Variation of Volcanic Gas Composition at a Persistently Degassing Asama Volcano, Japan
Abstract
Asama volcano at central Japan is a persistently degassing andesitic volcano and repeated eruptions every several years. The recent eruptions occurred in September 2004, August 2008 and February 2009 and are followed by increase of the volcanic gas emission during several months. The SO2 flux is typically 1,000-4,000 t/d during the high flux period after the eruption, whereas the flux is around 100 t/d during the low gas flux periods (JMA, 2013; Ohwada et al., in review). This study aims to understand the controlling process of volcano degassing based on the volcanic gas composition data. In particular, we focus to evaluate the gas composition contrast between the high and low gas flux periods. As the fumaroles and degassing vent locate in the summit crater of 500-m-diamter and are inaccessible, we estimated the gas composition by plume measurements with the Multi-GAS at the crater rim. The HCl/SO2 ratios are obtained by the alkali-filter trap. We started the repeat Multi-GAS measurements in 2004 and installed an automatic Multi-GAS monitoring station for a daily measurement at the western rim of the summit crater in 2010. The gas compositions obtained by the Multi-GAS measurements are often scattered even during the day of measurements, in particular during the low flux period and the scatter is likely due to variable contamination of gases from low-temperature fumaroles locating along the crater rim because the low-temperature fumaroles locate closer to the measurement site that the major degassing vent at the bottom of the crater. If we plot the gas concentration ratio, such as CO2/SO2 against SO2 concentration, the ratio commonly converges to a certain value at high SO2 concentration and this ratio is considered as representative of the major gas emission. The estimated molar ratios are CO2/SO2=1×0.5, HCl/SO2=0.2×0.1 and H2O/SO2=60×30 without clear contrast during the high and low flux periods. The CO2/SO2 ratios obtained based on a single day data tend to be higher than the average, however, the analyses with a larger data set, e.g., for a month, results in the average value. The HCl/SO2 ratios agree well with those obtained during the 2004 eruptive period by FT-IR and ash-leachate analyses with the range of 0.15-0.2 and 0.1-0.2, respectively (Mori and Notsu, 2005; Nogami et al., 2004). The H2O/SO2 ratios also tend to be higher during the low flux period and this can be due to a larger contribution of meteoric water during the low flux period. The constant gas composition regardless of the large variation of the gas flux suggests that the degassing process and its condition remains the same for the high and low flux periods. The similar HCl/SO2 ratio obtained during both the eruptive period by FT-IR and ash leachate and the persistent degassing stage in this study indicates that persistent degassing is fed by low pressure gas separation from continuously ascending magmas, consistent with the conduit magma convection model (Ohwada et al., in review; Shinohara, 2008). Based on the conduit magma convection model, the large flux changes without variation of the gas composition can be caused by the change of the magma convection rate with similar degassing pressure and magma composition.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V24C..02S
- Keywords:
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- 8430 VOLCANOLOGY Volcanic gases;
- 8419 VOLCANOLOGY Volcano monitoring