Sulfur contents of the parental metallic cores of magmatic iron meteorites

Abstract

Magmaticiron meteorites are thought to be samples of the central metallic coresof asteroid-sized parent bodies. Sulfur is believed to have been animportant constituent of these parental cores, but due to the lowsolubility of S in solid metal, initial S-contents for the magmaticgroups cannot be determined through direct measurements of the ironmeteorites. However, experimental solid metal-liquid metal partitioncoefficients show a strong dependence on the S-content of the metallicliquid. Thus, by using the experimental partition coefficients to modelthe fractional crystallization trends within magmatic iron meteoritegroups, the S-contents of the parental cores can be indirectlyestimated. Modeling the Au, Ga, Ge, and Ir fractionations in four ofthe largest magmatic iron meteorite groups leads to best estimates forthe S-contents of the parental cores of 12 ± 1.5 wt% forthe IIIAB group, 17 ± 1.5 wt% for the IIAB group, and 1± 1 wt% for the IVB group. The IVA elementalfractionations are not adequately fit by a simple fractionalcrystallization model with a unique initial S-content. These S-contentestimates are much higher than those recently inferred fromcrystallization models involving trapped melt. The discrepancy is duelargely to the different partition coefficients that are used by thetwo models. When only partition coefficients that are consistent withthe experimental data are used, the trapped melt model, and the lowS-contents it advocates, cannot match the Ge and Ir fractionations thatare observed in IIIAB ironmeteorites.