2018 JOHN A. BLACK AWARD: TOWARD DEVELOPING A CONCEPTUAL MODEL OF MAGMATISM AT MEDICINE LAKE VOLCANO, NORTHERN CALIFORNIA, USA

Medicine Lake Volcano (MLV) is the largest Cascade Volcano by volume (~600 km³) and has erupted 9 times during the Holocene. Detailed mapping by the USGS has shown that during the last 500 kyr MLV has erupted >200 lava flows, one ash-flow tuff (VEI 5-6), and built at least 17 scoria cones. Rhyolite, dacite, andesite, basaltic-andesite, and basalt have all erupted throughout the geologic record, with mafic vents making up 78.2% of the total mapped vents (408 of 522 vents). During the Holocene, the proportion of felsic vents is significantly higher (54.5%) than mafic vents (39.9%).

Although highly explosive eruptions at MLV are possible, new vent openings, accompanied by ballistic fallout and lava flows, are the most frequent and expected volcanic hazard. We use 522 vent locations, divided into subsets, to complete a series of spatial density calculations aimed at exploring the temporal and geochemical distribution of volcanism at MLV. This data is contoured to generate vent spatial density maps that reflect the lateral extent of melt in the subsurface. If magma ascent paths are assumed to be roughly vertical, these models can be used to estimate magma system boundaries in map view, and felsic vent subsets can be used to delineate pockets of evolved magma. Our analyses show that felsic vents cluster at the center of MLV (near and within the caldera) regardless of age. In contrast, Pleistocene mafic vents are widely distributed but become more focused during the Holocene. We then use whole-rock (ICP-OES) and mineral chemical (EPMA) analyses to constrain P-T storage conditions of key eruptive units, giving insight into crystallization depths prior to eruption. Thereby, providing us with insight into a 3^rd dimension (depth) which is key to developing a conceptual model of magmatism at MLV.