PETROGRAPHY AND GEOCHEMISTRY OF MOUNT ST. HELENS INCLUSIONS AND HOST DACITES: IMPLICATIONS FOR MAGMA TRANSPORT AND STORAGE BENEATH ARC VOLCANOES

Mount St. Helens (MSH), a Cascade Arc volcano, shows evidence of eruption since ~300 ka. While melts and major phases are young, suggesting rapid transport from generation to eruption, zircons require that material is stored in the magmatic system for extended periods and is then entrained and erupted by younger magmas. It is unclear how these magmas are stored: as solid rock or melt-bearing mush. Crystal-rich inclusions in erupted lavas may provide information on the conditions and state of stored material. We characterize petrography and geochemistry of dacites from the Pine Creek period and an inclusion to evaluate the physical character of the zircon-bearing materials stored in the MSH system.

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% SiO_2,  16.2-18.5 wt% Al2O3, 5.8-6.1 wt% Na₂O+K₂O) are chemically more evolved than inclusions (57-59 wt% SiO_2, ~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 735-767^oC (hosts) and 697-744^oC (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.