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

Mantle (spinel Iherzolite, dunite, harzburgite, websterite, orthopypoxenite ) and crustal (mafic granulite, pegmatitic gabbro) xenoliths occur in basal tic lavas of the Simcoe volcanic field, Cascades are. The mantle xenoliths are invaded by diverse alkali- and silica-rich glasses (phonolithic-trachyt ic) that are unrelated to the host lavas. Fluid inclusions occur in webster ite, orthopyroxenite, gabbro, and rarely in dunite. Microthermometry indica tes that these are nearly pure CO2, and provides constraints on the entrapm ent pressures and temperatures for representative lithologies. Phlogopite-b earing olivine orthopyroxenite and websterite xenoliths contain CO2 inclusi ons with densities between 1.07-0.59 g/cm(3). At independently estimated eq uilibration temperatures (similar to 1000 degrees C), entrapment pressures of the highest density inclusions must have been at least similar to 9 kbar (greater than or equal to 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 CO2 inclusions formed at pressures no greater than simil ar to 2 kbar. Inclusions recording intermediate pressures may reflect progr essive fluid entrapment over a range of depths during ascent; sources for t he low density secondary inclusions may be either retrapped, decrepitated p rimary inclusions or CO2 exsolved from the host lava. Gabbroic pegmatite an d mafic granulites also contain CO2 inclusions with densities between 1.02- 0.57 and 0.96-059 g/cm(3), respectively; assuming entrapment temperatures o f 900 +/- 100 degrees C, these inclusions likely formed at pressures (simil ar to 5-7 kbar) equivalent to mid-to-lower crustal depths (< 20-25 km). Bas ed on our depth estimates and other petrologic information. Simcoe xenolith s were derived from a previously metasomatized domain within the Cascades s ubarc lithospheric mantle. Any relation between the earlier metasomatic eve nt(s) and Cascadia subduction remains tenuous. (C) 1999 Elsevier Science B. V. All rights reserved.