The origin of Zn isotope fractionation in sulfides

Isotope fractionation of Zn between aqueous sulfide, chloride, and carbonate species (Zn2+, Zn(HS)2, Zn(HS)3-, Zn(HS)42-, ZnS(HS)−, ZnCl+, ZnCl2, ZnHCO3+, and ZnCO3) was investigated using ab initio methods. Only little fractionation is found between the sulfide species, whereas carbonates are up to 1‰ heavier than the parent solution. At pH > 3 and under atmospheric-like CO2 pressures, isotope fractionation of Zn sulfides precipitated from sulfidic solutions is affected by aqueous sulfide species and the δ66Zn of sulfides reflect these in the parent solutions. Under high PCO2PCO2 conditions, carbonate species become abundant. In high PCO2PCO2 conditions of hydrothermal solutions, Zn precipitated as sulfides is isotopically nearly unfractionated with respect to a low-pH parent fluid. In contrast, negative δ66Zn down to at least −0.6‰ can be expected in sulfides precipitated from solutions with pH > 9. Zinc isotopes in sulfides and rocks therefore represent a potential indicator of mid to high pH in ancient hydrothermal fluids.