The origin of Zn isotope fractionation in sulfides

Abstract

Isotope fractionation of Zn between aqueous sulfide, chloride, and carbonate species (Zn[2+], Zn(HS)_2, Zn(HS)_4[2-], ZnHCO_3[+], ZnS(HS)[−] ZnCl[+] , ZnCl_2, Pco_2, and ZnCO_3) 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 CO_2 pressures, isotope fractionation of Zn sulfides precipitated from sulfidic solutions is affected by aqueous sulfide species and the δ[66]Zn of sulfides reflect these in the parent solutions. Under high PCO_2 conditions, carbonate species become abundant. In high PCO_2 conditions of hydrothermal solutions, Zn precipitated as sulfides is isotopically nearly unfractionated with respect to a low-pH parent fluid. In contrast, negative δ[66]Zn 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.