PETROGRAPHY AND GEOCHEMISTRY OF MOUNT ST. HELENS INCLUSIONS AND HOST DACITES: IMPLICATIONS FOR MAGMA TRANSPORT AND STORAGE BENEATH ARC VOLCANOES
Pine Creek age dacite hosts (3.9-3.3ka) are porphyritic (30-40% phenocrysts) with large subhedral plag (0.5-2mm) and cpx (~0.75mm) in a fine groundmass. Inclusions have larger, uniform crystals (60-70% phenocrysts) showing no evidence of resorption, including euhedral plag (0.1-0.9mm), acicular apmh (≤4mm) and pyx. Hosts (62-66 wt% SiO2, 16.2-18.5 wt% Al2O3, 5.8-6.1 wt% Na2O+K2O) are chemically more evolved than inclusions (57-59 wt% SiO2, ~17.3 wt% Al2O3, 5.4-6.2 wt% Na2O+K2O). Trace elements are more similar: ~360-460ppm Sr,300-340ppm Ba, and 252-307ppm Zr in hosts and 350-480ppm Sr, 244-315ppm Ba, and 205-284ppm Zr in inclusions. Host plagioclase show calcic cores and sodic rims, while inclusions lack this concentric zoning. Inclusion zircons demonstrate one U-Th age (an anomaly among the complex populations typical of MSH rocks) of ~18ka, (age of Cougar eruptions). Al-in-amphibole barometry suggests inclusions crystallized at a shallower location while hosts formed deeper in the crust. Zircon saturation thermometry indicates 735-767oC (hosts) and 697-744oC (inclusions).
Textures, WR compositions, and plagioclase zoning suggest that the inclusion formed slowly at lower pressures than the main erupted magma. Together, these data suggest that a batch of Cougar Stage magma was stored for ~15 kyr, cooled to a crystal-rich mush, and was then entrained by the Pine Creek magma, remaining relatively unchanged as it traveled through the crust toward eruption. This has implications for the speed of magma transport and for the conditions and physical state of magma storage beneath active arc volcanoes.