Equilibrium nickel isotope fractionation in nickel sulfide minerals

Abstract Nickel is an important element on Earth, and a major element in the Earth's core, and plays important roles in many geological and biological systems. As an important sink of Ni, Ni sulfides are closely concerned with Ni migration in magma systems and the genesis and evolution of magmatic sulfide deposits. Ni isotopes of Ni sulfides may be a powerful geochemical tracer in magmatic processes and evolution of magmatic sulfide deposits. However Ni isotope fractionation factors of sulfides remain poorly known, which makes the applications of Ni isotopes to geological problems associated with sulfides difficult. In this study, the first-principles methods are used to compute Ni isotope fractionation parameters of polydymite (Ni 3 S 4 ), heazlewoodite (Ni 3 S 2 ), millerite (NiS), godlevskite (Ni 9 S 8 ) and vaesite (NiS 2 ). The reduced partition function ratios of 60 Ni/ 58 Ni ( 10 3 ln β 60 – 58 ) for these minerals decrease in the order of polydymite > heazlewoodite > millerite > godlevskite > vaesite. Ni isotope fractionations in these Ni sulfides show an approximately linear dependence on the average Ni S bond lengths, and have a significant negative correlation with the average Ni Ni bond lengths. Furthermore, a change in Fe/Ni ratio can also lead to Ni isotope fractionation, and with substitution Fe for Ni, the reduced partition function ratios of 60 Ni/ 58 Ni decrease.