11:00 AM - 11:15 AM
[SVC28-26] Time variation in the composition of volcanic gases at Mt Hakone (2018-2020)
Keywords:Mt. Hakone, Volcanic gas, Phreatic eruption
Mt. Hakone is an active volcano located in the western part of Kanagawa prefecture. Earthquake swarms began at the end of April 2015, and a small phreatic eruption occurred on June 29. After that, earthquake swarms occurred again in May 2019. Even now, volcanic gas is actively discharged from fumaroles in Owakudani geothermal area. In this study, we consider the volcanic activity of Mt. Hakone after 2018 based on the changes in the chemical composition of volcanic gas.
Volcanic gases were collected at three fumaroles in Owakudani (N, C) and Kamiyuba (S) once a month. The fumarole C was formed during a phreatic eruption in 2015. The chemical composition of C is similar to the steam composition of the adjacent No. 52 steam well with depth of 500 m, suggesting that the fumarole of C is representative of gas around 500 m beneath surface (Ohba et al, 2019). Titanium pipe was inserted into the fumarole to collect volcanic gas, and the interstice between the tube and the outlet of fumarole was closed with soil to prevent air contamination. Titanium pipe was connected to a rubber tube, and volcanic gas was collected by an absorption tube containing a mixed solution of potassium iodate and potassium iodide, and vacuum glass bottle containing potassium hydroxide. The composition of the collected volcanic gas was analyzed using gravimetric analysis for H2S and SO2, microdiffusion analysis for CO2, and gas chromatography for other components.
The CO2/H2S molar ratios at all points increased in harmony with the increase in the number of earthquakes observed by the Japan Meteorological Agency, peaking in August 2019. After that, it decreased at N and C, but the CO2/H2S molar ratio of S did not decrease, which increased slightly after the swarm earthquake. The SO2/H2S molar ratio increased at C to a peak in August 2019 and then decreased. In the SO2-CO2-H2S ternary diagram (Stix and de Moor, 2018), the positions of the three points were significantly different. N and S were distributed near the edge connecting the CO2 and H2S vertex because of the scarcity of SO2. Since C contains relatively high concentrations of SO2, it deviated from the edge connecting the CO2 and H2S vertices and was distributed on the line with CO2/SO2 molar ratio of about 5. N and S moved slightly toward the H2S vertex overall in 2018, but moved toward the CO2 vertex from January to August 2019, and then back toward the H2S vertex. C moved away from the H2S vertex along the line with CO2/SO2 molar ratio of about 5 from January to August 2019, and then back toward the H2S vertex.
From the variation of CO2/H2S and SO2/H2S molar ratios, it is estimated that the magmatic gas supply to the hydrothermal system of Mt. Hakone peaked in August 2019. This variation was in harmony with the increase and decrease in the number of earthquakes. After that, the activity was quieted down. Since the positions of N, S and C are significantly different in the ternary diagram, it is suggested that the process from the generation process of volcanic gas is significantly different. Fumarole C is considered to have a large magmatic contribution than other points because of the large variation in the CO2/H2S and SO2/H2S molar ratios and the high concentration of SO2. Fumarole S was distributed in the direction to the CO2 vertex compared to the other two points. The relatively small variation in the CO2/H2S ratio and the enrichment in CO2 suggest that the sulfur component of S was removed during its passage through the shallow aquifer and became relatively rich in CO2.
References
Ohba T, Yaguchi M, Nishino K, Numanami N, Daita Y, Sukigara C, Ito M, Tsunogai U (2019) Earth, Planets and Space, doi.org/10.1186/s40623-019-1027-5
Stix J, de Moor J M (2018) Earth Planets Space, doi.org/10.1186/s40623-018-0855-z
Volcanic gases were collected at three fumaroles in Owakudani (N, C) and Kamiyuba (S) once a month. The fumarole C was formed during a phreatic eruption in 2015. The chemical composition of C is similar to the steam composition of the adjacent No. 52 steam well with depth of 500 m, suggesting that the fumarole of C is representative of gas around 500 m beneath surface (Ohba et al, 2019). Titanium pipe was inserted into the fumarole to collect volcanic gas, and the interstice between the tube and the outlet of fumarole was closed with soil to prevent air contamination. Titanium pipe was connected to a rubber tube, and volcanic gas was collected by an absorption tube containing a mixed solution of potassium iodate and potassium iodide, and vacuum glass bottle containing potassium hydroxide. The composition of the collected volcanic gas was analyzed using gravimetric analysis for H2S and SO2, microdiffusion analysis for CO2, and gas chromatography for other components.
The CO2/H2S molar ratios at all points increased in harmony with the increase in the number of earthquakes observed by the Japan Meteorological Agency, peaking in August 2019. After that, it decreased at N and C, but the CO2/H2S molar ratio of S did not decrease, which increased slightly after the swarm earthquake. The SO2/H2S molar ratio increased at C to a peak in August 2019 and then decreased. In the SO2-CO2-H2S ternary diagram (Stix and de Moor, 2018), the positions of the three points were significantly different. N and S were distributed near the edge connecting the CO2 and H2S vertex because of the scarcity of SO2. Since C contains relatively high concentrations of SO2, it deviated from the edge connecting the CO2 and H2S vertices and was distributed on the line with CO2/SO2 molar ratio of about 5. N and S moved slightly toward the H2S vertex overall in 2018, but moved toward the CO2 vertex from January to August 2019, and then back toward the H2S vertex. C moved away from the H2S vertex along the line with CO2/SO2 molar ratio of about 5 from January to August 2019, and then back toward the H2S vertex.
From the variation of CO2/H2S and SO2/H2S molar ratios, it is estimated that the magmatic gas supply to the hydrothermal system of Mt. Hakone peaked in August 2019. This variation was in harmony with the increase and decrease in the number of earthquakes. After that, the activity was quieted down. Since the positions of N, S and C are significantly different in the ternary diagram, it is suggested that the process from the generation process of volcanic gas is significantly different. Fumarole C is considered to have a large magmatic contribution than other points because of the large variation in the CO2/H2S and SO2/H2S molar ratios and the high concentration of SO2. Fumarole S was distributed in the direction to the CO2 vertex compared to the other two points. The relatively small variation in the CO2/H2S ratio and the enrichment in CO2 suggest that the sulfur component of S was removed during its passage through the shallow aquifer and became relatively rich in CO2.
References
Ohba T, Yaguchi M, Nishino K, Numanami N, Daita Y, Sukigara C, Ito M, Tsunogai U (2019) Earth, Planets and Space, doi.org/10.1186/s40623-019-1027-5
Stix J, de Moor J M (2018) Earth Planets Space, doi.org/10.1186/s40623-018-0855-z