Mass-dependent fractionation of nickel isotopes in meteoritic metal

Abstract

We measured nickel isotopes via multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) in the bulk metal from 36 meteorites, including chondrites, pallasites, and irons (magmatic and non-magmatic). The Ni isotopes in these meteorites are mass fractionated; the fractionation spans an overall range of approximately 0.4 amu^(-1). The ranges of Ni isotopic compositions (relative to the SRM 986 Ni isotopic standard) in metal from iron meteorites (approximately 0.0 to approximately 0.3 amu^(-1)) and chondrites (approximately 0.0 to approximately 0.2 amu^(-1)) are similar, whereas the range in pallasite metal (approximately -0.1 to approximately 0.0 amu^(-1)) appears distinct. The fractionation of Ni isotopes within a suite of fourteen IIIAB irons (approximately 0.0 to approximately 0.3 amu^(-1)) spans the entire range measured in all magmatic irons. However, the degree of Ni isotopic fractionation in these samples does not correlate with their Ni content, suggesting that core crystallization did not fractionate Ni isotopes in a systematic way. We also measured the Ni and Fe isotopes in adjacent kamacite and taenite from the Toluca IAB iron meteorite. Nickel isotopes show clearly resolvable fractionation between these two phases; kamacite is heavier relative to taenite by approximately 0.4 amu^(-1). In contrast, the Fe isotopes do not show a resolvable fractionation between kamacite and taenite. The observed isotopic compositions of kamacite and taenite can be understood in terms of kinetic fractionation due to diffusion of Ni during cooling of the Fe-Ni alloy and the development of the Widmanstätten pattern.