Effect of Fe content on reduced partition function ratios of Ni isotopes in olivine: A first-principles study

Abstract Olivine is one of the most abundant minerals in the Earth’s mantle and is widely distributed in meteorites and mafic–ultramafic igneous rocks. In recent years, Ni isotopes in olivine have been extensively used to trace geological processes such as mantle evolution, crust-mantle interaction, ore formation, and planetary differentiation. However, the mechanism by which the Ni isotope fractionation properties of olivine respond to variations in Fe content remains poorly understood, limiting its effectiveness as a geochemical tracer. In this study, we employed first-principles calculations to determine the reduced partition function ratios of 60Ni/58Ni (β60-58 factors) in olivine with varying Fe contents. Our calculations show that, at a constant Ni concentration, increasing Fe content significantly expands the unit cell volume and lengthens the average Ni–O bond, which in turn leads to a decrease in the β60-58 factor. A significant negative correlation is observed between the average Ni–O bond length and the β60-58 factor. In contrast, when Ni/(Ni+Fe+Mg) ≤ 2/32, variations in Ni content have a negligible effect on the β60-58 factor. Given the generally low Ni concentrations in natural olivine, it is inferred that variations in δ60/58Ni values are unlikely to be influenced by Ni content. The results not only clarify the mechanism by which Fe content influences Ni isotope fractionation properties of olivine, but also provide essential data for the application of Ni isotopes in tracing geological evolution processes on Earth and in extraterrestrial bodies.