The effect of core composition on iron isotope fractionation between planetary cores and mantles

•Planetary core formation causes a fractionation of Fe isotopes.•The mantles of the smaller terrestrial planets and asteroids are isotopically light.•Core-bound impurity elements cause shortening of nearest-neighbor distances.•Shorter, stiffer bonds increase the preference of Fe-alloys for heavy Fe isotopes.•Volatile depletion processes are not needed to explain Fe isotope variations.