal heating) and small, time-dependent downwellings, which for 40 large super-Earths merge into broad downwellings. In the context of planetary evolution, if, 41as is likely, a super-Earth was extremely hot/molten after its formation, it is thus likely that 42 even after billions of years its deep interior is still extremely hot and possibly substantially 43 molten with a "super basal magma ocean" -a larger version of the proposal of (Labrosse et 44 al., 2007), although this depends on presently unknown melt-solid density contrast and 45 solidus. 46

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