11:30 AM - 11:45 AM
[SVC28-09] Temperature Estimation of the Hydrothermal System which caused the Phreatic Eruption at Kuju Volcano in 1995 based on coupled geochemical-calorimetric analyses
Keywords:Kuju Volcano, Rayleigh Distillation, Hydrothermal System, Temperature Estimation
Phreatic eruptions generally occur on a smaller scale than magmatic eruptions, but can cause injuries and deaths. A typical example of this is the eruption of Mt.Ontake in 2014. In addition, According to the Japan Meteorological Agency (2018), a phreatic eruption occurred at Kusatsu-Shirane volcano in 2018 without any specific earthquake precursors (e.g. earthquakes or tectonic movements) being observed, resulting in one death and 11 injuries. Therefore, it is essential to understand not only the detection of earthquakes and crustal movements but also the behaviors of magmatic steam and geothermal systems in order to reduce the damage caused by phreatic eruptions.
Also at Kuju Volcano, a phreatic eruption occurred in 1995 on the eastern ridge of the summit of Mt.Hossho. This eruption site is located southeast of "Kuju-Iwoyama", a fumarolic area that continuously releases volcanic gases. But now, volcanic gas emissions from the new craters have subsided and decreased, despite the ongoing volcanic activity. According to the Plan of Volcanic Eruption Prediction of the Japanese Council for Science and Technology Policy, Kuju-Iwoyama is designated as an active volcano with potential explosive activity. Therefore, it is very important to investigate the triggers of past phreatic eruptions at Mt.Kuju and to prepare for future eruptions.
According to Mizutani et al. (1986), fumarolic gases of up to 508°C were observed at Mt.Kuju in 1960, and the condensate of gases at that time was isotopically almost pure magmatic water, but in 1984, it was estimated to contain about 50% meteoric water. Kitaoka (1996) proposed a hydrothermal model for Mt. Kuju. This model explains that as water seeps from the surface and moves deeper, much of the evaporated vapor enters the relatively low-pressure pathway which includes magmatic vapor, and becomes hot superheated steam above a critical temperature, and mixes with magmatic steam. According to this model, hydrothermal water in a gas-liquid two-phase system is formed by the mixing of magmatic vapor and natural water. In this research, we determined the mixing rate between magmatic vapor and water from geochemical analysis of the δD-δ18O water isotopic composition of the fumarole, and estimated the temperature of the hydrothermal water which caused the phreatomagmatic eruption in 1995 from calorimetric analysis.
We focused on the relationship between apparent equilibrium temperature (AETs) and fumarolic temperature in the D-region, where phreatic eruptions occurred. The results showed that approximately two years after the eruption, the AETs decreased while the fumarolic temperature increased. The temperature of the fumaroles immediately after the eruption was close to 100°C and the Cl concentration in the fumarole at that time was relatively low. This state suggests that the Cl component remained in the hydrothermal water in a gas-liquid two-phase system, and this fact is considered to be the indication of decompression boiling. The subsequent increase in both Cl concentration and fumarole temperature also confirms this indication. In this research, we estimated the temperature of the hydrothermal water before the flash boiling occurred.
Analysis of the water isotopic composition (δD-δ18O diagrams) of fumarolic condensate and other fluids showed that the mixing ratio of magmatic vapor and meteoric water is around 6:4-7:3. By using this mixing ratio and calorimetric analysis, the temperature of the hydrothermal fluid just before flash boiling was estimated to be between 336°C and 374°C, which is close to the critical temperature, and this was suggested to be the cause of the 1995 phreatic eruption.
Also at Kuju Volcano, a phreatic eruption occurred in 1995 on the eastern ridge of the summit of Mt.Hossho. This eruption site is located southeast of "Kuju-Iwoyama", a fumarolic area that continuously releases volcanic gases. But now, volcanic gas emissions from the new craters have subsided and decreased, despite the ongoing volcanic activity. According to the Plan of Volcanic Eruption Prediction of the Japanese Council for Science and Technology Policy, Kuju-Iwoyama is designated as an active volcano with potential explosive activity. Therefore, it is very important to investigate the triggers of past phreatic eruptions at Mt.Kuju and to prepare for future eruptions.
According to Mizutani et al. (1986), fumarolic gases of up to 508°C were observed at Mt.Kuju in 1960, and the condensate of gases at that time was isotopically almost pure magmatic water, but in 1984, it was estimated to contain about 50% meteoric water. Kitaoka (1996) proposed a hydrothermal model for Mt. Kuju. This model explains that as water seeps from the surface and moves deeper, much of the evaporated vapor enters the relatively low-pressure pathway which includes magmatic vapor, and becomes hot superheated steam above a critical temperature, and mixes with magmatic steam. According to this model, hydrothermal water in a gas-liquid two-phase system is formed by the mixing of magmatic vapor and natural water. In this research, we determined the mixing rate between magmatic vapor and water from geochemical analysis of the δD-δ18O water isotopic composition of the fumarole, and estimated the temperature of the hydrothermal water which caused the phreatomagmatic eruption in 1995 from calorimetric analysis.
We focused on the relationship between apparent equilibrium temperature (AETs) and fumarolic temperature in the D-region, where phreatic eruptions occurred. The results showed that approximately two years after the eruption, the AETs decreased while the fumarolic temperature increased. The temperature of the fumaroles immediately after the eruption was close to 100°C and the Cl concentration in the fumarole at that time was relatively low. This state suggests that the Cl component remained in the hydrothermal water in a gas-liquid two-phase system, and this fact is considered to be the indication of decompression boiling. The subsequent increase in both Cl concentration and fumarole temperature also confirms this indication. In this research, we estimated the temperature of the hydrothermal water before the flash boiling occurred.
Analysis of the water isotopic composition (δD-δ18O diagrams) of fumarolic condensate and other fluids showed that the mixing ratio of magmatic vapor and meteoric water is around 6:4-7:3. By using this mixing ratio and calorimetric analysis, the temperature of the hydrothermal fluid just before flash boiling was estimated to be between 336°C and 374°C, which is close to the critical temperature, and this was suggested to be the cause of the 1995 phreatic eruption.