11:45 AM - 12:00 PM
[SVC34-05] Detection and monitoring of magma-origin gas in hot-spring water
Keywords:hot-spring water, magma-origin gas, Giggenbach diagram
We would like to discuss our current attempts to detect magma-derived gases dissolved in hot spring water at Uchinomaki hot spring in the Aso caldera and Futagawa fault.
The composition of hot spring water in the vicinity of volcanoes may change as volcanic activity progresses. This is because some chemicals are released not only through the vent but also through the volcanic body. Giggenbach (1992) plotted the nitrogen/helium and nitrogen/argon ratios of geothermal and volcanic gases with high helium isotope ratios on a triangular diagram (Giggenbach’s diagram). These points scattered among the three end members of andesitic-magma origin (A-component), basaltic-magma origin (B-component), and meteoric water origin (M-component), indicating that they can be classified as a mixture of these end members.
We decided to try to detect and monitor the deep-origin gas (A component + B component) in the hot spring water at Uchinomaki hot spring in the Aso caldera, which is now becoming more active. The same observations has been carrying out at a hot spring directly above the Futagawa fault, which is far from Mt. Aso, for comparison.
For the observation, dissolved gases were extracted from the hot spring water and analyzed by a quadrupole mass spectrometer in a fully automated manner. Since atmospheric contamination in the apparatus was prevented as much as possible but not completely, the amount of atmospheric contamination in the apparatus was corrected by using the acquired oxygen concentration as an index, as in Giggenbach (1992). The nitrogen/helium and nitrogen/argon ratios were calculated and plotted on the Giggenbach diagram. The corrected gas concentrations were calculated as a mixture of the three end members.
About 80% of dissolved gas in the hot spring water was M-component, about 20% was A-component, and less than a few percent was B-component. These values were stable in August and September 2021, but from October, the M-component gradually decreased, and the A-component correspondingly increased. Volcanic tremor of Mt. Aso became active from October 2021, and the relatively large eruption occurred on October 20, 2021.
If the Giggenbach's analysis like this study is able to detect changes in volcanic activity, it may be applied to future monitoring and assessment of volcanoes. We would like to investigate whether the relation between dissolved gas and volcanic activity obtained in this study are causal or not through further observations.
The composition of hot spring water in the vicinity of volcanoes may change as volcanic activity progresses. This is because some chemicals are released not only through the vent but also through the volcanic body. Giggenbach (1992) plotted the nitrogen/helium and nitrogen/argon ratios of geothermal and volcanic gases with high helium isotope ratios on a triangular diagram (Giggenbach’s diagram). These points scattered among the three end members of andesitic-magma origin (A-component), basaltic-magma origin (B-component), and meteoric water origin (M-component), indicating that they can be classified as a mixture of these end members.
We decided to try to detect and monitor the deep-origin gas (A component + B component) in the hot spring water at Uchinomaki hot spring in the Aso caldera, which is now becoming more active. The same observations has been carrying out at a hot spring directly above the Futagawa fault, which is far from Mt. Aso, for comparison.
For the observation, dissolved gases were extracted from the hot spring water and analyzed by a quadrupole mass spectrometer in a fully automated manner. Since atmospheric contamination in the apparatus was prevented as much as possible but not completely, the amount of atmospheric contamination in the apparatus was corrected by using the acquired oxygen concentration as an index, as in Giggenbach (1992). The nitrogen/helium and nitrogen/argon ratios were calculated and plotted on the Giggenbach diagram. The corrected gas concentrations were calculated as a mixture of the three end members.
About 80% of dissolved gas in the hot spring water was M-component, about 20% was A-component, and less than a few percent was B-component. These values were stable in August and September 2021, but from October, the M-component gradually decreased, and the A-component correspondingly increased. Volcanic tremor of Mt. Aso became active from October 2021, and the relatively large eruption occurred on October 20, 2021.
If the Giggenbach's analysis like this study is able to detect changes in volcanic activity, it may be applied to future monitoring and assessment of volcanoes. We would like to investigate whether the relation between dissolved gas and volcanic activity obtained in this study are causal or not through further observations.