QUANTIFYING THE MAGMA MIXING EVENT OF THE 1915 LASSEN PEAK, CALIFORNIA ERUPTION USING THE MAGMA CHAMBER SIMULATOR

Lassen Peak Volcano, located in northern California, is the southern-most volcano in the Cascade volcanic arc. The 1915 Lassen eruption produced four distinct volcanic products: (1) light dacite (~65-68 wt.% SiO₂), (2) black dacite (~64-66 wt.% SiO₂), (3) dark andesite (~59-61 wt.% SiO₂), and (4) andesitic inclusions (~57-59 wt.% SiO₂). These products have varied amounts of quartz, plagioclase (An_30-70), olivine (Fo_80-84), enstatite, augite, hornblende, biotite, and other minor minerals. It is hypothesized that the four products formed from complex mixing of variable proportions of host dacite and intruding basaltic andesite (Clynne, 1999). Rhyolite MELTS (Gualda et al., 2012) was used to determine the pressures and initial water concentrations for Clynne’s (1999) calculated mixing end member compositions (basaltic andesite and dacite). Best-fit models suggest the dacite magma chamber was relatively shallow (0.25 GPa), and the magma had relatively low initial H₂O (~0.75 wt.%). Mineral identity and compositions (e.g., olivine and augite) for the basaltic andesite are best reproduced at 0.45 GPa, and the best fit models suggest the magma was hydrous (~6 wt.% H₂O). The Magma Chamber Simulator (MCS) (Bohrson et al., 2014), a mass and energy balanced thermodynamic model, was used to investigate the mixing hypothesis. To computationally produce the 4 products, different masses of basaltic andesite were hybridized with dacite at 0.25 GPa. To determine the best fit models, MCS major elements and mineral identities were compared to the whole rock major element ranges and the mineralogy presented in Clynne (1999). The best fit recharge (basaltic andesite) to resident (dacite) magma mixing ratios are: light dacite = 0.2, black dacite = 0.5, dark andesite = 1.0, and the andesitic inclusions = 2.0. Upon recharge, orthopyroxene forms, consistent with its rare presence in the rocks, and the anorthite content of simulated plagioclase is similar to that observed. Continued fractional crystallization produces other observed minerals such as clinopyroxene and results in the magma reaching volatile saturation, consistent with the vesiculated nature of the andesitic inclusions. There is excellent agreement between best fit simulations and the composition and mineralogy of the 4 products confirms the mixing hypothesis.