Aqueous geochemistry at subduction zone conditions: Experimental constraints

Despite the importance of fluids in high pressure environments, the behavior of most solutes in H₂O-rich solutions at the relevant pressures and temperatures has not been measured. The revised Helgeson-Kirkham-Flowers equations of state (HKF-EOS) depends on empirical solute data and is the method the most widely used for calculating the thermodynamic properties of aqueous species at elevated pressures and temperatures. There are, however, empirical data on only a few species over the entire interval over which the HKF model may be used; it is necessary to support this predictive approach through experiments. This dissertation provides experimental data on the solubility of major rock forming elements at 0.5–2.0 GPa, 350–500°C and on the dissociation of H₂S_aq and HCl at elevated pressures and temperatures.; This study's diaspore solubility measurements greatly increase the range of conditions where the solubility of aluminum has been measured. In this system, the neutral aluminum species, Al(OH)₃0 predominates and an expression: GJ AlOH ⁰3=-98 2552-24799.4PGPa+ -285+76PGPa TK was derived to allow calculation of aluminum solubility in near-neutral solutions over a range of pressures and temperatures and demonstrate that aluminum is fairly mobile in near-neutral solutions.; Albite-paragonite-quartz solubility experiments probe the mobility of silica, sodium and aluminum in these high pressure environments. The measured solubilities were high and increase with both pressure and temperature. These results corroborate the strong increase in solubility with high pressures expected from lower pressure experiments.; As natural solutions are seldom pure H₂O, the dissociation of H₂S_aq at 0.5 GPa, 350°C and of HCl_aq at 1.0 GPa, 470°C and 1.2 GPa, 450°C were measured. The dissociation constant of H₂S_aq was found to be much lower than predicted, indicating that HS− dominates under more basic conditions than previously believed. The dissociation of HCl was determined to be about ten times lower than some predictions, agreeing with the lowest predicted values.; The results from these experiments demonstrate that mineral solubilities are comparatively high at elevated pressures. These experiments also demonstrate that the dissociation of the acids HCl_aq and H ₂S_aq are considerably more associated than predicted and will therefore play a less significant role in mass transport at high pressures than has been believed.