A petrologic investigation of mafic inputs into the Augustine volcano (Alaska) magma system over the past 2,200 years

The Augustine volcano (Alaska) magma reservoir has been penetrated by numerous injections of mafic magma over the past ∼2,200 years as recorded by quenched basaltic enclaves hosted in andesite lavas. Enclaves are described as low-K basalts to basaltic andesites with calc-alkaline differentiation trends and a petrogenetic source characterized by equal portions of N-MORB and lower continental crust originating ∼110 km below the volcanic front. Despite the frequency of geochemically similar injections of mafic magma at Augustine, a significant transition in the preservation of this mafic magma is observed. Whereas the oldest exposed eruption deposits of Southeast Point contains evidence for abundant (up to 26 vol%) mafic injection in the form enclaves within host andesite, deposits emplaced thereafter contain less than 3 vol% enclaves. Thus, younger deposits require that injecting mafic magma mixed thoroughly with the host magma. This study shows that the transition from enclave to non-enclave bearing magmas is not due to a profound geochemical change in the mafic or silicic magma but rather to a change in the manner that mafic and silicic magmas interacted as facilitated by the geometry of the chamber that the mafic magma intruded. Field relations, geochemical analysis, and the eruptive history at Augustine support a transition from a small, narrow and ephemeral chamber that promoted the generation and preservation of enclaves to a larger and more sustained chamber that promoted more thorough chemical and thermal equilibrium between the two magmas sometime after 2,200 years ago.