IRON ISOTOPE CONSTRAINTS ON PLANETESIMAL CORE FORMATION IN THE EARLY SOLAR SYSTEM

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 Fe-57/Fe-54 with respect to the silicate phase, with Delta Fe-57(metal-silicate) = 0.08% +/- 0.04 for Mount Egerton and 0.09 +/- 0.02% for Norton County. These data, combined with new measurements of Fe-57/Fe-54 of IIIAB iron meteorites, are used to constrain the origins of the high Fe-57/Fe-54 exhibited by all classes of iron meteorites. We find that if the parent bodies of the iron meteorites had chondritic bulk Fe-57/Fe-54 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 Fe-57/Fe-54 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. (C) 2018 Elsevier Ltd. All rights reserved.