The effect of Ni on element partitioning during iron meteorite crystallization

Iron meteorites exhibit a large range in Ni concentrations, from only 4% to nearly 60%. Most previous experiments aimed at understanding the crystallization of iron meteorites have been conducted in systems with about 10% Ni or less. We performed solid metal/liquid metal experiments to determine the effect of Ni on partition coefficients for 20 trace elements pertinent to iron meteorites. Experiments were conducted in both the end-member Ni-S system as well as in the Fe-Ni-S system with intermediate Ni compositions applicable to high-Ni iron meteorites. The Ni content of the system affects solid metal/liquid metal partitioning behavior. For a given S concentration, partition coefficients in the Ni-S system can be over an order of magnitude larger than in the Fe-S system. However, for compositions relevant to even the most Ni-rich iron meteorites, the effect of Ni on partitioning behavior is minor, amounting to less than a factor of two for the majority of trace elements studied. Any effect of Ni also appears minor when it is compared to the large influence S has on element partitioning behavior. Thus, we conclude that in the presence of an evolving S-bearing metallic melt, crystallization models can safely neglect effects from Ni when considering the full range of iron meteorite compositions.