Quartz, albite and diopside solubilities in H2O-NaCl and H2O-CO2 fluids at0.5-0.9 GPa

The solubility of quartz and incongruent dissolution ('apparent solubility' ) of albite and diopside in H2O-NaCl and H2O-CO2 fluids have been determine d at pressures up to 0.9 GPa (9 kbar) and temperatures of 500-900 degreesC. Solubilities of quartz and albite decrease with increasing salt concentrat ion [X(NaCl)](salt-out effect), whereas the solubility of diopside increase s with increasing X(NaCl)(salt-in effect). Quartz solubilities in the syste ms H2O-NaCl and H2O-CO2 are very similar and are proportional to X(H2O)(2). Quartz solubility in NaCl-rich brines does not change with pressure under the P-T conditions of our experiments. At 0.9 GPa and 800 degreesC, albite solubility in pure water is higher (similar to 100 g/ kg H2O) than that of quartz (similar to 74 g/kg H2O), but at NaCl concentrations > 6 mol% these solubilities are very similar. Albite dissolution is slightly incongruent; formation and composition of secondary and quench phases (paragonite, Na-ma rgarite, amorphous quench spheres) indicate that the solution is enriched i n SiO2 and Na2SiO3. As fluid composition changes from H2O towards more CO2- rich fluids or NaCl-rich brines, the solubilities of albite and quartz decr ease by about one order of magnitude, but are still measurable at X(H2O)< 0 .5. A thermodynamic analysis of new quartz solubility data for H2O-CO2 flui ds at 0.9 GPa indicates that complexing is dominated by SiO2.4H(2)O for wat er-rich fluids, but for X(H2O)<0.7 the mean solvation number decreases to l ess than or equal to 3. This speciation is similar to that reported previou sly for lower pressures, and is in agreement with recent high P-T spectrosc opic data for the system H2O-SiO2. For dissolution of quartz in both H2O-CO 2 and H2O-NaCl fluids, the molecular proportions of silica to water are alm ost the same at any X(H2O). Assuming similar non-ideal water-salt interacti ons irrespective of whether the water is in the fluid or is complexed with silica, then the speciation of silica appears to be similar in both H2O-CO2 and H2O-NaCl fluids under the experimental conditions. We conclude that th e speciation of silica in both H2O-CO2 and H2O-NaCl fluids at 0.9 GPa and 8 00 degreesC is comparable, is dominated by Si(OH)(4).2H(2)O and/or hydrated species with lower hydration numbers, and is also comparable to that propo sed for lower P-T conditions.