First-principles calculations of equilibrium inter-mineral nickel isotope fractionation in the mantle

Abstract

The nickel (Ni) isotopic systematics has been increasingly utilized to understand the processes governing planetary differentiation. However, there is a lack of fundamental knowledge regarding equilibrium Ni isotope fractionation among mantle minerals, which hampered further applications of Ni isotopes in this domain. In this study, we conducted first-principles calculations to determine the reduced partition function ratios of ⁶⁰Ni/⁵⁸Ni (10³lnβ_60–58) in major Ni-doped mantle minerals, including olivine, orthopyroxene, clinopyroxene, pyrope and spinel. Our calculations reveal that the 10³lnβ_60–58 is insensitive to the Ni concentration within the mineral Ni concentration ranges investigated here, and the 10³lnβ_60–58 increases in the order of pyrope < clinopyroxene < orthopyroxene < olivine < spinel. Moreover, the 10³lnβ_60–58 demonstrates a predominantly linear relationship with the average force constant (< F >) of Ni that is mainly influenced by the average NiO bond length. By comparing the measured inter-mineral Ni isotopic differences in mantle rocks with our predicted results, we find that olivine, spinel and orthopyroxene in most peridotitic samples have attained Ni isotopic equilibrium. Conversely, clinopyroxene appears to fall out of equilibrium with other mineral phases, likely attributed to its metasomatic origin. Furthermore, in conjunction with the melt-olivine Ni isotope fractionation factor obtained from theoretical calculation and laboratory experiment, we demonstrate that mantle silicate melting produces melts with a heavier Ni isotopic signature compared to the source. Consequently, the systematically lighter Ni isotopic compositions observed in global basalts cannot be purely attributed to silicate melting-induced Ni isotope fractionation. An additional process, perhaps sulfide dissolution, must be invoked.