EQUILIBRIUM AND NONEQUILIBRIUM OXYGEN-ISOTOPE EFFECTS IN SYNTHETIC CARBONATES

A suite of divalent metal (Ca, Cd, Ba) carbonates was synthesized over the temperature range 10-40 degrees C by the classical method of slow ly bubbling N-2 through a bicarbonate solution. It was discovered that carbonates could be precipitated reproducibly in or out of isotopic e quilibrium with the environmental solution by varying the concentratio ns of bicarbonate and cation, Precipitation rate had little or no infl uence on the isotopic composition of the product. Relatively high init ial concentrations of up to 25 mM in both bicarbonate and cation were prepared by adding solid metal chlorides to solutions of NaHCO3. On th e 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 tempera tures: 10001n alpha(Calcite-H2O) = 18.03(10(3)T(-1))-32.42 where alpha is the fractionation factor, and T is in kelvins. Combining new data for low-temperature precipitations and the high-temperature equilibriu m fractionations published by O'Neil et al. (1969) results in a revise d expression for the oxygen isotope fractionation between octavite (Cd CO3) and water from 0 degrees to 500 degrees C: 10001n alpha(CdCO3-H2O ) = 2.76(10(6)T(-2))-3.96 The ability to produce nonequilibrium carbon ates allowed assessment to be made, for the first time, of the tempera ture dependence of nonequilibrium stable isotope fractionations in min eral systems. The temperature coefficients of cu(carbonate-water) for nonequilibrium divalent metal carbonates are greater than those for eq uilibrium carbonates, a finding that may bear on the interpretation of analyses of biogenic carbonates forming out of isotopic equilibrium i n nature. New determinations of acid fractionation factors (10001n alp ha) at 25 degrees C for calcite (10.44 +/- 0.10), aragonite (11.01 +/- 0.01), and witherite (10.57 +/- 0.16) are mildly to strongly differen t from those published by Sharma and Clayton (1965) and point to a con trol on this fractionation by some physical property of the mineral. R eproducible values for octavite (CdCO3) varied from 11.18 to 13.60 dep ending on the conditions of preparation of the carbonate. These new va lues need to be considered in determinations of absolute O-18/O-16 rat ios of international reference standards and in relating analyses of c arbonates to those of waters, silicates, and oxides. (C) 1997 Elsevier Science Ltd.