Young zircon ages and compositions at Oregon's Mount Hood as evidence of high-silica magma storage underneath an andesitic arc volcano

Abstract Magma storage under andesitic arc volcanoes is ambiguous due to the complexities of ascending mafic (or silicic) magma intruding preexisting crystal mush that could represent more than 100,000 yr of magmatic heritage. This interaction can lead to eruption of a hybridized or mingled magma, or it could merely yield hybridization without eruption. The record of some of these events is kept in the ages and compositions of trace phases like zircon. We collected zircon U-Th disequilibria ages from the three most recent eruptive periods at Mount Hood: Polallie (30–12 ka), Timberline (ca. 1.5 ka), and Old Maid (ca. 0.2 ka). We found that zircons in these lavas and tephras are dominantly less than 50 ka, with a small portion up to ca. 350 ka. Many zircon ages are within error of the eruption ages of the Timberline and Old Maid eruptions, suggesting active zircon crystallization in a high-silica crystal mush underneath Mount Hood that was constructed during and since the end of the Polallie Period. These zircons record the progressive fractionation of the Polallie Period magma body and the influence of mafic rejuvenation to produce hybrid andesite at Mount Hood. We propose a model wherein a silicic filter developed since ca. 50 ka and continues to form even as the eruption activity at Mount Hood has waned since ca. 12 ka.