Phosphoric acid fractionation factors for calcite and aragonite between 25 and 75 °C: Revisited

This paper reports the results of an investigation to re-determine the phosphoric acid fractionation factor for calcite and aragonite in order to improve the accuracy and limit the uncertainty of this very important quantity. The 18O/16O ratio of 100% of the oxygen in calcite and aragonite was determined directly by combining data from a two-step procedure involving thermal decomposition of the carbonate, followed by fluorination of the residual CaO using BrF5.The oxygen isotope composition of the CO2 released upon the thermal decarbonation of calcite and aragonite is enriched in 18O relative to that of the remaining CaO. Reproducibility of the oxygen isotope composition of the CO2 derived from each of the decarbonation and fluorination steps was 0.55‰ (1σ). Nevertheless, statistically reliable acid fractionation factors for calcite (se=0.07) and aragonite (se=0.10) were determined on the basis of a large set of combined data from conventional phosphoric acid reactions at three different temperatures, plus the decarbonation/fluorination procedure. The acid fractionation factors (αCO2(ACID)-carbonate) determined in this study for calcite (αCO2(ACID)–Calcite=1.01030) and aragonite (αCO2(ACID)–Aragonite=1.01063) at 25 °C, differ slightly from previously published values.On the basis of our new determinations at 25, 50, and 75 °C, revised expressions for the temperature dependence of the acid fractionation factors for calcite and aragonite are proposed for the temperature range of 25–75 °C:1000lnαCO2(ACID)–Calcite=3.59(103/T)−1.791000lnαCO2(ACID)–Aragonite=3.39(103/T)−0.83where αCO2(ACID)–carbonate is the phosphoric acid fractionation factor, and T is temperature in kelvin.