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[SVC29-P04] Change in fluoride ion concentration in Bandaiko hot spring at east flank of Kusatsu-Shirane volcano and its relationship with volcanic activity
Keywords:fluoride ion, Kusatsu Shirane Volcano, hydrothermal system, geothermal fluid, hot spring, active crater lake
In a phreatic eruption, apparent seismic activity and crustal movements that can be regarded as precursors of eruption may not be observed. Therefore, for volcanoes with extensive hydrothermal activity, geochemical observations on fumaroles and hot springs are important in addition to geophysical observations. Chloride ions and fluoride ions derived from HCl and HF in volcanic gas are dissolved into active crater lakes and volcanic hot spring water. Their dissolved concentrations and concentration ratios are often used as indicators of volcanic activity. In particular, HF reacts readily with rocks mainly composed of silicates and is easily incorporated into rocks. Thus, an increase in the concentration of fluoride ions in volcanic geothermal water can be a more remarkable indicator of volcanic activity than that of chloride ions.
On the other hand, due to fluoride ions' strong complex-forming ability, direct determination by using an ion chromatography or an ion-selective electrode may give erroneous results. Therefore, in volcanic geothermal water containing a high concentration of metal ions complex with fluoride ions, it is essential to isolate fluoride ions by distillation prior to the determination of fluoride ions. For example, a trimethylsilylating distillation method is known.1) However, since the distillation operation is troublesome and time-consuming, the determination of fluoride ions is often omitted. In this study, we investigated a simple method that directly determines fluoride ions with an ion-selective electrode without distillation by using a specific total ionic strength adjustment buffer and confirmed its precision and accuracy. After that, we applied the simple method to the water of Yugama, the crater lake of the Kusatsu-Shirane volcano, and the water of Bandaiko hot spring located on the eastern flank of the volcano stored in our laboratory, and reconstructed the time-series data of the fluoride ion concentrations over the past decades. Subsequently, we discussed the relationship between the fluoride ion concentrations and the activity of the Kusatsu-Shirane volcano.
The Kusatsu-Shirane volcano consists of three pyroclastic cones, Mt. Shirane, Mt. Moto-Shirane, and Mt. Ainomine. In January 2018, a phreatic eruption occurred at Mt. Moto-Shirane. The fluoride ion concentration of Bandaiko located on the eastern flank of Mt. Moto-Shirane gradually increased from 2012 to 2017, then increased significantly coincidently with the eruption at Mt. Moto-Shirane. After that, fluoride ion still maintains high concentration until 2020, while a small fluctuation in the concentration has been observed. In the case of chloride ion, which is also halide ion, a gradual increase in the concentration was observed from 2012 to 2017 as same as fluoride ion concentration. However, unlike fluoride ion, no increase in the concentration coincident with the eruption of Mt. Moto-Shirane was seen, and no increase was observed after that.
On the other hand, the concentrations of both fluoride and chloride ions in Yugama, the crater lake of Mt. Shirane, increased from 2014 to 2016 and then gradually decreased. There was no significant increase in their concentrations coincident with the eruption of Mt. Moto-Shirane in January 2018, unlike Bandaiko, but the concentrations began to increase from around June 2018. After that, the concentrations have remained at a high while a small fluctuation has been observed. The F/Cl concentration ratio also increased significantly from June to September 2018 and has remained high since then.
Seismic activity and ground inflation have been observed around Yugama intermittently after the eruption at Mt. Moto-Shirane. The fluctuation of fluoride ion concentration in Yugama and Bandaiko occur to follow those volcanic activities. However, the change in fluoride ion concentration coincident with the eruption of Mt. Moto-Shirane in January 2018 is observed in Bandaiko, but not in Yugama. Accordingly, the chemistry of Yugama reflects volcanic activity around the summit area of Mt. Shirane, while Bandaiko may reflect the activities of the entire Kusatsu-Shirane volcano or that of Mt. Moto-Shirane, which do not appear in Yugama. Since the changes in halide ion concentrations are different between Bandaiko and Yugama, we would like to keep an eye on future changes.
1)Tsuchiya et al. (1985) Separation of microamounts of fluoride coexisting with large amounts of aluminum and silica by improved trimethylsilylating distillation. Analytica Chimica Acta 176, 151-159.
On the other hand, due to fluoride ions' strong complex-forming ability, direct determination by using an ion chromatography or an ion-selective electrode may give erroneous results. Therefore, in volcanic geothermal water containing a high concentration of metal ions complex with fluoride ions, it is essential to isolate fluoride ions by distillation prior to the determination of fluoride ions. For example, a trimethylsilylating distillation method is known.1) However, since the distillation operation is troublesome and time-consuming, the determination of fluoride ions is often omitted. In this study, we investigated a simple method that directly determines fluoride ions with an ion-selective electrode without distillation by using a specific total ionic strength adjustment buffer and confirmed its precision and accuracy. After that, we applied the simple method to the water of Yugama, the crater lake of the Kusatsu-Shirane volcano, and the water of Bandaiko hot spring located on the eastern flank of the volcano stored in our laboratory, and reconstructed the time-series data of the fluoride ion concentrations over the past decades. Subsequently, we discussed the relationship between the fluoride ion concentrations and the activity of the Kusatsu-Shirane volcano.
The Kusatsu-Shirane volcano consists of three pyroclastic cones, Mt. Shirane, Mt. Moto-Shirane, and Mt. Ainomine. In January 2018, a phreatic eruption occurred at Mt. Moto-Shirane. The fluoride ion concentration of Bandaiko located on the eastern flank of Mt. Moto-Shirane gradually increased from 2012 to 2017, then increased significantly coincidently with the eruption at Mt. Moto-Shirane. After that, fluoride ion still maintains high concentration until 2020, while a small fluctuation in the concentration has been observed. In the case of chloride ion, which is also halide ion, a gradual increase in the concentration was observed from 2012 to 2017 as same as fluoride ion concentration. However, unlike fluoride ion, no increase in the concentration coincident with the eruption of Mt. Moto-Shirane was seen, and no increase was observed after that.
On the other hand, the concentrations of both fluoride and chloride ions in Yugama, the crater lake of Mt. Shirane, increased from 2014 to 2016 and then gradually decreased. There was no significant increase in their concentrations coincident with the eruption of Mt. Moto-Shirane in January 2018, unlike Bandaiko, but the concentrations began to increase from around June 2018. After that, the concentrations have remained at a high while a small fluctuation has been observed. The F/Cl concentration ratio also increased significantly from June to September 2018 and has remained high since then.
Seismic activity and ground inflation have been observed around Yugama intermittently after the eruption at Mt. Moto-Shirane. The fluctuation of fluoride ion concentration in Yugama and Bandaiko occur to follow those volcanic activities. However, the change in fluoride ion concentration coincident with the eruption of Mt. Moto-Shirane in January 2018 is observed in Bandaiko, but not in Yugama. Accordingly, the chemistry of Yugama reflects volcanic activity around the summit area of Mt. Shirane, while Bandaiko may reflect the activities of the entire Kusatsu-Shirane volcano or that of Mt. Moto-Shirane, which do not appear in Yugama. Since the changes in halide ion concentrations are different between Bandaiko and Yugama, we would like to keep an eye on future changes.
1)Tsuchiya et al. (1985) Separation of microamounts of fluoride coexisting with large amounts of aluminum and silica by improved trimethylsilylating distillation. Analytica Chimica Acta 176, 151-159.