Carbon isotope fractionation in synthetic magnesian calcite

Mg-calcite was precipitated at 25 degrees 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 delta C-13 values for the bulk solid, the carbon isotope fractionation factor for the Mg-calcite-HCO3(aq)- system (as 10(3) 1n alpha(-)(Mg-c1-HCO3(aq))) increased with average mol percentage of Mg (X-Mg) 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 10(3) 1n alpha(-)(Mg-c1-HCO3(aq)) did not vary for precipitation rates that ranged from 10(3.21) to 10(4.60) mu mol m(-2) h(-1), it was not possible to hold Mg-content of the solid constant, so kinetic effect on 10(3) 1n alpha could not be evaluated from these experiments. (c) 2005 Elsevier Inc. All rights reserved.