Water exchange process and bulk composition regulate slab dynamics and deep earthquakes

Water strongly influences deep mantle processes, including geochemical cycles, slab dynamics, and deep-focus earthquakes. Yet the stability and interactions of hydrous minerals with nominally anhydrous minerals (NAMs) under the water-undersaturated conditions of subducting slabs remain unclear. Here we show, using high-pressure and high-temperature experiments on MgO-SiO₂-H₂O systems (~2 wt% H₂O), that hydrous minerals progressively dehydrate in the mantle transition zone, transferring water to NAMs such as wadsleyite and ringwoodite, while the cold slab core stays nearly dry. Rapid dehydration near the top of the lower mantle may generate fluids linked to the deepest earthquakes. Our results indicate that dry olivine transformations, rather than dehydration embrittlement, likely trigger most deep-focus earthquakes, and that hydration variations in NAMs influence slab deformation and stagnation above 660 km. The bulk Mg/Si ratio controls hydrous mineral stability, suggesting harzburgite transports water more efficiently than peridotite.