Geocheminar spring-2016

Equilibrium Metal-Silicate Fe Isotope Fractionation and Implications for Planetary Differentiation

Magmatic iron meteorites exhibit an enrichment in 57Fe/54Fe relative to chondrites of ~0.2‰. Though not definitive, this is suggestive of heavy Fe partitioning into the cores of differentiated bodies. We aim to determine if core formation is accompanied by an isotopic signature. Understanding equilibrium Fe isotope fractionation between metal and silicate phases is fundamental to assessing the significance of the variation of Fe isotopes in the Solar System. Here we determine the equilibrium Fe isotope fractionation between the metal and silicate phases using the equilibrated aubrite meteorites Norton County and Mount Egerton. The calculated isotopic fractionation ?57Femetal-silicate is 0.08‰ ± 0.039 (2 SE) for Norton County and 0.09‰ ± 0.019 (2 SE) for Mount Egerton, indicating that the heavy isotopes of Fe partition into the metallic phase. Using the stable Fe isotope fractionation and the temperature of equilibration for these meteorites, we explore what we can learn about the core sizes and compositions of asteroid parent bodies. We conclude that the observed difference in isotopic composition between magmatic iron meteorites and chondrites can be explained if the parent bodies of these iron meteorites had relatively small cores.