Compositional heterogeneity of distal tephra deposits from the 1912 eruption of Novarupta, Alaska

Physical and chemical analyses of distal tephra from the 1912 eruption of Novarupta, Alaska, show considerable variations in glass and mineral compositions. A combination of a 150°C range in temperature deduced from iron-titanium oxide geothermometry, and curved patterns in bivariant element plots of glass compositions indicate that a chamber of compositionally zoned magma existed prior to the eruption. Magma-mixing cannot explain these features. The magma chamber may have resembled the model recently proposed by McBirney (1980). A highly silicic, quartz-phyric magma with mean phenocryst compositions of An₂₅ plagioclase, Fs₄₂ orthopyroxene, at a temperature of 880°C and a water pressure of 1.4 kbar, was located above a more mafic, hotter magma, bearing phenocrysts of An₄₅ plagioclase and Fs₃₅, orthopyroxene.Our results on distal tephras compare favorably with those from a recently completed study at source by Hildreth (1983), suggesting that useful petrologic information about distant volcanoes can be obtained from both types of deposits. Compositionally heterogeneous abyssal tephra layers are common in the Gulf of Alaska. Eruptions from chambers of zoned magma may account for many of these layers.