ERUPTION INITIATION MECHANISMS AND TIMESCALES ACROSS MULTIPLE LAVA FLOWS AT A VERY HIGH THREAT CASCADE VOLCANO, MOUNT BAKER (KOMA KULSHAN), WA

Understanding the timescales of pre-eruptive processes is key for improving forecasts of future eruptions. Mount Baker (Koma Kulshan), WA, is part of the northern Cascade volcanic arc and classified as a very high threat volcano. However, we lack eruption initiation timescales: the duration of time between the final magmatic process before eruption (potentially the process that led to eruption) and the eruption itself. This study focuses on (1) estimating eruption initiation timescales and (2) determining eruption initiation mechanisms for three andesitic lava flows from Mount Baker: Dobbs Creek (~119 ka), Dobbs Cleaver (~105 ka), and Swift Creek (~48 ka). The crystal cargo of each lava contains multiple co-crystallizing assemblages, characterized in prior work using textures and geochemistry of individual phenocrysts and crystal clusters (Escobar-Burciaga, 2016). Specifically, this study examines plagioclase and clinopyroxene present as individual phenocrysts and/or within crystal clusters representing each assemblage. Both plagioclase and clinopyroxene have thin (~10 to 20 μm) reversely zoned rims in equilibrium with a basaltic to basaltic andesite liquid composition, or normally zoned rims in equilibrium with a dacitic liquid composition. Common disequilibrium textures suggest multiple magma mixing events before eruption. Major and trace element chemical transects across interior to rim zone boundaries were collected via EPMA and LA-ICP-MS for diffusion chronometry modeling. Temperature of diffusion was estimated based on the mineral rim composition and paired equilibrium liquid with plagioclase-liquid and clinopyroxene-liquid thermometers using Thermobar (v.1.0.19, Wieser et al. 2021). Both chemical and textural evidence suggest mafic recharge and/or thermal mixing as eruption initiation mechanisms. Most significantly, eruption initiation timescales from Sr and Mg diffusion in plagioclase as well as Fe-Mg interdiffusion in clinopyroxene are very short, on the order of weeks to a few years. These eruption initiation timescales are similar across the Dobbs Creek, Dobbs Cleaver, and Swift Creek lavas and similar to initiation timescales constrained for other Cascade volcanoes, indicating a need to be prepared for short response times across the Cascades.