Grain size in the lower mantle: constraints from numerical modeling of grain growth in two-phase systems

The lower mantle is believed to deform in the grain size-sensitive creep regime. Analysis of physical processes in the convective mantle suggests that the grain size is probably very small immediately after the phase transformations at 660 km depth and is controlled by subsequent grain growth. It was proposed that the microstructural evolution of two-phase aggregates eventually reaches an asymptotic regime in which grain growth in both phases is coupled due to Zener pinning and obeys a power-law scaling relationship d∝ t^{1/ n}. We performed Monte Carlo simulations for a particular case in which grain growth is controlled by diffusion along grain boundaries ( n=4) and found a good agreement with theoretical predictions. On geological time scales the grain size of (Mg, Fe)-perovskite is controlled by Ostwald ripening of magnesiowüstite and Ca-perovskite. However, the physical parameters are poorly constrained and the grain size remains highly uncertain. If the rate-limiting process is silicon diffusion, the coarsening exponent is likely to be n=3 and the grain size is likely to be 100-1000 μm.