A glimpse into Augustine Volcano's Pleistocene past: Insight from the petrology of a massive rhyolite deposit

Activity at Augustine Volcano, Alaska, has been characterized by intermediate composition domes, flows, and tephras during the Holocene. Erosive lahars and pyroclastic flows associated with the 2006 eruption revealed large exposures of voluminous rhyolite pumice fall beneath glacial tills; the massive rhyolite deposit is evidence of anomalously large prehistoric eruptions. The rhyolite is petrologically and mineralogically different from more recent eruptive products, with abundant amphibole (calcic amphiboles and cummingtonite) and quartz. Three texturally and chemically distinct lithologies are present. Fe–Ti oxide equilibria suggest temperatures of ~765°C and oxygen fugacities of NNO+1.5. Melt inclusions indicate that magma representing the stratigraphically lowest lithology was crystallizing at ~260MPa with a contemporary mixed H2O–CO2 fluid phase becoming progressively more H2O-rich. Magma forming the other lithologies crystallized in the presence of this H2O-dominated fluid, as demonstrated by the presence of cummingtonite and little to no CO2 in melt inclusions. Amphibole and quartz–feldspar–melt equilibria models yield results indicating a range of crystallization pressures from ~400MPa to ~175MPa. Apatites and melt inclusions have lower chlorine contents than more recently erupted material at Augustine suggesting that the composition of Augustine's volatile phase has changed over time. Reheating textures in melt inclusions and phenocrysts, as well as the presence of xenocrysts of olivine and clinopyroxene containing mafic melt inclusions, signify the likelihood of mixing and/or mingling of magmas as an eruption trigger. The unique qualities of this Pleistocene rhyolite and the potential hazard of a similarly large eruption in modern times indicate that further study is warranted.