Meteoric water-basalt interactions. I: A laboratory study

Rates, stoichiometry and activation energy of dissolution, pH vs. time and activity-activity paths were determined by dissolving basaltic rocks under simulated natural conditions at 25 to 60°C. Dissolution follows a linear rate law, with basaltic glass dissolving about 10 times faster than crystalline basalt. Rates are independent of pH from 7 to 9.5. The basaltic glass dissolves near stoichiometrically at 25°C, but this is not so for the crystalline basalt. The average activation energy for dissolution of basaltic glass is 32 kJ/ mol (±3). For individual elements leached from crystalline basalt, it ranges from 35 to 15 kj/mol. This indicates that under experimental conditions reactions on the surfaces of the solids are the rate-determining step in the dissolution mechanism. Ca, Mg, and Si reached steady state or equilibrium concentrations in solutions at 45 and 65°C. Alteration minerals formed in vugs on the solids but did not form a continuous protective layer on the solids. Reaction paths for the experimental solutions are strongly dependent on their pH versus time evolution. Solutions sealed off from the atmosphere evolve into the Ca-zeolite field, whereas those open to the atmosphere evolve into the clay mineral fields. During the experiments the consumption of hydrogen by solids at 25°C (10^-5 mol/kg) is not sufficient to significantly change the hydrogen isotope ratio of the experimental solutions.