Nickel isotope fractionation factors between silicate minerals and melt

Klaver, M; Elliott, T; Ionov, DA; Bizimis, M; Berndt, J; Klemme, S

Research article (journal)

Abstract

Quantitative knowledge of inter-mineral and mineral–melt isotopic fractionation factors is of crucial importance to interpret variability of mass-dependent isotope ratios in terrestrial rocks and meteorites. The isotopic composition of Ni shows great promise as a tracer of planetary differentiation, but the lack of solid constraints on the sign and magnitude of equilibrium Ni isotope fractionation factors hampers its further development. We address this issue by studying Ni isotope fractionation between silicate minerals and melt using a combination of carefully selected natural samples and olivine crystallisation experiments. Mineral separates from xenoliths are used to determine the Ni isotope fractionation behaviour of pyroxenes, spinel, and garnet relative to olivine. Nickel isotope fractionation between olivine and orthopyroxene is negligible, whereas clinopyroxene shows large variability that does not reflect equilibrium. Spinel has a clear affinity for the heavier isotopes of Ni relative to olivine; garnet has the lightest Ni isotope composition at equilibrium. The key parameter that dictates the sign and magnitude of Ni isotope fractionation during partial melting and crystallisation, however, is the olivine–melt fractionation factor. We present data from a natural basalt that contains olivine phenocrysts in chemical equilibrium with the host glass, and furthermore employ olivine crystallisation experiments in a 1-atmopshere furnace. Although the experiments invariably suffer from Ni loss to the Pt wire used to suspend the experiments in the furnace, these effects were carefully considered and quantified. The experiments limit the olivine–melt Ni isotope fractionation factor to lie between zero to slightly negative, which agrees very well with the natural datum, and we derive a best estimate value for Δ60/58NiOl–melt of -0.142±0.031‰ at 1000 K. Hence, olivine has a subtle but clearly resolvable affinity for the lighter isotopes of Ni relative to basaltic melt. The implication is that equilibrium partial melts of mantle peridotite should have a slightly heavier Ni isotope composition than their source. Available Ni isotope data for mid-ocean ridge basalts show some overlap with modelled partial melts but extend to notably lighter Ni isotope compositions, potentially reflecting source heterogeneity or diffusive fractionation during melt transport.

Details zur Publikation

Release year: 2024
Language in which the publication is written: English
Link to the full text: https://www.sciencedirect.com/science/article/pii/S0016703723006014