Controls on long-term low explosivity at andesitic arc volcanoes: Insights from Mount Hood, Oregon

摘要

The factors that control the explosivity of silicic volcanoes are critical for hazard assessment, but are often poorly constrained for specific volcanic systems. Mount Hood, Oregon, is a somewhat atypical arc volcano in that it is characterized by a lack of large explosive eruptions over the entire lifetime of the current edifice (～500,000 years). Erupted Mount Hood lavas are also compositionally homogeneous, with -95% having SiO_2 contents between 58 and 66 wt.%. The last three eruptive periods in particular have produced compositionally homogeneous andesite-dacite lava domes and flows. In this paper we report major element and volatile (H_2O, CO_2, Cl, S, F) contents of melt inclusions and selected phenocrysts from these three most recent eruptive phases, and use these and other data to consider possible origins for the low explosivity of Mount Hood. Measured volatile concentrations of melt inclusions in plagio-clase, pyroxene, and amphibole from pumice indicate that the volatile contents of Mount Hood magmas are comparable to those in more explosive silicic arc volcanoes, including Mount St. Helens, Mount Mazama, and others, suggesting that the lack of explosive activity is unlikely to result solely from low intrinsic volatile concentrations or from substantial degassing prior to magma ascent and eruption. We instead argue that an important control over explosivity is the increased temperature and decreased magma viscosity that results from mafic recharge and magma mixing prior to eruption, similar to a model recently proposed by Ruprecht and Bachmann (2010). Erupted Mount Hood magmas show extensive evidence for mixing between magmas of broadly basaltic and dacitic-rhyolitic compositions, and mineral zoning studies show that mixing occurred immediately prior to eruption. Amphibole chemistry and thermobarometry also reveal the presence of multiple amphibole populations and indicate that the mixed andesites and dacites are at least 100 ℃ hotter than the high-SiO_2 resident magma prior to mixing. Viscosity models suggest that recharge by hot, mafic magma prior to eruption can lower magmatic viscosity by at least a factor of four. Lower viscosity during ascent delays fragmentation and allows volatile escape through degassing, thus lowering the potential for explosive eruptions. These results suggest that low explosivity should be more common in volcanoes where intermediate magmas are produced through mixing of mafic and silicic magmas shortly before eruption.