3:30 PM - 5:00 PM
[SVC35-P03] Volcanic activity and subsurface structure inferred from noble gas isotopic compositions of volcanic gases
Keywords:Volcanic gas, Noble gas, Helium, Isotope, Hydrothermal system
In the sub-theme 3, "Development of Geochemical Observation Techniques," of the project B "Development of Cutting-edge Volcano Observation Technology" of the Integrated Program for Next Generation Volcanic Research and Human Resource Development, geochemical analyses of volcanic plumes, fumaroles, dissolved gases in hot spring water, soil gases, etc. (these are widely called "volcanic gases" here) emitted from volcanoes and their surroundings is being conducted. Based on the geochemical observations of volcanic gases, we are developing methods to evaluate the activity of magma or hydrothermal systems and to understand the distribution of deep underground magmatic fluids. In this presentation, we will introduce a case study of the Kusatsu-Shirane volcano, where several interesting results using noble gases as tracers have been obtained.
The are several noble gas components in volcanic gas, which are derived from magma, crust, atmosphere, or groundwater. For example, helium isotope ratios (3He/4He) differ between magmatic and crustal origins, and the air-corrected 3He/4He can be obtained by estimating the amount of atmospheric 4He from the 4He/20Ne with an assumption that all the 20Ne is atmospheric in origin. Furthermore, the contributions of the magmatic component with high 3He/4He (up to about 8 RA for arc volcanoes) and of the crustal component with low 3He/4He (less than 0.1 RA) can be determined from the air-corrected 3He/4He. By applying the same calculations to other noble gases, the elemental ratios of the noble gases of magmatic origin can also be determined. Since the solubility of noble gases in magma varies with each element and depends on the degree of magma vesicularity, it is expected that changes in the pressure of magma can be detected from changes in the elemental ratios of magmatic noble gases.
At the Kusatsu-Shirane volcano, an increase in the 3He/40Ar* (3He is almost entirely magmatic in origin, and 40Ar* means the magmatic origin in the total 40Ar) associated with an increase in seismic activity was observed in the fumaroles on the north flank of Mt. Shirane, which means that the magma pressure was decreasing as seismic activity increased (Obase et al., 2022). This is the first example of detecting pressure changes that cannot be detected by geophysical observations such as earthquakes or crustal deformation, based on noble gases in volcanic gases. However, it is necessary to accumulate more observations to clarify whether the pressure change is due to the breakdown of the sealing just above the magma or to the rise of the magma-fluid interface.
There is an emission of gaseous species through soil at the surface of volcanic bodies. These gaseous components can be collected and analyzed as soil gas, and in-situ observations of gaseous components that are easy to measure, such as carbon dioxide and mercury, can be used to determine the diffusive emission rate to the atmosphere.
The release rate of mercury from the soil around Yugama at the summit of Mt. Shirane and noble gases in the soil gas indicate that magma-derived 3He is released even in areas where no clear volcanic gas release is observed on the surface, which are near the craters of past side eruptions (Terada et al., 2023). In other words, using 3He as an indicator, it may be possible to know in advance the locations that have high permeability and are connected to hydrothermal systems involving magmatic gases, i.e., potential future eruption sites.
References
Obase, T., Sumino, H., Toyama, K., Kawana, K., Yamane, K., Yaguchi, M., Terada, A., and Ohba, T. (2022) Monitoring of magmatic-hydrothermal Sci. Rep. 12, 17967.
Terada, A., Wakamatsu, U., Mizutani, N., Kakuno, H., Kohase, T., Oba, T., Takahashi, M., Taniguchi, M., Takahashi, Y. (2023) Assessment of lateral eruption hazards at Kusatsu-Shirane volcano: Mass transport in the subsurface around the crater suggested by the chemical characteristics of soil gas. Japan Geoscience Union 2023 Meeting
The are several noble gas components in volcanic gas, which are derived from magma, crust, atmosphere, or groundwater. For example, helium isotope ratios (3He/4He) differ between magmatic and crustal origins, and the air-corrected 3He/4He can be obtained by estimating the amount of atmospheric 4He from the 4He/20Ne with an assumption that all the 20Ne is atmospheric in origin. Furthermore, the contributions of the magmatic component with high 3He/4He (up to about 8 RA for arc volcanoes) and of the crustal component with low 3He/4He (less than 0.1 RA) can be determined from the air-corrected 3He/4He. By applying the same calculations to other noble gases, the elemental ratios of the noble gases of magmatic origin can also be determined. Since the solubility of noble gases in magma varies with each element and depends on the degree of magma vesicularity, it is expected that changes in the pressure of magma can be detected from changes in the elemental ratios of magmatic noble gases.
At the Kusatsu-Shirane volcano, an increase in the 3He/40Ar* (3He is almost entirely magmatic in origin, and 40Ar* means the magmatic origin in the total 40Ar) associated with an increase in seismic activity was observed in the fumaroles on the north flank of Mt. Shirane, which means that the magma pressure was decreasing as seismic activity increased (Obase et al., 2022). This is the first example of detecting pressure changes that cannot be detected by geophysical observations such as earthquakes or crustal deformation, based on noble gases in volcanic gases. However, it is necessary to accumulate more observations to clarify whether the pressure change is due to the breakdown of the sealing just above the magma or to the rise of the magma-fluid interface.
There is an emission of gaseous species through soil at the surface of volcanic bodies. These gaseous components can be collected and analyzed as soil gas, and in-situ observations of gaseous components that are easy to measure, such as carbon dioxide and mercury, can be used to determine the diffusive emission rate to the atmosphere.
The release rate of mercury from the soil around Yugama at the summit of Mt. Shirane and noble gases in the soil gas indicate that magma-derived 3He is released even in areas where no clear volcanic gas release is observed on the surface, which are near the craters of past side eruptions (Terada et al., 2023). In other words, using 3He as an indicator, it may be possible to know in advance the locations that have high permeability and are connected to hydrothermal systems involving magmatic gases, i.e., potential future eruption sites.
References
Obase, T., Sumino, H., Toyama, K., Kawana, K., Yamane, K., Yaguchi, M., Terada, A., and Ohba, T. (2022) Monitoring of magmatic-hydrothermal Sci. Rep. 12, 17967.
Terada, A., Wakamatsu, U., Mizutani, N., Kakuno, H., Kohase, T., Oba, T., Takahashi, M., Taniguchi, M., Takahashi, Y. (2023) Assessment of lateral eruption hazards at Kusatsu-Shirane volcano: Mass transport in the subsurface around the crater suggested by the chemical characteristics of soil gas. Japan Geoscience Union 2023 Meeting