GRANITE-HYDROTHERMAL INTERACTION: A SIMULTANEOUS ESTIMATION OF THE MINERAL DISSOLUTION RATE BASED ON THE ISOTOPIC DOPING TECHNIQUE

Abstract

We propose an experimental evaluation of the simultaneous dissolution rate of K-feldspar, biotite and plagioclase during the interaction between a granite and a fluid artificially enriched in the less abundant strontium and potassium isotopes. Experiments were carried out at 453 K and 10 bars, for a time of 6 months, putting powered granite in contact with fluids, spiked in 39K and 84Sr, and saturated with respect to kaolinite, low-temperature albite, prehnite, calcite, adularia and quartz. The evolution of the (Rb/39K)sol vs. (41K/39K)sol and (87Sr/86Sr)sol vs. (84Sr/86Sr)sol ratios allows us to identify a single rock end-member from the first month of interaction. This indicates that the dissolving mineral assemblage remains constant during the remaining 5 months of interaction. Associating the mass conservation law to the isotopic mixing equations for both Sr and K, we evaluate that the mass of dissolved plagioclase is 3-4 times higher the mass of dissolved biotite and 10-20 times higher the mass of dissolved sanidine. The rate of potassium dissolution is estimated assuming that the (41K/39K)sol ratio is continuously in equilibrium with the ratio in neogenic phases. Coupling the proportion of dissolved minerals to the overall rate of potassium dissolution and normalising to the mineral surface area, we finally estimate that, in our experimental conditions, plagioclase and biotite dissolve with a rate of, respectively, 3+/-1x10-12 and 6+/-2x10-13 mol m-2 s-1, while the K-feldspar dissolution rate is 4+/-2x10-13 mol m-2 s-1.