Melting Relationships in the System NaAlSi_3O_8-NaF-H_2O to 4 Kilobars Pressure

The phase relationships in the systems NaF-H_2O and NaAlSi_3O_8-NaF-H_2O were determined between 600° and 900° C. at pressures up to 4 kb., and those in the anhydrous system NaAlSi_3O_8-NaF were determined at 1 atm. The phases encountered were albite, villiaumite (NaF), liquid, and vapor. The liquid quenches to a glass containing skeletal NaF crystals. Primary villiaumite is readily distinguished from "quench" NaF. Albite crystals coexisting with liquid are several times larger than subsolidus crystals. The binary reaction villiaumite + vapor ↔ liquid occurs at 860° ± 7° C. at 1 kb., and the binary reaction albite + villiaumite ↔ liquid occurs at 860° ± 5° C. at 1 atm. The beginning of melting in the ternary system is the reaction albite + villiaumite + vapor ↔ liquid; this occurs at 753° ± 5° C. at 0.5 kb., at 688° ± 5° C. at 1 kb., at 640° ± 5° C. at 2 kb., at 630° ± 5° C. at 3 kb., and at 600° ± 5° C. at 4 kb. The composition of the univariant liquid in the system NaF-H_2O at 1 kb. is approximately 80 wt. per cent NaF and 20 wt. per cent H_2O; the composition of the univariant liquid in the anhydrous system is approximately 84 wt. per cent NaAlSi_3O_8 and 16 wt. per cent NaF. The composition of the univariant liquid in the ternary system varies with pressure. At 1 kb. the composition expressed in terms of the anhydrous components is approximately 86 wt. per cent NaAlSi_3O_8 and 14 wt. per cent NaF, and the water content is about 30 wt. per cent; at 4 kb. it is approximately 75 wt. per cent NaAlSi_3O_8 and 25 wt. per cent NaF, and with a water content of about 45 wt. per cent. Critical conditions are probably reached at a pressure not far above 4 kb. The solubility of the solids in the vapor phase at 4 kb. is about 40 wt. per cent. These results indicate that small variations in NaF content of a silicate magma can produce large variations in the water content of residual magmas and large variations in the amount of water required to saturate the magma. NaF causes residual magmas to persist to significantly lower temperatures than the final consolidation temperature if H_2O were the only dissolved volatile component