The evolution of Augustine: linking Pleistocene rhyolite and the overlying dacite
Abstract
Augustine Volcano, Alaska, has erupted intermediate composition products for at least the past few thousand years. But among the oldest and thickest units recognized at Augustine, is at least one large rhyolitic fall unit ( 25 ka; Coombs and Vazquez, 2014). In an effort to unravel the long-term history of Augustine, the rhyolite has recently become the subject of active research. Rhyolitic melt inclusion and amphibole compositions indicate rhyolite magma resided significantly deeper than recent magmas (≥ 9 km vs. 5 km). Both volatile content (higher) and volatile composition (lower Cl, higher CO2) are also different in the rhyolite compared to recent magmas, as is the modal mineralogy (Nadeau et al., 2015). Despite such differences and separation in time, the rhyolite may not be completely distinct from modern eruptions; deformation recorded by INSAR from 1992 to 2005 was best fit by a dual source Mogi model placing the larger, deeper source at depths similar to those indicated by the rhyolite (Lee et al, 2010). Zircons also link the rhyolite to dioritic inclusions erupted in andesite in 2006. The two were likely cogenetic as parts of crystal-rich dacite that un-mixed into rhyolite and dioritic crystal mush (Coombs and Vazquez, 2014). Major element Harker diagrams for the Augustine suite, particularly P2O5, support such a relationship (Wallace and Coombs, 2013; Coombs and Vazquez, 2014). Also plotting along the Harker unmixing trends is high-P dacite that overlies the rhyolite. Here we investigate the relationship of the rhyolite to the high-P dacite. We present data contrasting mineralogy, mineral zonation, melt inclusion chemistry, and volatile content to better elucidate the origins of the voluminous rhyolite. This work represents the beginning of what could be a longer term effort to understand the evolution of Augustine from basalt and rhyolite in its early stages to smaller, intermediate eruptions today, including potential for future large rhyolitic eruptions.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.T11D2652N
- Keywords:
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- 1037 Magma genesis and partial melting;
- GEOCHEMISTRYDE: 7280 Volcano seismology;
- SEISMOLOGYDE: 8178 Tectonics and magmatism;
- TECTONOPHYSICSDE: 8413 Subduction zone processes;
- VOLCANOLOGY