Carbon isotope fractionation in synthetic magnesian calcite

Mg-calcite was precipitated at 25 °C in closed system, free-drift experiments, from solutions containing NaHCO3, CaCl2 and MgCl2. The carbon stable isotope composition of bulk solid and solution were analyzed from subsamples collected during time course experiments of 24 h duration. Considering only the Mg-content and δ13C values for the bulk solid, the carbon isotope fractionation factor for the Mg-calcite–HCO3(aq)− system (as 103lnαMg-cl-HCO3(aq)-) increased with average mol percentage of Mg (XMg) in the solid at a rate of (0.024 ± 0.011) per mol% MgCO3. Extrapolation of this relationship to the pure calcite end member yields a value of 0.82 ± 0.09, which is similar to published values for the calcite–HCO3(aq)− system. Although 103lnαMg-cl–HCO3(aq)- did not vary for precipitation rates that ranged from 103.21 to 104.60 μmol m−2 h−1, it was not possible to hold Mg-content of the solid constant, so kinetic effect on 103 ln α could not be evaluated from these experiments.