Effects of H₂O on liquidus phase relations in the haplogranite system at 2 and 5 kbar

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. 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. For both pressures, H₂O solubilities decrease with decreasing X^In_H2O. H₂O activities (a_H2O), 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. The data for H₂O-saturated conditions are in good agreement with Tuttle and Bowen (1958) at 2 kbar. At 5 kbar, the quartz-feldspar field boundary is richer in normative quartz by 2-6 wt.%, compared with Luth et al. (1964). The rise in liquidus temperatures with decreasing H₂O contrast of the melt is more marked for Ab- than for Or-rich compositions. 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. -from Authors