Crystallization of magmatic iron meteorites: The effects of phosphorus andliquid immiscibility

Magmatic iron meteorites are commonly thought to have formed by fractional crystallization of the metallic cores of asteroid-sized bodies. As fraction al crystallization proceeds, light elements such as P and S become enriched in the molten portion of the core. The light element content of the metall ic liquid influences the partitioning behavior of trace elements and may ca use liquid immiscibility to occur. The elemental trends observed in magmati c iron meteorites may have been affected by both of these processes. We have examined experimentally the effect of P on the solid-metal-liquid-m etal 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 withi n 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, sim ple fractional crystallization calculations can reproduce the large variati on of Re and Os concentrations observed in four magmatic iron meteorite gro ups but have difficulty matching the later crystallizing portions of the tr ends. We have also conducted experiments with three phases-solid metal and two im miscible metallic liquids-to determine the location of the liquid immiscibi lity field near conditions thought to be relevant to magmatic iron meteorit es. Our results show a significantly smaller liquid immiscibility field as compared to the previously published Fe-P-S phase diagram. Our revised phas e diagram suggests that liquid immiscibility was encountered during the cry stallization of asteroidal cores, but much later during the crystallization process than predicted by the previously published diagram.