THE 1915 ERUPTION OF LASSEN PEAK, CALIFORNIA: MASS, THERMAL, AND COMPOSITIONAL IMPLICATIONS OF THE MAGMA MIXING EVENT DOCUMENTED USING THE MAGMA CHAMBER SIMULATION

Lassen Peak Volcano, located in northern California, is the southern-most volcano in the Cascade volcanic arc. The 1915 Lassen eruption produced four distinct products: (1) light dacite, (2) black dacite, (3) dark andesite, and (4) andesitic inclusions. These products contain varied amounts of quartz, plagioclase, olivine, pyroxenes, and other minerals. Clynne (1999) hypothesized that the four products were a result of variable mixing of host dacite and intruding basaltic andesite. The best-fit Rhyolite MELTS (Gualda et al., 2012) model suggests that the host dacite was relatively H₂O-poor (~0.75 wt.%) and that the magma storage zone was in the upper crust (0.25 GPa). This model produced similar plagioclase (An₃₁) to those observed (An_30-36) in the dacite (Clynne, 1999). Olivine (Fo_80-84) in the basaltic andesite was best reproduced at 0.45 GPa, and the best-fit model suggests that the magma was wet (~6 wt.% H₂O). The thermal, mass, and compositional implications of mixing were evaluated using the Magma Chamber Simulator (MCS, Bohrson et al., 2014). To reproduce the four products, the basaltic andesite and dacite end members hypothesized by Clynne (1999) were mixed in different ratios at 0.25 GPa. Trace element modeling utilized published mineral-melt partition coefficients. Dacite-basaltic andesite mixing ratios (expressed as a ratio of the mass of recharge magma to mass of initial host dacite) that best reproduced major and trace elements of the four products are: light dacite=0.2, black dacite= 0.5, dark andesite= 1.0, and the andesitic inclusions= 2.5. Simulated plagioclase (An_66-74) and clinopyroxene (En₄₃) formed upon mixing, and these compositions match observed microphenocryst data (An_63-70, En_43-44) as presented by Salisbury et al. (2008) and Clynne (1999). Continued cooling of several hundred degrees C allowed the magma to reach volatile saturation, consistent with observed vesiculated andesitic inclusions. This result is consistent with recent thermometry on mixed products from Chaos Crags (Scruggs and Putirka, 2018) that also shows hundreds of degrees of cooling prior to eruption. MCS results reveal an upper crustal mixing zone, document the consequences of variable mixing on melt and minerals, and illuminate the mass and thermal histories of 1915 Lassen magma mixing events.