Hydrous, silica-rich melts migrating through the mantle are preserved as glass inclusions in mantle minerals in xenoliths from Philippine are lavas. These melts, with chemistries that indicate an origin by very low degrees of melting of the subducted ocean crust, have altered; their host peridotites, yielding a metasomatized mantle. This ’fertilized’ mantle is the source region of the are magmas, which share continuous chemical trends with the melt inclusions, reflecting mixing and/or varying degrees of melting. These observations provide direct evidence for the importance of slab-mantle interactions in the genesis of island-are magmas.