Modeling Coupled Chemical and Isotopic Equilibration Rates

We present a model for coupled chemical and isotopic equilibration rates based on Transition State Theory (TST) type rate laws and apply it to Ca isotope kinetic fractionation in natural systems. The model, in contrast to the steady state model proposed by DePaolo (2011), explicitly considers isotopic equilibration in the absence of bulk chemical disequilibrium by making use of a solid-solution model for the precipitating (and potentially) dissolving mineral phase. The model is applied to the subsequent isotopic equilibration of the isotopically light bulk or surface layer on calcite formed as a result of kinetic Ca fractionation associated with precipitation. The model is then applied to Ca isotopic re-equilibration at marine Site 984 in the mid-Atlantic using the Ca isotope data of Turchyn and DePaolo (2011) based on a modification of the reactive transport model presented in Maher et al. (2006). The simulations with the new model CrunchTope demonstrate that it is impossible to match the observed Ca isotope profiles at Site 984 using the same rate constants as were used to describe calcite precipitation. To match the isotopic data requires re-equilibration rate constants that are about 4 orders of magnitude smaller than those for bulk precipitation using a linear TST rate law.