UCLA

Brines are potentially important agents of mass transfer in the lower crust and subduction zones. Experiments and models typically assume that alkali-halide solutions can be approximated by the system NaCl-H2O. However, KCl is also likely to be abundant in high pressure and temperature fluids, and the effect of this solute is unknown. We experimentally determined the solubility of quartz in H2O-KCl solutions at 7.5 and 10 kbar and 600-800°C. We used hydrothermal piston cylinder methods and determined solubility by weight loss.

At 10 kbar and 800°C, quartz solubility is 1.23 mol SiO2/kg H2O in pure H2O. Quartz solubility increases slightly with addition of a small amount of KCl up to 0.06 bulk KCl mole fraction (XKCl). At higher XKCl, quartz solubility decreases exponentially to 0.19 mol SiO2/kg H2O at sylvite saturation (XKCl = 0.70). Quartz solubility decreases with T at constant XKCl. At 700°C and 600°C, quartz solubility at sylvite saturation (XKCl = 0.60 and 0.54, respectively) is 0.22 and 0.17 mol SiO2/kg H2O. At 7.5 kbar, “salting in” or silica solubility enhancement is observed at XKCl < 0.025. Solubility then decreases monotonically.

Solubility variations at different P, T and salt types can be compared using the relative solubility (i.e., observed solubility divided by that in pure H2O at P, T). In H2O-NaCl solutions at 10 kbar, relative solubility decreases with rising NaCl mole fraction independent of temperature. This is not seen at high P in KCl-H2O solutions. Relative quartz solubility measurements in KCl-H2O solutions at 7.5 and 10 kbar are higher than in NaCl-H2O solutions at any given salt mole fraction. Values in KCl-H2O solutions are comparable to those in NaCl-H2O solutions at lower P of ~5 kbar. This could be a result of the formation of hydrated KCl-SiO2 complexes.

Textural and geochemical analyses of many petrologic examples indicate mass transport of silicates and mineral paragenesis enabled by alkali-rich, low-a(H2O) fluids. This newly discovered enhancement of silica solubility in KCl-H2O solutions implies the possibility of significant silica metasomatism involving potassic salt solutions in deeper environments.