Liquidus phase relations have been experimentally determined in the system Qz-Or-Ab at 2 and 5 kbar under both H2O-saturated and H2O-undersaturated conditions. Crystallization experiments were conducted with dry synthetic glasses as starting materials. Reversals used partially crystallized glasses. H2O-undersaturated conditions were obtained by using H2O-CO2 fluid mixtures. Six liquidus surfaces, corresponding to an initial fluid phase composition of 1, 0.7, and 0.5 at 2 kbar and 1, 0.85, and 0.7 at 5 kbar, were studied. Small ratios of fluid to silicate were used, and the equilibruim fluid phase composition [H2O/(H2O + CO2)] difers from . For both pressures, H2O solubilities decrease with decreasing . H2O activities (aH2O), calculated for compositions close to the thermal minimum of the corresponding liquidus surface are 1, 0.5, and 0.25 at 2 kbar and 1, 0.6, and 0.4 at 5 kbar. General agreement is noted between crystallization and reversal experiments, both in terms of liquidus temperatures and phase compositions.
Our data for H2O-saturated conditions are in good agreement with Tuttle and Bowen (1958) at 2 kbar. At 5 kbar, our quartz-feldspar field boundary is richer in normative quartz by 2–6 wt%, compared with Luth et al. (1964). At both pressures, decreasing the H2O content of the melt causes a rise in liquidus temperatures for all studied compositions and a progressive shift of minimum and eutectic compositions toward the Qz-Or join at approximately content normative quartz content. The rise in liquidus temperatures with decreasing H2O contrast of the melt is more marked for Ab- than for Or-rich compositions. The observed changes in phase relations contrast with the results of calculations using the model of Burnham and Nekvasil (1986) for hydrous aluminosilicate melts. Our results suggest a differential reactivity of Ab- and Or-forming units in the melt with H2O, consistent with higher H2O solubilities in Ab-rich than in Or-rich melts. Application of the results to natural magmatic systems allows the effect of pressure and of the H2O content of the melt on residual granitic liquid compositions to be specified individually.