Abstract

Two Recent Plinian eruptions in the Wrangell Mountains (southeast Alaska) gave rise to two distinct ash-fall deposits that are collectively known as the White River Ash and cover much of the Yukon Territory, northwest Canada. Analysis of the pumiceous glass indicates that the magma chamber was compositionally inhomogeneous prior to each eruption. No compositional stratigraphy has been detected in the deposits, indicating either thorough mixing in the eruption cloud or thorough reworking after deposition. Thus each individual sample of ash represents a large part of the magma chamber, whereas larger pumice fragments are more homogeneous. Variations in temperature, 950–990 and 995–1030 °C, respectively, for the older and younger eruptions, and −log fo2 values, 9.3–8.3 and 8.3–7.7, derived from the Fe–Ti oxides, support the conclusion that the magma chamber was inhomogeneous.

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