Ultramafic xenoliths in the southern Washington Cascades lavas provide a rare view of the upper mantle beneath a convergent margin volcanic arc. An unusual olivine orthopyroxenite from this xenolith suite shows evidence of extreme multistage metasomatism involving infiltration of high-K2O silicic melts and CO2 fluid, replacement of olivine by orthopyroxene, and entrapment of fluid, melt, and phlogopite inclusions within orthopyroxene—all prior to its entrainment in the host basaltic magma. Analyses of minerals, glass, and fluid inclusions show that the mantle source of this rock was ≥30 km below the surface and reached a temperature of at least 1000 °C—possibly in response to basaltic underplating. This region of the mantle was previously enriched in silica and potassium, but not necessarily due to subduction processes. The metasomatic melts may be derived from an accreted slab of oceanic lithosphere, as opposed to a subducting oceanic plate.