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

This study investigates the effects of variations in the relative fugacitie s 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 degrees C. Crucibles were fabricated from San Carlos olivine, and the fugacities of oxygen and sulfur were controlled by mixing CO2, CO, and SO2 gases. Experimental conditions ranged from log f(O2) = -7. 9 to -10.3 and from log f(S2) = -1.5 to -2.5. Our experimental results demo nstrate that, at a given temperature and pressure, the olivine-sulfide melt dihedral angle is controlled by the concentration of O dissolved in an ani on-rich melt. Trace amounts of O dissolve in sulfide melt at f(O2) conditio ns near the iron-wustite oxygen buffer and the dihedral angle is 90 degrees . At f(O2) conditions near the fayalite-magnetite-quartz oxygen buffer the concentration of dissolved O is near 9 wt% and the dihedral angle is 52 deg rees, allowing small amounts of sulfide melt to form an interconnected netw ork in olivine-rich rocks and to migrate via porous flow. These results ind icate that sulfide melt is likely to be mobile at current upper mantle f(O2 ) and f(S2) conditions. In mantle peridotite, the addition or removal of su lfide 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 mobil ity of sulfide melt at oxidizing conditions implies that the addition of ox idized chondritic material during the later stages of the accretion of the Earth may have facilitated the segregation of core-forming material by poro us flaw if temperatures were in excess of the sulfide solidus. (C) 1999 Els evier Science B.V. All rights reserved.