Stability of hydrous phases in subducting oceanic crust

Experiments in the basalt-H₂O system at 600–950°C and 0.8–3.0 GPa, demonstrate that breakdown of amphibole represents the final dehydration of subducting oceanic tholeiite at T≥650°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°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°C/km, H₂O stored in hydrated low-potassium, metabasaltic layers cannot be subducted to depths greater than 100 km, then released to generate arc magma.