Monitoring of volcanic gas composition at Asama volcano, Japan, during 2004-2014

The composition of the volcanic gases discharged from the summit crater of Asama volcano has been monitored since 2004 by Multi-GAS and alkaline-filter techniques. The persistent degassing activity at Asama volcano is characterized by large variation of SO2 flux. The CO2/SO2 and H2O/SO2 ratios did not show clear variation irrespective of the SO2 flux variation and a few eruptions that occurred during active degassing periods. The estimated ratios have large uncertainty due to variable contribution of the different fumaroles in the summit crater to the volcanic plume and lack of a systematic variation can be due to the large uncertainty. The SO2/Cl ratio showed a systematic decrease after the eruption to the inactive period, suggesting that degassing pressure did not significantly increase after the eruption. Low-pressure degassing along with the continuous and intensive gas discharge suggests that the degassing is due to conduit magma convection. The apparently stable CO2/SO2 ratios imply a lack of significant volatile differentiation in the magma reservoir, such as CO2-rich bubble accumulation. The large variation of the SO2 flux along with stable gas composition implies that the large changes in magma convection rate are caused by changes in the radius of the convecting magma conduit.