Constraining the exsolved volatile fraction, the H2O/CO2 molar ratio, and magma compressibility of the 2006 Augustine eruption, Alaska
Abstract
Magma mobilized before and during volcanic eruptions can produce a surface deformation signal from which a magma volume change in the reservoir can be modeled. For volatile-rich, intermediate-silicic systems, this subsurface volume change is commonly 1-10 times smaller than the associated eruptive volume, which is calculated based on isopach mapping of eruptive units, their thicknesses, and densities. An exsolved volatile phase can cause magma to be compressible, such that the magma density (and resulting volume) will change as a function of pressure. Magma compressibility is commonly used to explain the discrepancy between deformation volumes and eruptive volumes. In this study we investigate magma compressibility and the exsolved volatile volume associated with storage conditions prior to the 2006 eruption of Augustine volcano, Alaska. We integrate volcanic gas, geodetic, and petrologic data to model the volume fraction of the exsolved volatile phase, constrain the volcanic gas H2O/CO2 composition, and calculate the magma compressibility. Based on the parameters for Augustine's magmatic system (pressure of 1200-1700 bar, a temperature of 880 ± 13 °C, and a phenocrysts volume percent of 40 ± 2%), we calculate that an exsolved volatile phase of 8.2 vol% and a magma compressibility of ~7.1 x 10-10 1/Pa provide the best fit to explain the observed eruptive volume of 72 million m3 compared to the 25 million m3 geodetic volume change at depth. In addition, we constrain the water budget of the eruption by combining our constraints on exsolved volatile volume fraction and observed SO2 and CO2 emissions. We estimate an H2O/CO2 molar ratio in syn-eruptive gas emissions of 28, and a total water emission of 5.5 x 106 tons. This ratio enables us to calculate a total gas emission for the duration of the eruption of 6.3 x 106 tons. These results agree well with previous estimates for the volume fraction of the exsolved gas phase, eruptive gas composition, and magma compressibility of similar systems and underline the importance of accounting for magma compressibility when analyzing geodetic data of volatile-rich volcanic systems.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V51I0182W
- Keywords:
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- 1038 Mantle processes;
- GEOCHEMISTRY;
- 1060 Planetary geochemistry;
- GEOCHEMISTRY;
- 8430 Volcanic gases;
- VOLCANOLOGY;
- 8450 Planetary volcanism;
- VOLCANOLOGY