Revisiting the Wasson fractional crystallization model for IIIAB iron meteorites with implications for the interpretation of their Fe isotope ratios

Abstract

AbstractThe trapped melt fractional crystallization model for the IIIAB iron meteorites put forward by J. T. Wasson two decades prior is revisited. The basic precepts upon which the model was based remain true, and the model can be implemented using Ir and Au solid/liquid distribution coefficients that are broadly consistent with experimental data. For this reason, the difference between the Wasson model and some more recent trapped melt models lies mainly with inferences about the S concentrations of the core of the IIIAB iron meteorite parent body. For the Wasson model, S bulk concentrations of about 2 wt% are implied. For the more recent model, much greater concentrations of between about 12–15 wt% are indicated. The two different trapped melt models profoundly influence the interpretation of high δ57Fe values relative to chondrites in the IIIAB irons. The Wasson model suggests that there should be more variations in δ57Fe than are observed among these meteorites, while the more recent trapped melt model relies on the crystallization of FeS from the trapped melt to raise the δ57Fe of the latter, thus minimizing the variability. The interpretation of Fe isotope ratios in the IIIAB meteorites therefore depends critically on the S concentration of the parent body core.