PETROLOGIC AND GEOCHEMICAL CONSTRAINTS ON PRE-ERUPTIVE STORAGE CONDITIONS OF MAGMAS ERUPTED DURING THE ~12.5KA FLARE UP OF MEDICINE LAKE VOLCANO, CA

Medicine Lake Volcano (MLV) in the California Cascades is a shield volcano superimposed by numerous basaltic to rhyolitic vents, primarily aligned with N-S trending normal faults. A significant portion of the post-glacial volcanic activity at MLV is comprised of the ~12.5 ka “flare up” eruptions of basalt and basaltic andesite magmas from seven discrete flank vents. Prior work documented surface areas (0.02-198 km²), volumes (0.0001-4.35 km³), and eruption styles (Hawaiian to Strombolian) (Donnelly-Nolan, 2010) of the ~12.5 ka deposits. This study uses new geologic mapping of near-vent features, whole-rock compositions of lavas and pyroclastic material, in situ elemental concentrations of olivine and CPX phenocrysts, and CPX geothermobarometry to constrain pre-eruptive storage conditions of flare up magmas. Lavas from vents are basalt and basaltic-andesite, with whole-rock compositions ranging from 49.3 to 54.2 wt% SiO₂ and 6.0 to 8.5 wt% MgO. Samples from eastern vents, Ribbon and Valentine Cave, form trends that are offset from central and western vent lavas. Lavas from eastern vents also have higher concentrations of Na₂O, P₂O₅ and TiO₂ and lower concentrations of MgO, CaO, Al₂O₃ and K₂O/P₂O₅ compared to lavas erupted from vents on the central and western MLV flanks. The mineral assemblage of flare-up lavas is olivine and plagioclase with less CPX and Fe-Ti oxides. Some crystals display disequilibrium textures, such as partially resorbed olivine, reversely zoned olivine, and dusty-sieved and coarsely-sieved plagioclase. Most olivine crystals have core compositions of >Fo₈₀, but core regions of olivine crystals in deposits erupted from all flare-up vents range from to Fo₆₆ to Fo₈₈. CPX geothermobarometry modeling (using the method of Putirka et al., 1996) of lavas from eastern vents record crystallization pressures of 0.5-17.3 kbar and temperatures of 1156.7-1270°C, whereas lavas from central and western vents record deeper crystallization pressures (0.6-10.5 kbar) but similar crystallization temperatures (1192.4 -1307.3°C). Geochemical and petrological results are consistent with a ~12.5 ka flare up eruption that tapped multiple magma systems beneath MLV, where deeper TiO₂-rich magmas erupted from vents in the east compared to shallower MgO-rich magmas that erupted from western and central vents.