Evaluation of the aspect ratio of ferropericlase under lower-mantle conditions

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The rheology of the lower mantle is key to understanding the dynamics and evolution of the Earth. The lower mantle consists of bridgmanite (Brg), with smaller amounts of ferropericlase (fPc). The question of which phase controls lower-mantle rheology is essential, because fPc is 2~3 orders of magnitude less viscous than Brg. If fPc grains are isolated, Brg will control the bulk lower-mantle viscosity. If fPc grains are interconnected, fPc will control the bulk lower-mantle viscosity despite its small fraction. The interconnectivity of fPc will occur if fPc grains are elongated. Large total strains on geologic time scales should produce elongated grains. This argument however does not consider that grains will become round over time to decrease surface energy. It is expected that rounding due to surface energy minimization should overcome elongation under the extremely low strain-rate conditions in the lower mantle. In this project, we will evaluate the aspect ratio of fPc at a given strain rate under the physical conditions of the lower mantle to provide necessary data to predict the effective lower-mantle viscosity by numerical modelling. Samples will be deformed by newly developed deformation assemblages with stepped diamond pistons at a pressure of 23 GPa and a temperature of 2000 K in a conventional multi-anvil press to elongate fPc grains. The fPc aspect ratios in the deformed samples will be measured using a scanning electron microscope. From the fPc aspect ratio and bulk shear strain, the elongation rate will be obtained as a function of the strain rate. Samples with elongated fPc grains will then be annealed under quasi-hydrostatic conditions at pressures of 23, 38 and 53 GPa and temperatures of 1800, 2100 and 2400 K for durations of 3 and 30 hours using an ultrahigh-pressure multi-anvil press. The fPc aspect ratios of the annealed samples will then be measured again using a scanning electron microscope again. To evaluate the contribution of dislocation and diffusion creep to rounding, the dislocation density will be determined by transmission electron microscopy. The rounding rate will be formulated according to the aspect ratios before and after annealing, annealing duration, dislocation density, pressure and temperature. We will finally evaluate the fPc aspect ratio under lower-mantle conditions based on this information.