Altered oceanic crust begins to dehydrate at depths ~ 50 km or less, as chlorite, phengite, and other hydrous phyllosilicates decompose
Further dehydration takes place at greater depths as other hydrous phases become unstable, including amphibole at about 3 GPa.
The slab crust is successively converted to blueschist, amphibolite, and finally anhydrous eclogite as it reaches about 80-100 km depth
In most (mature) arcs, the temperature in the subducted crust is below the wet solidus for basalt, so the released water cannot cause melting, and most of the water is believed to rise into the overlying mantle wedge, where it reacts with the lherzolite to form a pargasitic amphibole and probably phlogopite (yellowish area)
Slightly hydrous mantle immediately above the slab is carried downward by induced convective flow where it heats up, dehydrates, and melts at A (120 km)