Degassing Activity of a Volcanic Crater Lake: Volcanic Plume Measurements at the Yudamari Crater Lake, Aso Volcano, Japan

Abstract

Surface degassing is an important volatile discharge process for an active volcanic crater lake. The compositions of volcanic gases discharged from a lake surface (lake gas) were quantified by volcanic plume measurements using a Multi-GAS and alkaline-filter techniques at the Yudamari crater lake, Aso volcano, Japan. Compositions of the lake gases were quite variable and are clearly different from the gases from adjacent fumaroles. Differentiation processes of the lake gas, the lake water, and the fumarolic gases are evaluated based on their compositions. Concentrations of HCl in the lake gas and the lake water indicate that the lake gas composition is controlled by the equilibrium evaporation of the lake water at the lake temperature. Contrasting compositions of the lake gas and the lake water indicate that sulfate and elemental sulfur formation controls chemical differentiation in the lake. The composition of the hydrothermal fluids supplied to the lake is estimated based on mass balance of the lake gas and the lake water. The hydrothermal fluids have similar H2O/S and H2O/Cl ratios but lower CO2/S ratios than the fumarolic gases. This composition contrast indicates that the fumarolic gases are a mixture of magmatic gases and a vapor phase separated from the hydrothermal fluids supplied to the lake.