Chemical diffusivity of perovskite and post-perovskite from studies of fluoride analogues.

Abstract:

The physical properties of post-perovskite are thought to strongly influence the dynamics of the D’’ region at the base of the Earth’s mantle, however many of these properties are difficult to directly measure on MgSiO₃ post-perovskite. Ab initio simulations (1) predicted that chemical diffusivity of Mg and Si in post-perovskite MgSiO₃ is significantly anisotropic (with 8 orders of magnitude difference between the fast [001] and slow [010] directions) and with the fast direction some four orders of magnitude faster than diffusion in perovskite. While the simulations reproduced the available experiments on perovskite very well the surprising result for post-perovskite merits testing.

Post-perovskite CaIrO₃ does not show strong anisotropy in Ir-Pt interdiffusivity by either experiment or simulation (2) but the fluoride analogue systems are predicted by ab initio simulations to show very similar anisotropy to MgSiO₃ and the difference between perovskite and post-perovskite are, likewise, similar to the silicate system. Here we present measured and simulated diffusivities for NaXF₃ perovskite and post-perovskite (X = Zn, Mn, Co, Ni).

References:

1) Ammann, M.W., Brodholt, J.P., Wookey, J. and Dobson, D.P. First Principles Constraints on Diffusion in Lower Mantle Minerals and a Weak D’’ Layer. Nature, 465, 462-465, 2010.

2) McCormack, R. Ph.D thesis University College London, 2012.