Iron isotope constraints on planetesimal core formation in the early solar system

Abstract

Abstract We determined the Fe isotope fractionation between the metal and silicate phases of two aubrite meteorites, Norton County and Mount Egerton. We find that the metallic phase is high in 57Fe/54Fe with respect to the silicate phase, with Δ57Femetal-silicate = 0.08‰ ± 0.04 for Mount Egerton and 0.09 ± 0.02‰ for Norton County. These data, combined with new measurements of 57Fe/54Fe of IIIAB iron meteorites, are used to constrain the origins of the high 57Fe/54Fe exhibited by all classes of iron meteorites. We find that if the parent bodies of the iron meteorites had chondritic bulk 57Fe/54Fe values, their cores must have been unusually small (≤8% by mass). Relaxing the constraint that the bodies were chondritic in their bulk iron isotope ratios allows for larger core mass fractions commensurate with usual expectations. In this case, the elevated 57Fe/54Fe values of iron meteorites are due in part to evaporation of melt during the accretion stages of the parent bodies and not solely the result of metal-silicate differentiation.