Shallow-storage conditions for the rhyolite of the 1912 eruption at Novarupta, Alaska

Recent studies have proposed contrasting models for the plumbing system that fed the 1912 eruption of Novarupta, Alaska. Here, we investigate the conditions under which the rhyolitic part of the erupted magma last resided in the crust prior to eruption. Geothermometry suggests that the rhyolite was held at ∼800-850 °C, and analyses of melt inclusions suggest that it was fluid saturated and contained ∼4 wt% water. Hydrothermal, water-saturated experiments on rhyolite pumice reveal that at those temperatures the rhyolite was stable between 40 and 100 MPa, or a depth of 1.8-4.4 km. These results suggest that pre-eruptive storage and crystal growth of the rhyolite were shallow; if the rhyolite ascended from greater depths, it did so slowly enough for unzoned phenocrysts to grow as it passed through the shallow crust.