Crystallization of magmatic iron meteorites: The effects of phosphorus and liquid immiscibility

Abstract

Abstract— Magmatic iron meteorites are commonly thought to have formed by fractional crystallization of the metallic cores of asteroid‐sized bodies. As fractional crystallization proceeds, light elements such as P and S become enriched in the molten portion of the core. The light element content of the metallic liquid influences the partitioning behavior of trace elements and may cause liquid immiscibility to occur. The elemental trends observed in magmatic iron meteorites may have been affected by both of these processes.We have examined experimentally the effect of P on the solid‐metal‐liquid‐metal partitioning behavior of Ag and Pd, Re and Os, two element pairs used to date iron meteorite processes. Phosphorus has no effect on the partition coefficient of either Ag or Pd, which are incompatible and identical within experimental error. Compatible Re and Os also have identical partitioning behavior, within experimental error, and show increasing compatibility in the solid metal with increasing P content of the metallic liquid. Including the effects of both S and P on the partitioning behavior of Re and Os, simple fractional crystallization calculations can reproduce the large variation of Re and Os concentrations observed in four magmatic iron meteorite groups but have difficulty matching the later crystallizing portions of the trends.We have also conducted experiments with three phases—solid metal and two immiscible metallic liquids—to determine the location of the liquid immiscibility field near conditions thought to be relevant to magmatic iron meteorites. Our results show a significantly smaller liquid immiscibility field as compared to the previously published Fe‐P‐S phase diagram. Our revised phase diagram suggests that liquid immiscibility was encountered during the crystallization of asteroidal cores, but much later during the crystallization process than predicted by the previously published diagram.