How do hydrous rocks form in subduction zones

Oceanic plates become hydrated on or below the seafloor when seawater alters basalt and peridotite (e.g., serpentinization). As the plate subducts, pressure–temperature changes stabilize hydrous minerals such as serpentine, chlorite, amphibole and—at higher pressures—lawsonite.

Progressive metamorphism and devolatilization then release fluids that percolate into the overlying mantle wedge and the slab itself, producing metasomatism and new hydrous assemblages (blueschist and lawsonite-eclogite facies). In warm subduction settings, mafic oceanic crust undergoes punctuated major dehydration at depths overlapping episodic tremor and slip; in colder geotherms, lawsonite persists deeper, transporting water to greater depths.

These reactions explain fluid pathways, intermediate-depth seismicity (via dehydration embrittlement), and the prevalence of hydrous minerals in exhumed subduction complexes.