Interconnectivity of Fe–O–S liquid in polycrystalline silicate perovskite at lower mantle conditions

Abstract

An important aspect of planetary core formation concerns whether interconnectivity of liquid metal can occur in crystalline silicates, which at low melt fractions requires that the dihedral angle between the two phases is <60°. [Shannon, M.C., Agee, C.B., 1998. Percolation of core melts at lower mantle conditions. Science 280, 1059–1061] previously reported that dihedral angles in mantle assemblages decrease from 108° at upper mantle conditions to 71° at lower mantle conditions as a result of mineral phase transformations. Furthermore [Terasaki, H., Frost, D.J., Rubie, D.C., Langenhorst, F., 2005. The effect of oxygen and sulphur on dihedral angle between Fe–O–S melt and solid silicates under high pressure: implications for Martian core formation. Earth Planet. Sci. Lett. 232, 379–392] observed that dihedral angles between Fe–O–S liquid and solid silicates (olivine and ringwoodite) decrease to 66° at high oxygen and sulphur fugacities. Therefore, it may be possible for liquid metal to form an interconnected network at lower mantle conditions at high O and S fugacities. We have investigated the effects of the FeO content of perovskite, with Mg/(Mg + Fe) (Mg#) = 0.84–1.00, on the dihedral angle up to 23.5 GPa and 2223 K. Observed dihedral angles decrease significantly from 102° to 79° with increasing FeO content of the perovskite phase. This tendency is in good agreement with our previous dihedral angle results for olivine and ringwoodite. The dihedral angle is, however, still higher than the critical value of 60° at pressures of the top of the lower mantle, i.e. at this depth efficient core–mantle differentiation is not possible by a percolation mechanism.