EQUILIBRIUM AND NONEQUILIBRIUM OXYGEN ISOTOPE EFFECTS IN SYNTHETIC CARBONATES

Abstract

A suite of divalent metal (Ca, Cd, Ba) carbonates was synthetized over the temperature range 10-40°C by the classical method of slowly bubbling N2 through a bicarbonate solution. It was discovered that carbonates could be precipitated reproducibly in or out of isotopic equilibrium with the environmental solution by varying the concentrations of bicarbonate and cation. Precipitation rate had little or no influence on the isotopic composition of the product. Relatively high initial concentrations of up to 25 mM in both bicarbonate and cation were prepared by adding solid metal chlorides to solutions of NaHCO3. On the basis of results of equilibrium experiments and a new determination of the acid fractionation factor, a new expression is proposed for the oxygen isotope fractionation between calcite and water at low temperatures: 10001nα(Calcite-H2O) = 18.03(103T-1) - 32.42 where α is the fractionation factor, and T is in kelvins. Combining new data for low-temperature precipitations and the high-temperature equilibrium fractionations published by O'Neil et al. (1969) results in a revised expression for the oxygen isotope fractionation between octavite (CdCO3) and water from 0° to 500°C: 10001nα(CdCO3-H2O) = 2.76(106T-2) - 3.96The ability to produce nonequilibrium carbonates allowed assessment to be made, for the first time, of the temperature dependence of nonequilibrium stable isotope fractionations in mineral systems. The temperature coefficients of α(carbonate-water) for nonequilibrium divalent metal carbonates are greater than those for equilibrium carbonates, a finding that may bear on the interpretation of analyses of biogenic carbonates forming out of isotopic equilibrium in nature.New determinations of acid fractionation factors (10001nα) at 25°C for calcite (10.44 +/- 0.10), aragonite (11.01 +/- 0.01), and witherite (10.57 +/- 0.16) are mildly to strongly different from those published by Sharma and Clayton (1965) and point to a control on this fractionation by some physical property of the mineral. Reproducible values for octavite (CdCO3) varied from 11.18 to 13.60 depending on the conditions of preparation of the carbonate. These new values need to be considered in determinations of absolute 18O16O ratios of international reference standards and in relating analyses of carbonates to those of waters, silicates, and oxides.