Grain-growth kinetics of ferropericlase at high-pressure

Grain-growth kinetics of (Mg_0.85Fe_0.15)O ferropericlase was investigated at pressures of 5 and 10 GPa and temperatures of 1673-1873 K using a Kawai-type multi-anvil apparatus. Presintered ferropericlase aggregate with an average grain size of 4.0 μm was used as starting material of grain-growth annealing experiments. The grain-growth kinetics of ferropericlase is described by Gⁿ - G₀ⁿ = k₀ t exp [ - (E^* + P V^*) / R T ], where G is the average grain size at annealing time t, G₀ the initial average grain size, P the pressure, R the gas constant and T is the absolute temperature, with n = 2.6 ± 0.4, log₁₀ k₀ = -7.5 ± 1.7 m^2.6/s, E^* = 262 ± 30 kJ/mol, V^* = 4.7 ± 0.7 cm³/mol. Compared at same pressure and temperature, grain-growth rate of ferropericlase is similar to olivine and faster than those of wadsleyite and ringwoodite. The present results show that, at the top of the lower mantle (P = 25 GPa and T = 1773 K) grain size of ferropericlase in single phase system evolves to ∼2 × 10^-2 m after significant geological time (10 My) while a previous study predicts that grain size of MgO in two-phase system is as small as ∼8 × 10^-6 m at same condition.