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
Ki. Shmulovich et Cm. Graham, 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, Contributions to Mineralogy and Petrology, 124(3-4), 1996, pp. 370-382
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.