The dependence of labradorite dissolution and Sr isotope release rates on solution saturation state

Labradorite dissolution kinetics and Sr release rates were measured as a fu nction of the saturation state of weathering solutions in column reactors. During the first 750 hours, rapid, nonstoichiometric dissolution was observ ed. Once steady state had been reached, both the overall dissolution and Sr release became stoichiometric. Under steady state conditions that were far from being in equilibrium with the labradorite, we measured the log of the overall labradorite dissolution rate (mol mineral/m(2)/s) to be -10.6 +/- 0.1 while the Sr release rate was - 13.2 +/- 0.1 (mol Sr/m(2)/s). The isoto pic ratio of the output solutions did not vary with time as both the early Sr-87/Sr-86 ratios and the later, steady state ratios were all essentially the same as that of the bulk labradorite (0.704671). As the saturation state of the solution in the columns increased from -16 t o -4.5 kcal/mol, the labradorite dissolution rate decreased by a factor of similar to 4.5. To quantify this decrease, we determined a function that de scribed the dependence of the labradorite dissolution rate on the solution saturation state. Using an implicit finite difference model to predict the chemical evolution of the solution passing through the column, we found tha t the dependence of labradorite dissolution rate on solution saturation sta te that best agrees with our experimental data was Rate = -kmin{0.76*[1 - exp((1.3 x 10(-17))*(\Delta G(r)\/RT)(14))] - 0.24 * [1 - exp(-0.35 * \Delta G(r)\/RT)]}, where k(min) is the far from equilibrium rate constant (mol/m(2)/s), Delta G(r) is the Gibbs free energy of the dissolution reaction (kcal/mol), R is the gas constant (kcal/mol K) and T is the temperature (K). The rate depend ence described by this equation suggests that under far from equilibrium co nditions, dissolution occurs primarily by etch pit formation at defect site s. Closer to equilibrium, etch pit formation becomes less important and dis solution becomes more uniform across the crystal surface. The dependence of the Sr release rates on solution saturation could also be described by the above equation where k(min) was the far from equilibrium Sr release rate. Changes in the solution saturation state did not, however, affect the isoto pic ratio of the Sr released during weathering. Quantifying the rates of Sr release during plagioclase weathering and the effect that the solution sat uration state has on those rates has important implications in terms of the use of Sr isotopes as a proxy for chemical weathering rates and the establ ishment of a more rigorous relationship between variations in the marine Sr isotopic record, average global weathering rates and atmospheric CO2 conce ntrations. Copyright (C) 2000 Elsevier Science Ltd.