A petrogenetic study of the dacite from the 1912 eruption of Novarupta, Katmai National Park, Alaska: implications for magma storage locations

Abstract

The 1912 eruption o f Novarupta Volcano involved three compositionally distinct magmas: rhyolite, dacite, and andesite. Compositional gaps and a lack of linearity in whole-rock, glass and mineral trends across the 1912 suite negate the hypothesis that the dacite is a mixture of the andesite and the rhyolite. Plagioclase zoning profiles, estimated water contents, and whole-rock and glass com positions suggest that the 1912 dacite did not evolve by crystal fractionation from the 1912 andesite, but that both the andesite and the dacite evolved by crystal fractionation from other magmas. The same data suggest that the dacite and andesite resided in separate but mutually proximal small magma chambers at -7 km depth beneath Trident Volcano prior to eruption. Studies by other workers suggest that the rhyolite magma resided underneath Novarupta at the head of the Valley of Ten Thousand Smokes.