11:18 〜 11:43
[AHW23-03] Temporal variations of gas geochemistry at Mt. Ontake after the 2014 eruption
★Invited Papers
キーワード：volcanic activity、hydrothermal system、Mt. Ontake、noble gases
Mt. Ontake is a strato-volcano located in central Honshu Island, Japan. The volcano suddenly erupted on September 27, 2014 without any apparent precursory phenomena, and caused severe damage to surrounding travelers. It was a hydro-volcanic eruption which is difficult to be predicted because precursory anomaly such as edifice inflation and increase of seismic activity are usually very scarce. For investigation of volcanic activity, helium isotopes can be used as sensitive geochemical tracers because 3He/4He ratios precisely reflect the addition of mantle-derived components. Before and after the 2014 eruption, noble gas compositions in spring gases around the central cone were measured for investigating the structure of Ontake hydrothermal system and any anomaly related to increase in volcanic activity (Sano et al., 2015). As a result, they found that the 3He/4He ratios of the closest site to the central cone (Nigorigo: 4.2 km away from the cone) increased significantly from June 2003 to November 2014, while those of distant springs showed no valuable changes. This 3He enhancement may have been caused by increased input of magmatic volatiles with high heat which gradually increases water vapor pressure in the volcanic conduit by heating aquifer, finally causing the hydro-volcanic eruption in 2014. Unfortunately, it was difficult to identify this anomaly before the eruption because there had been no similar observations, but now, periodic observations of gas geochemistry are important for assessing the current and the future Mt. Ontake volcanic activity.
In order to assess Mt. Ontake activity after the 2014 eruption, temporal variations of gas geochemistry of spring gases were analyzed in this study. Gas samples at seven spring sites were collected between 2018 and 2024. The gas samples were collected using lead glass containers. In Atmosphere and Ocean Research Institute, the University of Tokyo, a portion of the gas sample was introduced into a vacuum line for purification. Subsequently 4He/20Ne ratios were measured using a quadrupole mass spectrometer, and 3He/4He ratios were analyzed using noble gas mass spectrometers Helix-SFT or VG5400. The obtained noble gas data were calibrated against air data. After that, a portion of gas samples was used for measurement of gas compositions and 40Ar/36Ar ratios with quadruple mass spectrometers. Then carbon and nitrogen isotope ratios (δ13C and δ15N values) in the rest gas samples were measured using a continuous flow GC-IRMS system (IsoPrime 100 with vario-EA system).
For the Nigorigo site, comparing the obtained data with the values reported before 2016 (Kagoshima et al., 2016), significant decrease in the air-corrected 3He/4He ratios were observed from 2014 to 2024. In November 2014, just after the eruption, the 3He/4He ratio of 7.43 Ra (1 Ra: the atmospheric 3He/4He ratio of 1.4×10-6) was observed in Nigorigo, and it dropped to be 6.75 Ra in November 2024. The recent 3He/4He ratio is comparable to the values observed before beginning of 3He enhancement around 2003, implying dissipation of the "excess" magmatic volatiles with high 3He/4He ratios accumulated in the volcanic conduit during Mt. Ontake activation processes. This indicates a possibility that Mt. Ontake is now in stable condition after ten years of the eruption. In the presentation, chemical data obtained at other spring sites will be also compared for further discussion.
(References)
Kagoshima et al. (2016) JVGR 325, 179-188.; Sano et al. (2015) Sci. Rep. 5, 13069.
In order to assess Mt. Ontake activity after the 2014 eruption, temporal variations of gas geochemistry of spring gases were analyzed in this study. Gas samples at seven spring sites were collected between 2018 and 2024. The gas samples were collected using lead glass containers. In Atmosphere and Ocean Research Institute, the University of Tokyo, a portion of the gas sample was introduced into a vacuum line for purification. Subsequently 4He/20Ne ratios were measured using a quadrupole mass spectrometer, and 3He/4He ratios were analyzed using noble gas mass spectrometers Helix-SFT or VG5400. The obtained noble gas data were calibrated against air data. After that, a portion of gas samples was used for measurement of gas compositions and 40Ar/36Ar ratios with quadruple mass spectrometers. Then carbon and nitrogen isotope ratios (δ13C and δ15N values) in the rest gas samples were measured using a continuous flow GC-IRMS system (IsoPrime 100 with vario-EA system).
For the Nigorigo site, comparing the obtained data with the values reported before 2016 (Kagoshima et al., 2016), significant decrease in the air-corrected 3He/4He ratios were observed from 2014 to 2024. In November 2014, just after the eruption, the 3He/4He ratio of 7.43 Ra (1 Ra: the atmospheric 3He/4He ratio of 1.4×10-6) was observed in Nigorigo, and it dropped to be 6.75 Ra in November 2024. The recent 3He/4He ratio is comparable to the values observed before beginning of 3He enhancement around 2003, implying dissipation of the "excess" magmatic volatiles with high 3He/4He ratios accumulated in the volcanic conduit during Mt. Ontake activation processes. This indicates a possibility that Mt. Ontake is now in stable condition after ten years of the eruption. In the presentation, chemical data obtained at other spring sites will be also compared for further discussion.
(References)
Kagoshima et al. (2016) JVGR 325, 179-188.; Sano et al. (2015) Sci. Rep. 5, 13069.