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

Abstract

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 °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 l 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.