AN EXPERIMENTAL STUDY OF OXYGEN ISOTOPE FRACTIONATION BETWEEN INORGANICALLY PRECIPITATED ARAGONITE AND WATER AT LOW TEMPERATURES

Abstract

To determine oxygen isotope fractionation between aragonite and water, aragonite was slowly precipitated from Ca(HCO3)2 solution at 0 to 50°C in the presence of Mg2+ or SO42-. The phase compositions and morphologies of synthetic minerals were detected by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The effects of aragonite precipitation rate and excess dissolved CO2 gas in the initial Ca(HCO3)2 solution on oxygen isotope fractionation between aragonite and water were investigated. For the CaCO3 minerals slowly precipitated by the CaCO3 or NaHCO3 dissolution method at 0 to 50°C, the XRD and SEM analyses show that the rate of aragonite precipitation increased with temperature. Correspondingly, oxygen isotope fractionations between aragonite and water deviated progressively farther from equilibrium. Additionally, an excess of dissolved CO2 gas in the initial Ca(HCO3)2 solution results in an increase in apparent oxygen isotope fractionations. As a consequence, the experimentally determined oxygen isotope fractionations at 50°C indicate disequilibrium, whereas the relatively lower fractionation values obtained at 0 and 25°C from the solution with less dissolved CO2 gas and low precipitation rates indicate a closer approach to equilibrium. Combining the lower values at 0 and 25°C with previous data derived from a two-step overgrowth technique at 50 and 70°C, a fractionation equation for the aragonite-water system at 0 to 70°C is obtained as follows: 103lnα=20.44x103/T-41.48.This equation represents the first experimental calibration of oxygen isotope fractionation between inorganically precipitated aragonite and water at low temperatures. By considering the kinetic mechanism of oxygen isotope disequilibrium, we argue that this equation is a close proxy for thermodynamic equilibrium fractionation in the low-temperature mineral. Therefore, the discrepancies in CaCO3-H2O fractionation factors between different synthesis experiments may imply that some of the studies reflect steady-state fractionations during aragonite precipitation and subsequent polymorphic transition to calcite at different run conditions.