Deep magmatic degassing versus scrubbing: Elevated CO2 emissions and C/S in the lead-up to the 2009 eruption of Redoubt Volcano, Alaska

Abstract

[1] We report CO2, SO2, and H2S emission rates and C/S ratios during the five months leading up to the 2009 eruption of Redoubt Volcano, Alaska. CO2 emission rates up to 9018 t/d and C/S ratios ≥30 measured in the months prior to the eruption were critical for fully informed forecasting efforts. Observations of icemelt rates, meltwater discharge, and water chemistry suggest that surface waters represented drainage from surficial, perched reservoirs of condensed magmatic steam and glacial meltwater. These fluids scrubbed only a few hundred tonnes/day of SO 2, not the >2100 t/d SO2 expected from degassing of magma in the mid- to upper crust (3-6.5 km), where petrologic analysis shows the final magmatic equilibration occurred. All data are consistent with upflow of a CO2-rich magmatic gas for at least 5 months prior to eruption, and minimal scrubbing of SO2 by near-surface groundwater. The high C/S ratios observed could reflect bulk degassing of mid-crustal magma followed by nearly complete loss of SO2 in a deep magmatic-hydrothermal system. Alternatively, high C/S ratios could be attributed to decompressional degassing of low silica andesitic magma that intruded into the mid-crust in the 5 months prior to eruption, thereby mobilizing the preexisting high silica andesite magma or mush in this region. The latter scenario is supported by several lines of evidence, including deep long-period earthquakes (-28 to -32 km) prior to and during the eruption, and far-field deformation following the onset of eruptive activity.