4:30 PM - 4:45 PM
[SVC35-10] Theoretical and Observational Research on Heterogeneous Geocheimcal Equilibria in the Hydrothermal System of Active Volcanoes
Keywords:Kuju Volcano, Phreatic Eruption, Hydrothermal System, Low-temperature Fumarolic Gas
Not only the detection of volcanic seismic activity, ground deformation, and magmatic gas, but also the understanding of the behavior of hydrothermal systems within the volcano is essential in order to reduce human casualties from sudden phreatic eruptions.
In this study, we focused on the 1995 phreatic eruption at Kuju volcano, where geochemical observation data were available for high-temperature volcanic gases, low-temperature fumaroles linked to hydrothermal systems, and hot spring waters. Based on the theory of the physical chemistry of gas-liquid two-phase systems, we attempted to extract critical information about the volcanic hydrothermal system which causes phreatic eruptions, such as temperature and steam fraction, by analyzing the water isotope and gas compositions of fumaroles.
The 1995 eruption of Kuju volcano (primarily a phreatic eruption) occurred in the D-region, with no previous fumarolic activity, which is located approximately 300 meters away from existing fumarolic areas. By referencing existing materials and analyzing water isotope compositions (δD-δ18O), we found that the mixture ratio of underground water and magmatic steam was approximately 37:63. Using this result, the hydrothermal water temperature was calculated to be around 372°C, based on the calorimetric analyses.
On the other hand, focusing on the relatively low-temperature (approximately 95°C) fumarolic gases with low SO2/H2S ratios, continuously being emitted from existing fumarolic areas (A, B, C-regions), we considered two chemical equilibria in the hydrothermal reservoir: CH4 + 2H2O = CO2 + 4H2, and H2S + 4H2O = 4H2 + 2H+ + SO42-. Considering the distribution of gas components between the gas and liquid phases, we calculated the temperature and steam fraction of the hydrothermal reservoir. As a result, we found that the reservoir was in a hydrothermally dominated state (steam fraction was approximately less than 0.1) both before and after the eruption, with temperatures ranging from 264 to 277°C.
The hydrothermal temperature of 372°C indicated by the former method suggests 212 atm at the D-region's underground hydrothermal reservoir, corresponding to a depth of 814m by assuming a gas-liquid two-phase state and lithostatic pressure. This result is consistent with previous research indicating a shallow source at 500m above sea level (around 1000m depth) during the 1995 Kuju volcano eruption (Nishi et al., 1996), and the pressure source may have been groundwater heated to near the critical temperature by magmatic steam.
On the other hand, based on the estimated temperatures (264–277°C) of the existing fumarolic areas (A, B, C-region) obtained using the latter method, it is inferred that the two-phase (vapor-liquid) reservoir that has continuously existed within the volcanic body since before the eruption is a separate hydrothermal system. This system is characterized by lower pressure (49–60 atm) and shallower depth (at a depth of 189–231 m under lithostatic pressure equilibrium) compared to the one identified beneath the D-region in this study. In the existing fumarolic areas, it is assumed that the vapor pressure of the hydrothermal fluids was low, the pressure was relieved by continuous fumarolic emissions, and the surrounding rock had sufficient strength to stably maintain the two-phase (vapor-liquid) reservoir. Such factors are considered to have contributed to the stability of the hydrothermal system, which prevented steam eruptions in the existing fumarolic areas.
In this study, we focused on the 1995 phreatic eruption at Kuju volcano, where geochemical observation data were available for high-temperature volcanic gases, low-temperature fumaroles linked to hydrothermal systems, and hot spring waters. Based on the theory of the physical chemistry of gas-liquid two-phase systems, we attempted to extract critical information about the volcanic hydrothermal system which causes phreatic eruptions, such as temperature and steam fraction, by analyzing the water isotope and gas compositions of fumaroles.
The 1995 eruption of Kuju volcano (primarily a phreatic eruption) occurred in the D-region, with no previous fumarolic activity, which is located approximately 300 meters away from existing fumarolic areas. By referencing existing materials and analyzing water isotope compositions (δD-δ18O), we found that the mixture ratio of underground water and magmatic steam was approximately 37:63. Using this result, the hydrothermal water temperature was calculated to be around 372°C, based on the calorimetric analyses.
On the other hand, focusing on the relatively low-temperature (approximately 95°C) fumarolic gases with low SO2/H2S ratios, continuously being emitted from existing fumarolic areas (A, B, C-regions), we considered two chemical equilibria in the hydrothermal reservoir: CH4 + 2H2O = CO2 + 4H2, and H2S + 4H2O = 4H2 + 2H+ + SO42-. Considering the distribution of gas components between the gas and liquid phases, we calculated the temperature and steam fraction of the hydrothermal reservoir. As a result, we found that the reservoir was in a hydrothermally dominated state (steam fraction was approximately less than 0.1) both before and after the eruption, with temperatures ranging from 264 to 277°C.
The hydrothermal temperature of 372°C indicated by the former method suggests 212 atm at the D-region's underground hydrothermal reservoir, corresponding to a depth of 814m by assuming a gas-liquid two-phase state and lithostatic pressure. This result is consistent with previous research indicating a shallow source at 500m above sea level (around 1000m depth) during the 1995 Kuju volcano eruption (Nishi et al., 1996), and the pressure source may have been groundwater heated to near the critical temperature by magmatic steam.
On the other hand, based on the estimated temperatures (264–277°C) of the existing fumarolic areas (A, B, C-region) obtained using the latter method, it is inferred that the two-phase (vapor-liquid) reservoir that has continuously existed within the volcanic body since before the eruption is a separate hydrothermal system. This system is characterized by lower pressure (49–60 atm) and shallower depth (at a depth of 189–231 m under lithostatic pressure equilibrium) compared to the one identified beneath the D-region in this study. In the existing fumarolic areas, it is assumed that the vapor pressure of the hydrothermal fluids was low, the pressure was relieved by continuous fumarolic emissions, and the surrounding rock had sufficient strength to stably maintain the two-phase (vapor-liquid) reservoir. Such factors are considered to have contributed to the stability of the hydrothermal system, which prevented steam eruptions in the existing fumarolic areas.