Experimental studies of low-T oxygen isotope fractionation between carbonate minerals and water

Abstract

Oxygen isotope fractionation factors between calcium carbonates and water have been applied to ancient marine geochemistry principally for the purpose of geothermometry. The problem was encountered, however, with respect to the direction and magnitude of O isotope fractionation between calcite and aragonite at thermodynamic equilibrium. This study presents a series of synthesis experiments at 0–70 C to resolve these intriguing issues. The experiments were designed with oneand two-step approaches, respectively, corresponding to the two complex anions of dissolved carbonate prior to aragonite precipitation. Witherite and metastable aragonite were directly synthesized by the direct precipitation and the overgrowth techniques, respectively. The twostep experiments delivered equilibrium fractionations between the precipitated carbonate and water, whereas one-step experiments did not. Significant difference was observed in experiment-derived calcite–aragonite fractionations at above 25 C; the two-step experiments at different periods from 20 to 60 days demonstrate that the dissolved [CO3] 2 and H2O have achieved O isotope equilibrium prior to the aragonite precipitation. Thus this difference cannot be ascribed to rapid (disequilibrium) precipitation of aragonite from either [CO3] 2 or [HCO3] dominant solutions. The experimental results show that the equilibrium fractionations derived from the two-step experiments are independent of the effects of high solution pH, the low O/O ratio of [CO3] 2 and rapid precipitation. Our experiments suggest negligible effect of Mg concentrations on the O isotope fractionations within the analytical error, indicating that no salt effect of Mg cation occurred under the experimental conditions. The O isotope composition of precipitated aragonite does not depend on the concentration of dissolved carbonate species in aqueous solutions provided that the O isotope equilibrium is really achieved thermodynamically between dissolved carbonate and water. Although the effects of the chemical composition of solutions may occur in synthesis experiments on the carbonate–water fractionations, enough attention must be paid to the kinetic mechanism of O isotope exchange and equilibrium in carbonate-water systems. It is of critical importance in judging the achievement of O isotope equilibrium between the dissolved carbonate complex anions and water prior to the precipitation of either aragonite or calcite. According to the currently available observations, it appears that the direction of O isotope fractionation between calcite and aragonite is positive rather than negative at thermodynamic equilibrium.