Oxygen isotope fractionation during smithsonite formation from aqueous solutions

Oxygen isotope fractionation between carbonate minerals and water is used as an environmental proxy to estimate mineral formation temperatures or isotopic composition of precipitating fluids. To date no experimental data on the oxygen isotope fractionation factor between smithsonite (ZnCO₃) and water, α(¹⁸O)_{smithsonite-water} = (¹⁸O/¹⁶O)_smithsonite / (¹⁸O/¹⁶O)_water, exist. Therefore, in the present study experimental work on smithsonite synthesis in the temperature range between 25 and 80 °C is coupled with ab-initio based theoretical calculations. Laboratory precipitation experiments took place in titanium reactors at elevated pCO₂ (~10 atm) in order to induce the formation of smithsonite from hydrozincite (Zn₅(CO₃)₂(OH)₆), which is the precursor phase initially formed at 25 °C and low pCO₂ (pH ~ 6.8). The constant α(¹⁸O)_{smithsonite-water} = (1000 + δ¹⁸O_smithonite) / (1000 + δ¹⁸O_water) value reached at a reaction time ≥ 10,000 min (7 days) suggests near equilibrium conditions. Based on the experimentally obtained temperature relation of α(¹⁸O)_{smithsonite-water} at 25, 40, 60, and 80 °C the integrated equation can be linearly described by the function: 10³lnα¹⁸O_{smithsonite−water}=2.79∗10⁶/T²–0.95±0.06∗10⁶/T²+0.60 where the temperature is in Kelvin. The ab initio calculations suggest that this relation can be described in the temperature range from 0 to 100 °C as: 10³lnα_{smithsonite−liquid water}=3.21∗10⁶/T²–3.63±0.025∗10⁶/T²+0.90 The α(¹⁸O)_{smithsonite-water} values from the experimental approach fit within error to the theoretical relationship from the literature and the above ab initio calculations. Difference in slope between the experiment and theoretical obtained equation likely reflects modeling inaccuracies, whereas kinetic effects cannot be completely ruled out in the experimental approach. The obtained α(¹⁸O)_{smithsonite-water} values match with the general sequence of Zn²⁺ < Fe²⁺ < Mn²⁺ < Ca²⁺ for mono-cation trigonal Me-carbonate minerals suggesting incorporation of lighter oxygen isotopes in the carbonate mineral at increasing cation radius as indicated from thermodynamic considerations. Potential applications of oxygen isotope fractionation during smithsonite formation for natural aqueous surroundings are discussed.