The origin of Zn Isotope Fractionation in Sulfides 5

2 Abstract: 23 Isotope fractionation of Zn between aqueous sulfide, chloride, and carbonate species 24 (Zn2+, Zn(HS)2, Zn(HS)3−, Zn(HS)42−, ZnS(HS)−, ZnCl+, ZnCl2, ZnHCO3+, and ZnCO3) 25 was investigated using ab initio methods. Only little fractionation is found between the 26 sulfide species, whereas carbonates are up to 1 ‰ heavier than the parent solution. At 27 pH>3 and under atmospheric-like CO2 pressures, isotope fractionation of Zn sulfides 28 precipitated from sulfidic solutions is affected by aqueous sulfide species and the δ66Zn 29 of sulfides reflect these in the parent solutions. Under high 2CO P conditions, carbonate 30 species become abundant. In high 2CO P conditions of hydrothermal solutions, Zn 31 precipitated as sulfides is isotopically nearly unfractionated with respect to a low-pH 32 parent fluid. In contrast, negative δ66Zn down to at least −0.6 ‰ can be expected in 33 sulfides precipitated from solutions with pH>9. Zinc isotopes in sulfides and rocks 34 therefore represent a potential indicator of mid to high pH in ancient hydrothermal 35 fluids. 3