Insight into the Evolving Composition of Augustine Volcano's Source Magma from a Low-K Dacite

Augustine Volcano, located in southern Alaska, has erupted a wide compositional variety of volcanic rocks over its ~26,000 year lifespan. We performed an investigation of the mineralogy and petrology of the low-K dacite (LKD), the oldest yet-unstudied unit in the Augustine tephrostratigraphic succession, with the goal of creating a hypothesis of how the unit formed, as well as finding the unit's relationship to other ancient Augustine tephras. Our work was based on examination of hand samples, grain mounts, and thin sections of the LKD, the latter examined with both petrographic microscopes and with the electron microprobe at the American Museum of Natural History. We found a mineral assemblage in the LKD including plagioclase, calcic amphiboles, cummingtonite, magnetite, and ilmenite. Modeled values for the temperature, pressure, oxygen fugacity, and water content of the LKD's parent melt led us to the conclusion that the unit was derived from mixing of a rhyolitic parent magma similar to that of the ~30 m thick rhyolite erupted by Augustine in the Pleistocene, together with deeply-sourced mafic material of a similar geochemical character to the mafic component of the high-P dacite hypothesized by other research. Further work is needed to fully characterize this unit, as well as those above it in the Augustine tephrostratigraphic column, in order to arrive at a complete understanding of the relationship between ancient and modern Augustine tephras.