Planetary accretion and core formation inferred from Ni isotopes in enstatite meteorites

Nickel is a siderophile and near-refractory element, making its isotopes a potential tool for tracing planetary accretion and differentiation. However, the origin of the Ni stable isotope difference between bulk silicate Earth (BSE) and chondrites remains enigmatic. To address this problem, we report high precision Ni stable isotope data of enstatite chondrites and achondrites that possess similar mass independent O and Ni isotope compositions like the Earth-Moon system. Bulk enstatite chondrites have 560/58Ni values of 0.24 +/- 0.08 %o (2 s.d., n =13). Enstatite achondrites, including main-group aubrites, Shallowater and Itqiy, show relatively large 560/58Ni variations, ranging from 0.03 +/- 0.02 %o to 0.57 +/- 0.04 parts per thousand . This could reflect fractionations between sulfide and metal phases, as is evidenced by correlation between their S/Ni ratios and delta Ni-60/58 values. In enstatite achondrites, Ni is mainly hosted in metal and to a lesser extent in sulfides, so delta Ni-60/58 values in enstatite achondrites may represent the Ni isotopic values of the cores of their parent bodies. The overlapping delta Ni-60/58 values between bulk enstatite achondrites and enstatite chondrites indicate limited Ni stable isotope fractionation during core formation processes on reduced, sulfur-rich parent bodies. The Ni stable isotope gap between chondrites and the BSE could be possibly explained by chondrule-rich accretion model.