Stability of hydrous phases in subducting oceanic crust

Experiments in the basait-H2O system at 600-950 degrees C and 0.8-3.0 GPa, demonstrate that breakdown of amphibole represents the final dehydration of subducting oceanic tholeiite at T greater than or equal to 650 degrees C; the dehydration H2O occurs as a free fluid or in silicate melt co-existing with an anhydrous eclogite assemblage. In contrast, about 0.5 wt% of H2O is stored in lawsonite at 600 degrees C, 3.0 GPa. Our results suggest that slab melting occurs at depths shallower than 60 km for subducting young oceanic crust; along a subduction zone with an average thermal gradient higher than 7 degrees C/km, H2O stored in hydrated low-potassium, metabasaltic layers cannot be subducted to depths greater than 10D km, then released to generate are magma.