4:15 PM - 4:30 PM
[SVC35-09] The nature of fumarolic gases collected at Ojigokudani, Iwate volcano, in September 2024
Keywords:Iwate volcano, Fumarolic gas, Direct Sampling, Chemical analysis
Iwate Volcano in northern Honshu, Japan, can be divided into two volcanoes, Nishi-Iwate and Higashi-Iwate, based on its topographical features. Over the last 7,000 years, Nishi-Iwate has experienced repeated phreatic eruptions, while Higashi-Iwate has experienced repeated magmatic eruptions, and these eruptions remain a concern for volcanic disaster prevention [1]. In 1998, an increase in earthquakes, crustal deformations, and fumarolic activity were observed at Iwate Volcano, which were suspected to be due to magma intrusion, but no eruptions occurred. Recently, since February 2024, deep inflation of Iwate volcano, an increase in volcanic earthquakes around Mt. Kurokura, and crustal deformations indicating shallow inflation around Ojigokudani fumarolic area on Nishi-Iwate have been observed; on October 2nd, the Volcanic Alert Level was raised from 1 to 2 [2]. The ratio of magmatic to hydrothermal components in hydrothermal systems can be a useful indicator of activity in volcanoes that produce phreatic eruptions, and analyzing the chemical and stable isotope ratios of fumarolic gases is one effective means to investigate these ratios [3]. This presentation presents the characteristics of the chemical and stable isotope compositions of the fumarolic gases collected from the Ojigokudani fumarolic area on September 5, 2024.
Fumarolic gas samples were collected from a sulfur chimney on the south side of Ojigokudani and a nearby fumarole in a September 5, 2024 sampling. For gas sampling, a titanium pipe connected to a rubber tube was inserted into the fumarole and sealed the gap, guiding the gases into sampling glassware with prevent atmospheric air contamination. Sampling was performed following the procedures of Ozawa [4] and Giggenbach [5]. Briefly, the gas was absorbed in an alkaline solution sealed in an evacuated glass bottle, and to accurately determine the SO2/H2S ratio, the gas was passed through a KI-KIO3 solution to oxidize SO2 to SO4 and H2S to S, and then brought back to the laboratory.
In the September 2024 fieldwork, the temperature of the fumaroles in the Ojigokudani was about the same as the boiling point at the local elevation, and the sampled gases were also 95.5-96.4 °C. The SO2/H2S ratios, which generally have a large value in high-temperature gases, were 0.01-0.02, and the apparent equilibrium temperatures (AETs) calculated from the reaction SO2 + 3H2 = H2S + 2H2O were estimated to be 212-227°C. The δD (-68.6 - -58.3‰) and δ18O values (-7.5 - -4.5‰) of water in gases were in the region of mixing between local meteoric water and typical magmatic gas [6-7], but the meteoric water component predominated as of September 2024. On the other hand, during the 1998 unrest, the nature of the fumarolic gases changed significantly in less than a year from June 1998, when gas observations began, to May 1999, the maximum fumarole temperature rose from 135°C to 143°C, the SO2/H2S ratio rose from 0.05 to over 0.2, the AETs reached 642°C from 464°C, and the δD and δ18O values of the water approached those of typical magmatic gas [8]. Chemical observations of fumarolic gases appear to be effective at this volcano as well, and gas monitoring should be continued.
*This study was partially supported by ERI JURP 2024-KOBO16 in the Earthquake Research Institute, University of Tokyo.
[1] Itoh J., Doi N. (2005) Geological map of Iwate volcano, GSJ, 7pp. [2] JMA (2025) Bulletins on Volcanic Activity (Iwate volcano, Annu Rep), 19pp. [3] Ohba T. (2022) Chikyukagaku, 56, 64-75. [4] Ozawa T. (1968) Geochem. Int., 5:939–947. [5] Giggenbach W.F. and Goguel R.L. (1989) NZ DSIR Chem. Rep. 2401:1-82. [6] Kusakabe M., Matsubaya O. (1986) Bull. Vol. Soc. Japan, 30, S267-S283. [7] Taran Y. et al., (1989) Dokl. Acad. Sci. USSR 304, 440-443. [8] Ohba T. et al. (2011) Annals of Geophysics, 54, 187-197.
Fumarolic gas samples were collected from a sulfur chimney on the south side of Ojigokudani and a nearby fumarole in a September 5, 2024 sampling. For gas sampling, a titanium pipe connected to a rubber tube was inserted into the fumarole and sealed the gap, guiding the gases into sampling glassware with prevent atmospheric air contamination. Sampling was performed following the procedures of Ozawa [4] and Giggenbach [5]. Briefly, the gas was absorbed in an alkaline solution sealed in an evacuated glass bottle, and to accurately determine the SO2/H2S ratio, the gas was passed through a KI-KIO3 solution to oxidize SO2 to SO4 and H2S to S, and then brought back to the laboratory.
In the September 2024 fieldwork, the temperature of the fumaroles in the Ojigokudani was about the same as the boiling point at the local elevation, and the sampled gases were also 95.5-96.4 °C. The SO2/H2S ratios, which generally have a large value in high-temperature gases, were 0.01-0.02, and the apparent equilibrium temperatures (AETs) calculated from the reaction SO2 + 3H2 = H2S + 2H2O were estimated to be 212-227°C. The δD (-68.6 - -58.3‰) and δ18O values (-7.5 - -4.5‰) of water in gases were in the region of mixing between local meteoric water and typical magmatic gas [6-7], but the meteoric water component predominated as of September 2024. On the other hand, during the 1998 unrest, the nature of the fumarolic gases changed significantly in less than a year from June 1998, when gas observations began, to May 1999, the maximum fumarole temperature rose from 135°C to 143°C, the SO2/H2S ratio rose from 0.05 to over 0.2, the AETs reached 642°C from 464°C, and the δD and δ18O values of the water approached those of typical magmatic gas [8]. Chemical observations of fumarolic gases appear to be effective at this volcano as well, and gas monitoring should be continued.
*This study was partially supported by ERI JURP 2024-KOBO16 in the Earthquake Research Institute, University of Tokyo.
[1] Itoh J., Doi N. (2005) Geological map of Iwate volcano, GSJ, 7pp. [2] JMA (2025) Bulletins on Volcanic Activity (Iwate volcano, Annu Rep), 19pp. [3] Ohba T. (2022) Chikyukagaku, 56, 64-75. [4] Ozawa T. (1968) Geochem. Int., 5:939–947. [5] Giggenbach W.F. and Goguel R.L. (1989) NZ DSIR Chem. Rep. 2401:1-82. [6] Kusakabe M., Matsubaya O. (1986) Bull. Vol. Soc. Japan, 30, S267-S283. [7] Taran Y. et al., (1989) Dokl. Acad. Sci. USSR 304, 440-443. [8] Ohba T. et al. (2011) Annals of Geophysics, 54, 187-197.