Phosphoric acid fractionation factors for smithsonite and cerussite between 25 and 72°C

The intramolecular kinetic oxygen isotope fractionation between CO₂ and CO₃²⁻ during reaction of phosphoric acid with natural smithsonite (ZnCO₃) and cerussite (PbCO₃) has been determined between 25 and 72°C. While cerussite decomposes in phosphoric acid within a few hours at 25°C, smithsonite reacts very slowly with the acid at 25°C providing yields of CO₂ < 25% after 2 weeks. The low yields result in a low precision for oxygen isotope measurements of the acid-liberated CO₂ (±1.65‰, 1σ, n = 9). The yield and reproducibility of oxygen isotope values of the acid-liberated CO₂ from smithsonite can be improved, the latter to ∼±0.15‰, by increasing the reaction temperature to 50°C for 12 h or to 72°C for 1 h. Our new phosphoric acid fractionation factor for natural cerussite at 25°C deviates significantly from a previously published value on synthetic material. The temperature dependence of the oxygen isotope factionation factor, α between acid-liberated CO₂ and carbonate at 25 to 72°C is given by the following equations