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)(4)(2-), 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 parts per thousand 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 delta Zn-66 of sulfides reflect these in the parent solutions. Under high P-CO2 conditions, carbonate species become abundant. In high PCO2 conditions of hydrothermal solutions, Zn precipitated as sulfides is isotopically nearly unfractionated with respect to a low-pH parent fluid. In contrast, negative delta Zn-66 down to at least -0.6 parts per thousand 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. (C) 2011 Elsevier Ltd. All rights reserved.