THE EFFECT OF AL2O3 ON FE-MG PARTITIONING BETWEEN MAGNESIOWUSTITE AND MAGNESIUM SILICATE PEROVSKITE

Abstract

We have measured the partitioning of Fe2+, Fe3+ and Mg2+ between magnesiowustite and magnesium silicate perovskite in both Al2O3-bearing and Al2O3-free systems. Starting compositions with varying Fe/(Fe+Mg) molar ratios were equilibrated in a multianvil apparatus between 24 and 25 GPa and 1650-1900°C. A time study indicated that run durations of at least 8 h were required for the partitioning to reach equilibrium at 1650°C. Multi-chamber rhenium metal capsules were employed that provided relatively oxidising conditions and allowed up to eight starting materials to be equilibrated in a single experiment. In the Al2O3-free system Fe partitions preferentially into magnesiowustite. In the Al2O3-bearing system, however, the proportion of Fe that partitions into perovskite increases with the perovskite Al2O3 content. The Fe-Mg distribution between the phases has been parameterised as a function of the perovskite Al2O3 concentration and we observe the Al2O3 dependence to be non-linear. The Fe3+/ΣFe ratios of coexisting perovskite and magnesiowustite crystals were measured using electron energy-loss near-edge structure (ELNES) spectroscopy. In the Al2O3-bearing system perovskite Fe3+/ΣFe ratios range between 60 and 80%, which is over three times greater than that measured for Al2O3-free perovskite. The results imply that increasing the Al2O3 content of perovskite only increases the Fe3+ solubility and has no measurable effect on the Fe2+ partitioning between magnesiowustite and perovskite. The variation in the Fe3+ solubility with the Al3+ content of perovskite is non-linear. We propose that this results from substitution of Fe3+ onto the perovskite six-fold coordinated site for low Al3+ concentrations but substitution of a FeAlO3 component at higher Al3+ contents, where Fe3+ is on the eight-fold site charge balanced by Al on the six-fold coordinated site. A comparison with our experimental data suggests that the Fe partitioning between some (Mg,Fe)SiO3 and (Mg,Fe)O inclusions found in diamonds is quite consistent with their origin as perovskite and magnesiowustite in the lower mantle. The partitioning between some (Mg,Fe)O and (Mg,Fe,Al)(Al,Si)O3 diamond inclusions, however, is inconsistent with our results.