Mass‐dependent fractionation of nickel isotopes in meteoritic metal

Abstract

available online at http://meteoritics.org 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 ≈0.4‰ amu −1 . The ranges of Ni isotopic compositions (relative to the SRM 986 Ni isotopic standard) in metal from iron meteorites (≈0.0 to ≈0.3‰ amu −1 ) and chondrites (≈0.0 to ≈0.2‰ amu −1 ) are similar, whereas the range in pallasite metal (≈−0.1 to 0.0‰ amu −1 ) appears distinct. The fractionation of Ni isotopes within a suite of fourteen IIIAB irons (≈0.0 to ≈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 ≈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. Mass-dependent fractionation of nickel isotopes in meteoritic metal

Diffusion‐driven kinetic isotope effect of Fe and Ni during formation of the Widmanstätten pattern

“…The average of these data is 0.031‰/amu relative to IRMM-014 (Zhu et al 2001;Kehm et al 2003;Poitrasson et al 2005;Schoenberg and von Blanckenburg 2006;Cook et al, Forthcoming). There is no clear evidence for Fe isotope heterogeneity at a bulk scale in iron meteorites (Dauphas and Rouxel 2006).…”

“…This is the topic of the present paper, which builds on preliminary results reported at the same meeting (Dauphas 2006). (Poitrasson et al 2005;Cook et al, Forthcoming;Horn et al 2006a). All studies agree that there is little or no significant Fe isotopic fraction between taenite and kamacite (<0.1‰/amu).…”

Isotope Fractionation Processes of Selected Elements

“…In the metal phase of meteorites, Ni isotopes fractionate between kamacite (Fe-rich phase) and taenite (Ni rich phase), the former being isotopically heavier than the latter (Cook et al 2007). To investigate potential Ni isotope fractionation between core and mantle, Lazar et al (2012) determined Ni isotope fractionations between Ni metal and Ni talc silicate.…”

Experimental determination of equilibrium nickel isotope fractionation between metal and silicate from 500°C to 950°C

“…Measurable equilibrium inter-mineral fractionation of Fe isotopes at high temperature has been demonstrated in the laboratory (Shahar et al, 2008) and recent studies have revealed mass-dependent isotope fractionation of stable Ni isotopes in meteorites and terrestrial materials (Tanimizu and Hirata, 2006;Cook et al, 2007;Moynier et al, 2007;Steele et al, 2011). Given the similarities in relative mass differences between Fe and Ni isotopes, there is reason to expect that Ni isotopes would also exhibit measurable equilibrium fractionations at high temperatures.…”