Abstract

The solubility behavior of K2O, Na2O, Al2O3, and SiO2 in silicate-saturated aqueous fluid and coexisting H2O-saturated silicate melts in the systems K2O-Al2O3-SiO2-H2O and Na2O-Al2O3-SiO2-H2O has been examined in the 1-2 GPa pressure range at 1100°C. Glasses of tetrasilicate compositions (NS4 and KS4) with 0, 3 and 6 mol % Al2O3 were used as starting materials. In both systems, the oxides dissolve incongruently in both aqueous fluid and silicate melt. The aqueous fluids are enriched in alkalis and depleted in silica and alumina. The extent of incongruency is more pronounced in the K2O-Al2O3-SiO2-H2O system than in the K2O-Al2O3-SiO2-H2O system.

The partition coefficients of the oxides, Dfluid/meltoxide, are linear and positive functions of the oxide concentration in the fluid for each composition. There is a slight dependence of the partition coefficients on bulk composition No effect of pressure could be discerned. For alkali metals the fluid/melt partition coefficients range from 0.06 to 0.8. For Al2O3 this range is 0.01-0.2, and for SiO2 0.01-0.32. For all compositions, Dfluid/meltK2O ~ Dfluid/meltNa2O > Dfluid/meltSiO2 > Dfluid/meltAl2O3 for the same oxide concentration in the fluid. The Dfluid/meltK2O, Dfluid/meltNa2O, and Dfluid/meltSiO2 correlate negatively with the Al2O3 content of the systems. This correlation is consistent with a solubility model of alkalis that involve associated KOH°, NaOH°, and aluminate complexes. The behavior of Dfluid/meltSiO2 can modeled with a solute that contains depolymerized silicate complexes associated with alkalis in the fluid.