Connections between eruption style and magmatic reservoir evolution: Insights from Augustine Volcano, Alaska, USA

At a single volcano, eruptive behavior can change through time as the plumbing system evolves. Augustine Volcano, a frequently active intermediate stratovolcano in the Alaska-Aleutian arc (USA), is an ideal setting to investigate magma reservoir processes due to its frequent modern and Holocene eruption history. Its most recent 2006 CE eruption included mixed effusive and moderately explosive (VEI 3) events and has been studied in detail. Proximal fall deposits from this eruption were generally mixed fine ash to lapilli with variable thickness, but typically ~5 cm on the island (Wallace et al., 2010). The Pre-contact 'Tephra M' was deposited by a potentially more explosive eruption about ~750 years before present (ybp), and appears as well-sorted on-island medium to coarse lapilli deposits as thick as 95 cm. Although clearly a more significant eruption, Tephra M and other Pre-contact eruptions (lapilli to bomb pumice fall) have not been studied in detail. Using Fe-Ti oxide thermometry, groundmass glass chemistry, mineral chemistry, and modal mineralogy, I determine that Tephra M is comprised of mostly a light tan pumice and a lesser proportion of light tan and grey banded pumices that correspond to two distinct magmatic components: a basaltic-andesite (BA, approx. 55.3 wt. % SiO2) and a high-silica andesite-dacite (HSAD, 61.5 to 64.8 wt. % SiO2). Tephra M was triggered by the mingling and partial mixing of a hotter BA (866 to 977 degreesC (±50; 1σ) degreesC) and a volumetrically larger, cooler, viscous HSAD magma (846 to 870 degreesC), promoting explosivity. The difference in eruption style of the Post-contact eruptions (2006/1986 CE) and Pre-contact eruptions (Tephra M) can be explained by different plumbing system configurations, magma compositions, and temperatures. The 2006 eruption was the result of four or more compositionally variable components (low- to high-silica andesites) that were generally hotter in temperature than the two compositionally and thermally contrasting BA and HSAD magmas of Tephra M. Tephrostratigraphy suggests that smaller Post-contact style eruptions may have occurred in between the main Pre-contact eruptions. This could mean Augustine and its eruptive behavior bay be controlled by variable plumbing configurations of discrete batches of intermediate magmas.