Nickel isotope heterogeneity in the early Solar System

We report small but significant variations in the Ni-58/Ni-61-normalised Ni-60/Ni-61 and Ni-62/Ni-61 ratios (expressed as epsilon Ni-60 and epsilon Ni-62) of bulk iron and chondritic meteorites. Carbonaceous chondrites have variable, positive epsilon Ni-62 (0.05 to 0.25), whereas ordinary chondrites have negative epsilon Ni-62 (-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 epsilon Ni-60 versus epsilon Ni-62 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 epsilon Ni-62 reflect distinct nucleosynthetic components in precursor solids that have been variably mixed, but some of the epsilon Ni-60 variability could reflect a rachogenic component from the decay of Fe-60. Comparison of the epsilon Ni-60 of iron and chondritic meteorites with the same epsilon Ni-62 allows us to place upper limits on the Fe-60/Fe-56 of planetesimals during core segregation. We estimate that carbonaceous chondrites had initial Fe-60/Fe-56 <1 x 10(-7). Our data place less good constraints on initial Fe-60/Fe-56 ratios of ordinary chondrites but our results are not incompatible with values as high as 3 x 10(-7) as determined by in-situ measurements. We suggest that the Ni isotope variations and apparently heterogeneous initial Fe-60/Fe-56 results from physical sorting within the protosolar nebula of different phases (silicate, metal and sulphide) that carry different isotopic signatures. (C) 2008 Elsevier B.V. All rights reserved.