The Role of Oxygen Fugacity in Fractionating Parent-Daughter Pairs between Basaltic and Sulfidic Liquids

Abstract:

Here we examine the effect of oxygen fugacity on trace element partitioning between basaltic and sulfidic liquids. We specifically focus on parent-daughter pairs (Sm-Nd, Re-Os, Lu-Hf, Hf-W, U-Pb, and Th-Pb), such that the isotopic effects associated with sulfide fractionation can be predicted. This work is motivated by recent experiments and observations that suggest Earth experienced massive sequestration of a sulfide liquid to its core during the accretion phase, possibly under extremely reduced conditions. Experiments were run in graphite capsules using a piston-cylinder apparatus (1500°C, 1GPa). Starting compositions comprised ~2/3 of a synthetic MORB and ~1/3 FeS by weight. Oxygen fugacity was varied by adding the Fe component of the MORB starting composition as either FeO or FeSi₂. Trace elements were added either as solutions or metal powders. Run durations ranged between one and four hours. The recovered samples were polished using either water or ethanol for lubrication, and then carbon-coated prior to analysis. Major elements were analyzed using a combination of EDS and WDS techniques. Trace element analyses are currently underway.

Experiments with iron added as FeSi₂ have relatively lower concentrations of O in the sulfide, lower concentrations of Fe in the basalt, and higher concentrations of S in the basalt. These same experiments contained sub-micron CaS and MgS phases within the FeS phase. These observations are consistent with the achievement of very low oxygen fugacity for experiments with FeSi₂ added compared to experiments with FeO added. Once trace element partition coefficients are determined, they will be coupled to radiogenic isotope evolution models associated with sulfide fractionation under varying redox conditions.