Kinetic fractionation of carbon-13 during calcium carbonate precipitation

Kinetic isotope fractionation of 13C during precipitation of CaCO3 under open system conditions has been investigated. The isotope enrichment factor epsilonsub(HCO3--CaCO3) varies between - 0.35 +- 0.23 and - 3.37 +- 0.36 parts per thousand at 250C depending on the rate of precipitation and mineralogy, the enrichment of 13C in the solid carbonate phase decreasing with increasing precipitation rate. An estimate of equilibrium epsilonsub(HCO3--Calcite) of between - 1.83 +- 0.32 and - 2.26 +- 0.31 parts per thousand is calculated from slow precipitation runs. A surface diffusion crystal growth model is used to describe the combination of kinetic isotope effects on thermodynamic isotope fractionation during rapid diffusion controlled crystal growth. Under slow precipitation conditions epsilonsub(Calcite-Aragonite) was estimated as - 1.4 parts per thousand; however, during rapid precipitation this fractionation appears to diminish and aragonite becomes less enriched in 13C. (author)