Stability of the assemblage albite plus forsterite at high temperatures and pressures with petrologic implications

Abstract

The maximum limits of the assemblage albiteforsterite have been determined experimentally at high pressures and temperatures. At subsolidus temperatures, albite plus forsterite is replaced at high pressures by jadeitic clinopyroxene and enstatitic orthopyroxene. The boundary for this reaction lies within experimental uncertainity of that for jadeite=albite+nepheline. Melting of albite+forsterite at high pressures produces enstatite+liquid, which is different from the low-pressure eutectic behavior. Melting rates are very slow and several hundred hours are required to establish equilibrium near the solidus. The subsolidus boundary for albite plus forsterite lies near that for sanidine plus forsterite, but with a shallower slope which more closely matches that of anorthite plus forsterite. Both albite plus forsterite and anorthite plus forsterite are replaced at high pressures by an assemblage containing clinopyroxene plus orthopyroxene, unlike sanidine plus forsterite, which is replaced by a feldspathoid plus orthopyroxene. The presence of sodium enlarges the depth region over which plagioclase lherzolite can stably exist; it may also stabilize alkali feldspar plus olivine in crustal rocks.