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Nickel Isotope Investigation by MC-ICP-MS and PTIMS

Introduction: The evidence for the presence of short-lived Fe (decaying into Ni, with the half life of 1.5 Ma) in the early system has recently received considerable attention, both experimentally and theoretically. Evidence for Ni excesses in eucrites, without correlation to Fe/Ni has been obtained by Shukolyukov and Lugmair [1, 2]. Recently, based on ion microprobe studies of phases with very high Fe/Ni, direct evidence for the in situ decay of Fe has been obtained [3-5]. These studies indicate a significant initial abundance of Fe/Fe, in the solar system (1.1± 0.2×10 and 1.7±0.5×10 for Bishunpur and Krymka [3] and 7±3×10 for Semarkona [4, 5]). An extensive recent review [6] of the abundances of short-lived nuclides in the early solar system and their astrophysical production sites and rates concludes that the initial Fe/Fe by an AGB source would be significant, at 10 to 2×10, for a dilution factor of ~4×10 (the ratio of the contaminating mass to the solar parental cloud) [6]. A supernova can be the source or Mn and possibly of Fe [6]. Tachibana et al. [7] have determined evidence for Fe in ferromagnesian chondrules from Semarkona and Bishunpur, with initial Fe/Fe of (2.2-3.7)×10. In addition, early data on Ni in Allende refractory inclusions indicated the presence of general Ni isotope anomalies [8], although mass interference from Zn affected Ni, and interferences at other Ni isotopes could not be completely eliminated. Results: We have developed analytical techniques for the measurement of Ni isotopes by MC-ICP-MS (using the Thermo/ Finnigan Neptune, at Caltech) and by TIMS (the Thermo/Finnigan Triton, at JPL). The advantage of MC-ICP-MS rests with high ionization efficiency in the plasma source (coupled with improvements in sample transmission efficiency, based on the use of a desolvating nebulizer (ESI ApexAreo), which also reduces oxides and some molecular interferences. The disadvantages lie dominantly in potentially significant mass interferences, precisely due to the high ionization efficiency for all elements and the presence of molecular ion species. For ICP-MS, interferences at mass 58 arise from Fe and from ArO. The presence of dominant ArO does not permit the adequate monitoring of Fe interference. To minimize the effects of molecular interferences, we obtain data under intermediate mass resolution and by centering the magnetic field to the narrow mass region on the peak tops, where the molecular ion beams are occluded by the collector slits [9, 10]. The Ni in the samples is chemically separated from Fe by ion exchange. The Ni solution concentrations for runs are at the level of a few ppm, which yields over 10 Amps, at intermediate mass resolution, at ~50 μL/min. We have also reduced significant mass interference from Zn (affecting Ni), by identifying its source as the purified water, produced by distillation at Caltech. Water produced through mixedbed ion exchange and recirculation, at JPL, is considerably more Znfree. But, the interference on Ni was still significant,>1‰, for both the ICPMS and TIMS measurements. We are not reporting Ni data. Chemical cleanup of Zn has now been achieved, but not in time for this abstract. Given 62Ni/61Ni 3.26 3.27 3.28 58 N i/ 6 1 N i