Abstract.
The solubility of quartz and incongruent dissolution ('apparent solubility') of albite and diopside in H2O–NaCl and H2O–CO2 fluids have been determined at pressures up to 0.9 GPa (9 kbar) and temperatures of 500–900 °C. Solubilities of quartz and albite decrease with increasing salt concentration [X(NaCl)](salt-out effect), whereas the solubility of diopside increases with increasing X(NaCl)(salt-in effect). Quartz solubilities in the systems 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 °C, albite solubility in pure water is higher (~100 g/kg H2O) than that of quartz (~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-margarite, amorphous quench spheres) indicate that the solution is enriched in SiO2 and Na2SiO3. As fluid composition changes from H2O towards more CO2-rich fluids or NaCl-rich brines, the solubilities of albite and quartz decrease 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 fluids at 0.9 GPa indicates that complexing is dominated by SiO2.4H2O for water-rich fluids, but for X(H2O)<0.7 the mean solvation number decreases to ≤3. This speciation is similar to that reported previously for lower pressures, and is in agreement with recent high P–T spectroscopic data for the system H2O–SiO2. For dissolution of quartz in both H2O–CO2 and H2O–NaCl fluids, the molecular proportions of silica to water are almost the same at any X(H2O). Assuming similar non-ideal water-salt interactions 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 the speciation of silica in both H2O–CO2 and H2O–NaCl fluids at 0.9 GPa and 800 °C is comparable, is dominated by Si(OH)4·2H2O and/or hydrated species with lower hydration numbers, and is also comparable to that proposed for lower P–T conditions.
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Shmulovich, K., Graham, C. & Yardley, B. Quartz, albite and diopside solubilities in H2O–NaCl and H2O–CO2 fluids at 0.5–0.9 GPa. Contrib Mineral Petrol 141, 95–108 (2001). https://doi.org/10.1007/s004100000224
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DOI: https://doi.org/10.1007/s004100000224