CONNECTIONS BETWEEN ERUPTION STYLE AND MAGMATIC RESERVOIR EVOLUTION: INSIGHTS FROM AUGUSTINE VOLCANO, ALASKA, USA

Understanding the connection between magmatic reservoir evolution and eruption style is key to determining the potential for a hazardous eruption. Augustine Volcano, a frequently active andesitic stratovolcano in the Alaska-Aleutian arc (USA), is an ideal setting to investigate magma reservoir processes due to its frequent modern and Holocene eruptions. Its most recent eruption in 2006 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. In contrast, the late Holocene “Tephra M” was deposited by a more explosive eruption about 750 years before present, as evidenced by well-sorted on-island medium to coarse lapilli deposits as thick as 80 cm. Although clearly a more significant explosive eruption, Tephra M and other significant Holocene eruptions have not been studied in similar detail to the 2006 and other recent Augustine eruptions.

Here we are initiating a detailed study of the Tephra M deposits using a combination of major element bulk rock and glass compositions, plagioclase hygrometry, and Fe-Ti oxide (ilmenite-magnetite) thermometry. Preliminary Fe-Ti oxide thermometry suggest that the Tephra M magma ranged in temperature from approx. 870 to 980°C. These results overlap with published 860 to 1120°C Fe-Ti oxide temperatures from the 2006 eruption, but maximum temperatures reported for 2006 are notably higher than any we have found to date in Tephra M components. In addition, studies of the 2006 eruption have found evidence for the intrusion of hotter, mafic magma (56 – 59 weight % SiO₂) into stored cooler, felsic magma (62 – 63 weight % SiO₂) prior to the eruption. Available analysis of Tephra M pumice (62 – 64 weight % SiO₂; n = 5) are compositionally similar to the 2006 felsic endmember magma. Current efforts will explore if this maximum temperature difference is legitimate, and if the difference influenced the explosivity of Tephra M compared to 2006 eruption. We are studying components identified at 7 sample localities collected for Tephra M to better document potential compositional differences between the eruptions, along with plagioclase analyses to constrain volatile concentrations that may have affected eruptive behavior.