The ocean lithosphere suffers extensive hydrothermal alteration at the ridge, but the rocks eventually finish up subducting back into the mantle:
It is because these fluids are released in the Benioff Zone as the slab is subducted that magmas are generated in the mantle wedge above the subduction zone. It is fluid, not friction, which is responsible for active margin magmatism. But it is ridge processes which make it all possible.

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