NEUTRON-POOR NICKEL ISOTOPE ANOMALIES IN METEORITES

Abstract

We present new, mass-independent, Ni isotope data for a range of bulk chondritic meteorites. The data are reported as e 60 Ni58/61, e 62 Ni58/61, and e 64 Ni58/61, or the parts per ten thousand deviations from a terrestrial reference, the NIST SRM 986 standard, of the 58 Ni/ 61 Ni internally normalized 60 Ni/ 61 Ni, 62 Ni/ 61 Ni, and 64 Ni/ 61 Ni ratios. The chondrites show a range of 0.15, 0.29, and 0.84 in e 60 Ni58/61, e 62 Ni58/61, and e 64 Ni58/61 relative to a typical sample precision of 0.03, 0.05, and 0.08 (2 s.e.), respectively. The carbonaceous chondrites show the largest positive anomalies, enstatite chondrites have approximately terrestrial ratios, though only EH match Earth’s composition within uncertainty, and ordinary chondrites show negative anomalies. The meteorite data show a strong positive correlation between e 62 Ni58/61 and e 64 Ni58/61, an extrapolation of which is within the error of the average of previous measurements of calcium-, aluminium-rich inclusions. Moreover, the slope of this bulk meteorite array is 3.003 ± 0.166 which is within the error of that expected for an anomaly solely on 58 Ni. We also determined to high precision (∼10 ppm per AMU) the mass-dependent fractionation of two meteorite samples which span the range of e 62 Ni58/61 and e 64 Ni58/61. These analyses show that “absolute” ratios of 58 Ni/ 61 Ni vary between these two samples whereas those of 62 Ni/ 61 Ni and 64 Ni/ 61 Ni do not. Thus, Ni isotopic differences seem most likely explained by variability in the neutron-poor 58 Ni, and not correlated anomalies in the neutron-rich isotopes, 62 Ni and 64 Ni. This contrasts with previous inferences from mass-independent measurements of Ni and other transition elements which invoked variable contributions of a neutron-rich component. We have examined different nucleosynthetic environments to determine the possible source of the anomalous material responsible for the isotopic variations observed in Ni and other transition elements within bulk samples. We find that the Ni isotopic variability of the solar system cannot be explained by mixing with a component of bulk stellar ejecta from either SN II, Wolf–Rayet or, an asymptotic giant branch source and is unlikely to result from bulk mixing of material from an SN Ia. However, variable admixture of material from the Si/S zone of an SN II can create all the characteristics of Ni isotope variations in solar system materials. Moreover, these characteristics can also be provided by an SN II with a range of masses from 15 to 40 M� , showing that input from SN II is a robust source for Ni isotope variations in the solar system. Correlations of Ni isotope anomalies with O, Cr, and Ti isotope ratios and Pb/Yb in bulk meteorites suggest that the heterogeneous distribution of isotopic anomalies in the early solar system likely resulted from nebular sorting of chemically or physically different materials bearing different amounts of isotopes synthesized proximally to the collapse of the protosolar nebula.