Fluid inclusions in mantle and lower crustal xenoliths from the Simcoe volcanic field, Washington

Abstract

Mantle (spinel lherzolite, dunite, harzburgite, websterite, orthopyroxenite) and crustal (mafic granulite, pegmatitic gabbro) xenoliths occur in basaltic lavas of the Simcoe volcanic field, Cascades arc. The mantle xenoliths are invaded by diverse alkali- and silica-rich glasses (phonolithic–trachytic) that are unrelated to the host lavas. Fluid inclusions occur in websterite, orthopyroxenite, gabbro, and rarely in dunite. Microthermometry indicates that these are nearly pure CO 2, and provides constraints on the entrapment pressures and temperatures for representative lithologies. Phlogopite-bearing olivine orthopyroxenite and websterite xenoliths contain CO 2 inclusions with densities between 1.07–0.59 g/cm 3. At independently estimated equilibration temperatures (∼1000°C), entrapment pressures of the highest density inclusions must have been at least ∼9 kbar (≥30 km depth). Associated lherzolites cut by veins of websterite also must be derived from similar depths. Textural relations indicate that invasive melts were the immediate sources for these fluids. The lowest density CO 2 inclusions formed at pressures no greater than ∼2 kbar. Inclusions recording intermediate pressures may reflect progressive fluid entrapment over a range of depths during ascent; sources for the low density secondary inclusions may be either retrapped, decrepitated primary inclusions or CO 2 exsolved from the host lava. Gabbroic pegmatite and mafic granulites also contain CO 2 inclusions with densities between 1.02–0.57 and 0.96–0.59 g/cm 3, respectively; assuming entrapment temperatures of 900±100°C, these inclusions likely formed at pressures (∼5–7 kbar) equivalent to mid-to-lower crustal depths (<20–25 km). Based on our depth estimates and other petrologic information, Simcoe xenoliths were derived from a previously metasomatized domain within the Cascades subarc lithospheric mantle. Any relation between the earlier metasomatic event(s) and Cascadia subduction remains tenuous.