CONSTRAINTS ON THE PRE-ERUPTIVE STORAGE CONDITIONS AND ERUPTION INITIATION MECHANISMS OF MOUNT BAKER’S MOST RECENT EXPLOSIVE MAGMATIC ERUPTION, WASHINGTON, U.S

Mount Baker is a very high-threat Cascade stratovolcano situated in Northwestern Washington. This volcano last experienced an explosive magmatic eruption ~6.7 ka, producing the Black Ash (BA) tephra from Sherman Crater within the Baker edifice (VEI 6). Recent geochemical data on BA tephra glass shards (Machel, 2023) suggests that this main edifice eruption, and the associated BA tephra, is derived from a high-magnesium series magma, typical of edifice eruptions for the last 100 ka. In contrast, the penultimate ~9.8 ka Baker eruption was from the flank, fed by a low-Mg series magma. Interestingly, lava flows from both of these magma series (low-Mg and high-Mg) contain a subset of similar crystals and crystal clots (crystal cargo) interpreted to have been entrained from crystal mushes existing at various depths beneath the volcanic field. The entrained cargo comprises similar assemblages in lavas that span ages from 100 ka to at least 9.8 ka, suggesting that the crystal mushes are long-lived and laterally extensive, spanning both main edifice and flank. We are using textural information coupled with geochemistry of the crystal cargo to develop a conceptual model of the subvolcanic magmatic architecture prior to the 6.7 ka eruption. We focus on mineral textures, geochemical zoning profiles, thermobarometry, and phase equilibria of crystal clots, as well as loose crystals of plagioclase, clinopyroxene, orthopyroxene, and iron-titanium oxides. The conceptual model outcomes include magma storage depths, storage conditions, and eruption initiation mechanisms for the BA tephra. We test the hypothesis that migrating mafic magma rejuvenated a shallow silicic magma mush in the subsurface, triggering the eruption, similar to what has been shown for the penultimate ~9.8 ka flank eruption based on crystal populations and zoning profiles (see Garvey, 2022).