Textural equilibria of iron sulfide liquids in partly molten silicate aggregates and their relevance to core formation scenarios

Abstract

The influence of the coexistence of silicate and iron sulfide melts on the wetting of mantle olivine by iron sulfide melt is investigated. The dihedral angle between iron alloy and olivine is unaffected by the presence of silicate melt at 1.5 GPa and 1370–1410°C and is much greater than the maximum allowable dihedral angle for melt connectivity at low melt fractions. In experimental charges where silicate and sulfide melt occupy the same melt pocket, the sulfide melt adheres to olivine grain faces rather than forming discrete droplets. The relative interfacial energy between iron sulfide and basaltic silicate melt is lower than that of iron sulfide with olivine, but the contrast is not sufficient for basaltic silicate melt to wet FeS‐olivine grain boundaries. This result implies that very large silicate melt fractions are necessary to mobilize iron sulfide melts in partially molten silicate phase assemblages. Therefore segregation of Fe‐rich metallic melts by porous flow at low melt fraction is not a likely process for metal separation. The experiments also investigated the influence of the presence of a thermal gradient on sulfide melt migration. Although the sulfide melt can not migrate through the olivine network by porous flow, evidence in the experiments indicates mobility of sulfide through the interconnected silicate melt network. The sulfide melt pools in contact with silicate melt at the hot, top part of the experimental charges grow over time at the expense of the silicate melt pools in the cooler part of the charge. This points to a diffusive transport mechanism for local sulfide redistribution. We review literature data on metal‐olivine wetting angles and find that the metal composition has a dominant influence on the wetting behavior of a metallic liquid on an olivine matrix. Iron‐rich metallic liquid compositions are characterized by large dihedral angles (>90°). The metal‐olivine dihedral angle decreases with increasing amounts of dissolved light element (S, C, O). For light element contents in excess of ∼59 at %, the dihedral angle approaches 60°, and the wetting behavior of a metallic liquid is similar to that of a silicate liquid.