Plagioclase stability at elevated temperatures and water pressures

The breakdown of plagioclase to zoisite, kyanite, quartz, and a more sodic plagioclase in the system NaAlSi₃O₈–CaAl₂Si₂O₈–H₂O) has been investigated with reversed reactions at 8, 9, and 10 kbar. At these pressures all plagioclase compositions from ~An₄₀ to An₁₀₀ break down along the same P–T curve as pure anorthite, which can be described by the equation P = −4590 + 20.4T (P = bars, T = °C). The isobaric curves in the T–X section are thus represented by a horizontal line (constant temperature) between ~An₄₀ and An₁₀₀. At compositions more sodic than ~An₄₀ the curves dip rather sharply to lower temperatures. In the regions beneath the curves, the composition of sodic plagioclase along the down-dipping limb is in equilibrium with Zo + Ky + Qz. The remarkable behavior in the An₄₀–An₁₀₀ region suggests that under the hydrothermal conditions of this study, regions of ordered anorthite develop in the plagioclase and subsequently react with water as would independent crystals of An. Partial melting of pure An in the presence of water begins at 10.2 kbar and 725° C. At 10.8 kbar a region of partial melt extends at nearly the same temperature from An₃₀ to An₁₀₀.

The subsolidus experimental data compare favorably with observations on metamorphic rocks. Plagioclase compositions are significantly affected by P_H2O as well as temperature and it is probable that the configuration of the plagioclase breakdown curve at moderate to high pressures effectively cuts off the oligoclase limb of the peristerite gap. Except for very high grade or low pressure metamorphic conditions plagioclases in the range An₄₀ to An₁₀₀ are unstable with respect to zoisite or epidote.