Equilibrium fractionation of S, Fe, and Ni isotopes in Fe-Ni sulfides: A first-principles investigation

Abstract

Fe, Ni, and S isotopes are important chemical tracers, and they complement each other to improve our understanding of the Earth's evolution. As common accessory minerals in the Earth's interior and the most important group of ore minerals, sulfide minerals display a wide range of Fe and Ni contents. Using first-principles methods, we investigated the equilibrium fractionation factors of Fe, Ni, and S isotopes of the β-(Ni, Fe)S, FeS2-NiS2, and Fe3S4-Ni3S4 solid solutions, to constrain the effect of cation contents on Fe, Ni, and S isotopic signatures. For the β-(Ni, Fe)S solid solution, the iron β-factors, nickel β-factors, and sulfur β-factors all decrease approximately linearly with the increasing Fe content. For the FeS2-NiS2 solid solution, the iron β-factors, nickel β-factors, and sulfur β-factors show a positive linear relationship with the Fe content. For the Fe3S4-Ni3S4 solid solution, the iron β-factors and nickel β-factors have a strong negative correlation with the Fe content, while the sulfur β-factors have a weak negative correlation with the Fe content. In general, the trend of variation of the β-factors with Fe content is the opposite to that of the average bond lengths with the Fe content. Our results provide an improved understanding of Fe, Ni, and S isotopic fractionation in natural systems that may be used to better constrain physical, chemical, and geo-biological evolution on Earth.