Nickel isotopes in iron meteorites-nucleosynthetic anomalies in sulphides with no effects in metals and no trace of 60Fe

Iron-60 decays to Ni-60 with a half-life of 1.49 Myrs such that Ni isotopic studies of iron meteorites have the potential to provide powerful new constraints on the energy budgets and time-scales of planetesimal melting, differentiation and core formation. We report high-resolution MC-ICPMS Ni isotope compositions for the Fe-Ni metal phase from 33 iron meteorites as well as for 10 coexisting sulphides. The isotopic composition of every metal sample is indistinguishable from that of the standard within uncertainties, whereas several sulphides show an excess of Ni-61 correlated with a deficit in Ni-60. These latter effects are not explicable by currently known analytical artefacts. Nor can they be readily explained by spallation reactions or radioactive decay. Based on our sampling they seem more prevalent in, but not exclusive to, non-magmatic iron meteorites and could reflect admixing of less than 0.4 ppm pure s-process component into '' normal '' Ni on meteorite parent bodies. Sulphides do not show the excess (60)-Ni expected from their high Fe/Ni ratios if they formed within the first few million years of the solar system. The data provide evidence that sulphides in iron meteorites crystallized more than 10 Myrs after the start of the solar system. (c) 2005 Elsevier B.V. All rights reserved.