Using Chemical Zoning in Plagioclase to Understand Changes in Eruption Behavior at Augustine Volcano, Alaska

Chemical zoning in a mineral, analogous to tree growth rings, reflects magmatic evolutionary processes as magmas ascend from the mantle through the crust to finally be erupted. The mineral plagioclase commonly reflects this zoning and is prevalent in intermediate arc magmas. Augustine Volcano, a frequently active intermediate stratovolcano in the Aleutian Arc (AK) has a stratigraphic record that suggests prehistoric eruptions (pre-1800s) were more explosive than those observed in the past 100 years. Geochemical studies of erupted material, such as tephra, from 2200-1000 years B.P. compared to the most recent eruption in 2006 show a transition from a magma that has both silica-poor and silica-rich endmembers to a magma that has a more homogenous intermediate composition. Tephra B, an unstudied andesitic pumice unit that erupted 400 years B.P., represents a gap in the data and presents a key opportunity to use chemical zoning in plagioclase to interrogate the compositional and eruptive transitions observed at Augustine. This study tests the hypothesis that plagioclase zoning will reflect the mixing of magmas, driving the change in eruptive behavior at Augustine, and provide insight into how magma evolution contributes to changes in overall eruption style.