Nickel isotope heterogeneity in the early Solar System

We report small but significant variations in the ⁵⁸Ni/⁶¹Ni-normalised ⁶⁰Ni/⁶¹Ni and ⁶²Ni/⁶¹Ni ratios (expressed as ε⁶⁰Ni and ε⁶²Ni) of bulk iron and chondritic meteorites. Carbonaceous chondrites have variable, positive ε⁶²Ni (0.05 to 0.25), whereas ordinary chondrites have negative ε⁶²Ni (− 0.04 to − 0.09). The Ni isotope compositions of iron meteorites overlap with those of chondrites, and define an array with negative slope in the ε⁶⁰Ni versus ε⁶²Ni diagram. The Ni isotope compositions of the volatile-depleted Group IVB irons are similar to those of the refractory CO, CV carbonaceous chondrites, whereas the other common magmatic iron groups have Ni isotope compositions similar to ordinary chondrites. Only enstatite chondrites have identical Ni isotope compositions to Earth and so appear to represent the most appropriate terrestrial building material. Differences in ε⁶²Ni reflect distinct nucleosynthetic components in precursor solids that have been variably mixed, but some of the ε⁶⁰Ni variability could reflect a radiogenic component from the decay of ⁶⁰Fe. Comparison of the ε⁶⁰Ni of iron and chondritic meteorites with the same ε⁶²Ni allows us to place upper limits on the ⁶⁰Fe/⁵⁶Fe of planetesimals during core segregation. We estimate that carbonaceous chondrites had initial ⁶⁰Fe/⁵⁶Fe < 1 × 10^− 7. Our data place less good constraints on initial ⁶⁰Fe/⁵⁶Fe ratios of ordinary chondrites but our results are not incompatible with values as high as 3 × 10^− 7 as determined by in-situ measurements. We suggest that the Ni isotope variations and apparently heterogeneous initial ⁶⁰Fe/⁵⁶Fe results from physical sorting within the protosolar nebula of different phases (silicate, metal and sulphide) that carry different isotopic signatures.