Most magmas derived in subduction zones come from the mantle above the subducting plate when hydrous fluids are released from minerals that break down in the metamorphosed basalt, rise into the mantle, and initiate partial melting. However, Defant and Drummond recognized that when young oceanic crust (less than 25 million years old) is subducted, adakites are typically produced in the arc.
In subduction zones, most magmas come from the upper mantle above the plate being subducted, triggered by the release of hydrous fluids from broken-down basalt that have undergone metamorphism. Nevertheless, Defant and Drummond observed that when newly formed oceanic crust (less than 25 million years old) is submerged, adakites are commonly produced in the volcanic arc.