Ab initio study of iron isotope fractionation during Earth’s core-mantle segregation*

Recent studies have revealed that the iron isotope composition of mid-ocean ridge basalts (MORBs) is +0.1‰ richer in heavy Fe isotope (⁵⁶Fe) relative to primitive chondritic meteorites, while basalts from Mars and Vesta have similar Fe isotopic composition as these meteorites. Here we investigate the hypothesis that iron isotope fractionation may have occurred during core formation on Earth. In particular, we compute Fe isotope fractionation factors among the lower mantle phases, bridgmanite (Bdg) and ferropericlase (Fp), and the metal phase at relevant pressure-temperature conditions. We pay particular attention to the effect of the spin crossover in Fe in Bdg and Fp on these fractionation factors. In addition, Fe in Bdg can occupy more than one crystalline site and can be in more than one valence state. In the metal phase, we consider variable amounts of Ni, the other metallic element expected to alloy with Fe in the core. Considering all these possible states of Fe in the silicate, oxide, and metallic phase, we show that the spin crossover in Fe, which does not occur in Mars or Vesta, may have played an important role in the Fe isotope fractionation during core-mantle segregation in the Earth.