Subduction zones and volcanic arcs are the most tectonically active regions on Earth and are crucial to understanding, geochemical cycles, tectonic-climate coupling, ore genesis and natural hazards. Bimodal volcanism is a long-recognised characteristic of arc crust that has never been satisfactorily explained. This project tests the new hypothesis that the two types of magmas originate from distinct mantle sources. It takes the innovative approach of integrating novel high-pressure experiments with database analysis of natural volcanic rocks, covering magmatic systems from mantle source to volcano. This project will improve our understanding of arc processes, including the association of economic metals with arc volcanism.