Incomplete reaction of plagioclase in experimental dehydration melting of amphibolite

Dehydration melting of a hornblende‐plagioclase mixture of amphibolitic composition was investigated at 1000°C and at 800 MPa and 1200 MPa. At 1200 MPa the reaction products are garnet, clinopyroxene, melt and relatively Ab‐rich plagioclase (An₄₇). At 800 MPa the products are orthopyroxene, clinopyroxene, magnetite, amphibole (pargasite) and An‐rich plagioclase (An₇₅). The melts are rich in plagioclase components (especially in Ab) and, when compared to tonalites, relatively poor in silica. The grainsize of the starting materials was ≤?5 μm in the 800 MPa and ≤?10 μm in the 1200 MPa runs. All run products show unchanged plagioclase cores, which are the remnants of a very sluggish reaction assumed to be controlled by dissolution/precipitation processes at the plagioclase grain boundaries. The results indicate that only local equilibrium could have been obtained in recent investigations on dehydration melting experiments in plagioclase‐bearing systems. The results also suggest that plagioclase compositions once formed may persist for a very long time, even in hot magma chambers, if the prevailing water activity is low.