Grain growth of ε-iron: Implications to grain size and its evolution in the Earth's inner core

Knowledge of grain growth rate of ε-iron can put constraint on estimation of the grain size in the inner core. We determined grain growth rate of ε-iron at ∼55 GPa and 1200-1500 K by means of in-situ X-ray diffraction observation to be Gn−G0n=kt, where G (m) is the grain size at time t (s), G0 (m) is the initial grain size, n is growth exponent (fixed to 2) and k is the growth constant expressed as k=k0exp⁡(−H⁎/RT) with log k0(mn/s)=−5.8(±2.4) and activation enthalpy H⁎=221(±61)kJ/mol, and R is the gas constant and T is the absolute temperature. Extrapolation of the grain growth law of ε-iron to the inner core conditions suggests that the grain size in the inner core is in a range from several hundred meters to several kilometers, which is intermediate among the previous estimations, and hence the dominant deformation mechanism is considered to be Harper-Dorn creep rather than diffusion creep as pointed out by the previous work. This indicates the relatively uniform viscosity in the entire inner core.