OH defects in quartz in the system quartz-albite-water and granite-water between 5 and 25 kbar

The incorporation of OH defects in quartz from the systems quartz-water, quartz-albite-water and granite-water at pressures between 5 and 25 kbar and temperatures between 800 and 1,000 degrees C was investigated by IR spectroscopy. The two most important OH absorption features can be assigned to hydrogarnet defects (absorption band at 3,585 cm(-1)) and coupled substitutions involving Al3+ (Al-H defects, absorption bands at 3,310, 3,378 and 3,430 cm(-1)). Al incorporation in quartz is controlled by mineral/melt partitioning (D-Al(Qz/Melt) = 0.01) and exhibits a negative pressure dependence. This trend is not clearly reflected by the concentration of Al-H defects, which shows positive deviations from the theoretical 1:1 correlation of Al/H for some samples. In contrast to the Al-H defects, formation of hydrogarnet defects appears to be positively correlated to pressure and water activity, and may be used a petrological indicator. The overall water concentration exhibits only minor changes with pressure and temperature, but a clear correlation of water activity (controlled by various amounts of dissolved salts) and hydrogarnet substitution could be established.