MELTING OF ALBITE AND DEHYDRATION OF BRUCITE IN H2O-NACL FLUIDS TO 9 KBARS AND 700-900-DEGREES-C - IMPLICATIONS FOR PARTIAL MELTING AND WATER ACTIVITIES DURING HIGH-PRESSURE METAMORPHISM

The melting reaction: albite((solid)) + H2O(fluid) = albite-H2O(melt) has been determined in the presence of H2O NaCl fluids at 5 and 9.2 kb ar, and results compared with those obtained in presence of H2O CO2 fl uids. To a good approximation, albite melts congruently at 9 kbar, ind icating that the melting temperature at constant pressure is principal ly determined by water activity. At 5 kbar, the temperature (T)-mole f raction (X((H2O))) melting relations in the two systems are almost coi ncident. By contrast, H2O-NaCl mixing at 9 kbar is quite non-ideal; al bite melts similar to 70 degrees C higher in H2O-NaCl brines than in H 2O-CO2 fluids for X((H2O)) = 0.8 and similar to 100 degrees C higher f or X((H2O)) = 0.5. The melting temperature of albite in H2O-NaCl fluid s of X((H2O)) = 0.8 is similar to 100 degrees C higher than in pure wa ter. The P-T curves for albite melting at constant H2O-NaCl show a tem perature minimum at about 5 kbar. Water activities in H2O-NaCl fluids calculated from these results, from new experimental data on the dehyd ration of brucite in presence of H2O-NaCl fluid at 9 kbar, and from pr eviously published experimental data, indicate a large decrease with i ncreasing fluid pressure at pressures up to 10 kbar. Aqueous brines wi th dissolved chloride salt contents comparable to those of real crusta l fluids provide a mechanism for reducing water activities, buffering and limiting crustal melting, and generating anhydrous mineral assembl ages during deep crustal metamorphism in the granulite facies and in s ubduction-related metamorphism. Low water activity in high pressure-te mperature metamorphic mineral assemblages is not necessarily a criteri on of fluid absence or melting, but may be due to the presence of low a((H2O)) brines.