Effect of pressure on grain-growth kinetics of ferropericlase to lower mantle conditions

Grain-growth kinetics of (Mg_0.85Fe_0.15)O ferropericlase was investigated at a temperature of 1873 K up to a pressure of 25 GPa under dry conditions using a Kawai-type multi-anvil apparatus. The grain-growth kinetics of ferropericlase is described by Gⁿ-G ₀ⁿ = k₀ exp(-E^*+PV ^*/RT)t where G is the average grain-size at annealing time t; G₀, the initial average grain-size; P, pressure; R, the gas constant; and T, absolute temperature. Least squares fit of this equation for the present data and our previous data on the same material yielded n = 2.8 ± 0.2, k₀ = 10^-8.4±1.1m^2.8/s, E ^* = 273 ± 24 kJ/mol, and V^* = 4.5 ± 0.2 cm³/mol. The present results show that at the conditions at the top of the lower mantle (700 km depth), the grain-size of ferropericlase in a single phase system evolves to ∼1 cm after significant geological time of 1 My. The present results may be applied to regions where single phase ferropericlase aggregates are present, say by deformation-induced phase segregation. In these areas, the grain-growth kinetics is fast so that one expects a large grain-size that will results in strong lattice-preferred orientation causing significant seismic anisotropy.