Flux melting

In igneous petrology, flux melting occurs when water and other volatile components are added to hot solid rock. In engineering and metallurgy, flux is a substance, such as salt, that produces a low melting point mixture with a metal oxide. In the same way, the addition of water and other volatile compounds to rocks composed of silicate minerals lowers the melting temperature of those rocks.

In subduction zones, the ultramafic rock of the upper mantle is melted by the addition of volatiles from the down-going plate. The subducting slab of oceanic crust carries water and other volatiles into the mantle, where these volatiles are released by metamorphic dewatering into the overlying mantle wedge. The partial melting triggered by the incorporation of volatiles produces mafic magma which rises and differentiates forming the igneous and volcanic rocks of the overlying volcanic arc.