How do hydrous rocks affect mantle melting and magma genesis

Water lowers the peridotite solidus dramatically, so slab-derived fluids and/or hydrous melts flux the mantle wedge, initiating partial melting at lower temperatures and generating calc-alkaline arc magmas. 

Experimental petrology on H₂O-bearing lherzolite shows large solidus depression under water-saturated or damp conditions; modeling and experiments further indicate that modest H₂O contents shift melt productivity and major-element trends.

Some studies propose contributions from sediment-derived melts in addition to fluids, but the consensus remains that slab-released volatiles are critical to starting and sustaining wedge melting. Hydrous metasomatism also stabilizes amphibole/mica in the lithosphere, transiently storing up to a few wt% H₂O before further heating drives melting or dihydroxylation.