LIQUIDUS TEMPERATURES AND PHASE COMPOSITIONS IN THE SYSTEM QZ-AB-OR AT 5 KBAR AND VERY-LOW WATER ACTIVITIES

Liquidus phase relations have been experimentally determined in the sy stems Qz-Ab-Or-(H2O), Qz-Ab-(H2O) and Qz-Or-(H2O) at H2O-undersaturate d conditions (a(H2O) = 0.07) and P = 5 kbar. Starting materials were h omogeneous synthetic glasses containing 1 wt% H2O. The liquidus temper atures were bracketed by crystallization and dissolution experiments. The results of kinetic studies showed that crushed glasses are the bes t starting materials to overcome undercooling and to minimize the temp erature difference between the lowest temperature of complete dissolut ion (melting) and the highest temperature at which crystallization can be observed. At P = 5 kbar and a(H2O) = 0.07, the Qz-Ab eutectic comp osition is Qz(32)Ab(68) at 1095 degrees C (+/- 10 degrees C) and the Q z-Or eutectic is Qz(38)Or(62) at 1030 degrees C (+/- 10 degrees C). Th e minimum temperature of the ternary system Qz-Ab-Or is 990 degrees C (+/- 10 degrees C) and the minimum composition is Qz(32)Ab(35)Or(33) T he Oz content of the minimum composition in the system Qz-Ab-Or-H2O re mains constant with changing a(H2O) The normative Or content, however, increases by approximately 10 wt% with decreasing a(H2O) from 1 to 0. 07. Such an increase has already been observed in the system Qz-Ab-Or- H2O-CO2 at high a(H2O) and it is concluded that the use of CO2 to redu ce water activities does not influence the composition of the minima i n quartz-feldspar systems. The determined liquidus temperature in melt s with 1 wt% H2O is very similar to that obtained in previous nominall y ''dry'' experiments. This discrepancy is interpreted to be due to pr oblems in obtaining absolutely dry conditions. Thus, the hitherto publ ished solidus and liquidus temperatures for ''dry'' conditions are pro bably underestimated.