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

Abstract

We measured the Ni isotopic composition of metal from a variety of meteorite groups to search for variations in the ⁶⁰Ni abundance from the decay of the short-lived nuclide ⁶⁰Fe (t<inf>1/2</inf> = 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 massbias-corrected ⁶⁰Ni/⁵⁸Ni ratio (±0.15 ε 2ω) is comparable to similar studies using double-focusing MC-ICPMS. We report the first high-precision data for ⁶⁴Ni, the least abundant Ni isotope, obtained via MC-ICPMS. The external precision on the mass-bias-corrected ⁶⁴Ni/⁵⁸Ni ratio (±1.5 ε 2ω) is better than previous studies using thermal ionization mass spectrometry. No resolvable excesses relative to a terrestrial standard in the mass-bias-corrected ⁶⁰Ni/⁵⁸Ni 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 ⁶⁰Fe/ ⁵⁶Fe ratio of ∼1 × 10^-6 at the time of Fe/Ni fractionation in chondritic metal. A ⁶⁰Fe/⁵⁶Fe ratio of (4.6 ± 3.3) ×10⁷ 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. © 2006 American Chemical Society.