High precision measurements of non-mass-dependent effects in nickel isotopes in meteoritic metal via multicollector ICPMS

We measured the Ni isotopic composition of metal from a variety of meteorite groups to search for variations in the Ni-60 abundance from the decay of the short-lived nuclide Fe-60 (t(1/2) = 1.49 My) and for possible nucleosynthetic effects in the other stable isotopes of Ni. We developed a high-yield Ni separation procedure based on a combination of anion and cation exchange chromatography. Nickel isotopes were measured on a single-focusing, multicollector, inductively coupled mass spectrometer (MC-ICPMS). The external precision on the mass-bias-corrected Ni-60/Ni-58 ratio (+/- 0.15 epsilon; 2 sigma) is comparable to similar studies using double-focusing MC-ICPMS. We report the first high-precision data for Ni-64, the least abundant Ni isotope, obtained via MC-ICPMS. The external precision on the mass-bias-corrected Ni-64/Ni-58 ratio (+/-1.5 epsilon; 2 sigma) is better than previous studies using thermal ionization mass spectrometry. No resolvable excesses relative to a terrestrial standard in the mass-bias-corrected Ni-60/Ni-58 ratio were detected in any meteoritic metal samples. However, resolvable deficits in this ratio were measured in the metal from several unequilibrated chondrites, implying a Fe-60/Fe-56 ratio of similar to 1 x 10(-6) at the time of Fe/Ni fractionation in chondritic metal. A Fe-60/Fe-56 ratio of (4.6 +/- 3.3) x 10(-7) is inferred at the time of Fe/ Ni fractionation on the parent bodies of magmatic iron meteorites and pallasites. No clearly resolvable non-mass-dependent anomalies were detected in the other stable isotopes of Ni in the samples investigated here, indicating that the Ni isotopic composition in the early solar system was homogeneous ( at least at the level of precision reported here) at the time of meteoritic metal formation.