Partitioning of iron between magnesian silicate perovskite and magnesiowüstite at about 1 Mbar

The disproportionation products of olivine Fo₉₀ in diamond-anvil cell experiments at about 1 Mbar are magnesian silicate perovskite Mg# ~94 and a series of magnesiowüstites Mg# 85-90, yielding a recommended value of 0.4 ± 0.1 for the distribution coefficient that defines the exchange of Mg and Fe²⁺ between the two phases (K_D). The distinctive compositional trends which would signify that ferric iron had been stabilised during experiments are lacking. Pressure does not exert any significant effect on distribution behaviour, despite thermodynamic predictions of major reallocation of Fe²⁺ from magnesiowüstite into and in favour of perovskite, towards the base of the lower mantle. Nor does the presence of Al³⁺ in magnesian silicate perovskite in amounts appropriate for the lower mantle substantially modify distribution behaviour, and hence phase chemistry, provided the oxygen content of the system remains essentially constant. Wood's [Earth Planet. Sci. Lett. 174 (2000) 341] multi-anvil study has been used to link K_D with temperature and to calculate that magnesian silicate perovskite Mg# 93.0±0.6 would coexist with magnesiowüstite Mg# 82.6±0.9 just beyond the 660 km discontinuity (1800 K), whilst at the core-mantle boundary (2650 K) the corresponding values would be 91.9 ± 0.4 and 84.9 ± 0.8.