Wetting of mantle olivine by sulfide melt: implications for Re/Os ratios in mantle peridotite and late-stage core formation

Abstract

This study investigates the effects of variations in the relative fugacities of oxygen and sulfur on the wetting of mantle olivine by molten sulfide. Experiments were performed on mixtures of San Carlos olivine and synthetic FeS at 1 bar and 1350°C. Crucibles were fabricated from San Carlos olivine, and the fugacities of oxygen and sulfur were controlled by mixing CO 2, CO, and SO 2 gases. Experimental conditions ranged from log f O 2 =−7.9 to −10.3 and from log f S 2 =−1.5 to −2.5. Our experimental results demonstrate that, at a given temperature and pressure, the olivine–sulfide melt dihedral angle is controlled by the concentration of O dissolved in an anion-rich melt. Trace amounts of O dissolve in sulfide melt at f O 2 conditions near the iron–wüstite oxygen buffer and the dihedral angle is 90°. At f O 2 conditions near the fayalite–magnetite–quartz oxygen buffer the concentration of dissolved O is near 9 wt% and the dihedral angle is 52°, allowing small amounts of sulfide melt to form an interconnected network in olivine-rich rocks and to migrate via porous flow. These results indicate that sulfide melt is likely to be mobile at current upper mantle f O 2 and f S 2 conditions. In mantle peridotite, the addition or removal of sulfide melt by porous flow will variably fractionate Re/Os, U/Pb, and Th/Pb ratios because Os and Pb are more chalcophile than Re, U, and Th. The Re/Os ratio of the peridotite is especially sensitive to this process. The mobility of sulfide melt at oxidizing conditions implies that the addition of oxidized chondritic material during the later stages of the accretion of the Earth may have facilitated the segregation of core-forming material by porous flow if temperatures were in excess of the sulfide solidus.