[S-VC29] Hydrothermal systems of volcanoes

[SVC29-P02] Temporal variation of chemical composition of hot spring waters at Ebinokogen Ioyama Volcano in Kirishima Volcanic Group

*Keigo Mori¹, Jun-ichiro Ishibashi¹, Takeshi Matsushima¹, Harue Masuda², Syogo Oshima³, Saki Tsutsumi³, Yasuhisa Tajima⁴ (1.Graduate School of Science, Kyushu University, 2.Graduate school of Science, Osaka City University, 3.West Japan Engineering Consultants, Inc,, 4.Nippon Koei Co., Ltd,)

Keywords:fluid rock interaction, monitoring of time series change

Vigorous emanation of hot spring waters and fumarolic gases has been observed at Ebinokogen Ioyama Volcano, since the eruption in April 2018. We have conducted periodic sampling of hot waters emanating from the craters and fumarolic vents since July, 2018, and monitored temporal variation of their chemical composition which could be related to magmatic and hydrothermal activities. We report temporal variation of major cation compositions as well as anion compositions, to discuss factors which control fluid chemistry. Water samples were collected from W4 hot spring formed at the eruption crater located about 500 meters west from the Iwoyama crater, Y2a and Y2b hot springs formed at the eruption crater 50 meters south from the Iwoyama crater, and small fumarolic vents located in the area between them (Tajima et al., 2020). During the fluid sampling, water temperature, pH, ORP (oxidation-reduction potential) and EC (electrical conductivity) were measured on site using probes. Chemical composition of major cations (Na, K, Mg, Ca) and Si of the spring waters was determined by ICP-OES (inductivity coupled plasma optical emission spectrometry) and that of major anions (Cl, SO₄) was determined by IC (ion chromatography).

Long-term variation of major cations for two and half years duration basically follows the trend observed in that of major anions (such as Cl/SO₄ ratio). The most drastic change in W4 hot spring water chemistry was recognized at the early winter at year 2020; concentrations of major cations were high till Dec. 2019, while they significantly decreased since Feb. 2020. To characterize behavior of each cation, the observed concentration in the W4 hot spring water (defined as Ci) is compared with the concentration expected from complete dissolution of andesite from Ioyama by interaction with the acidic hot spring water (defined as Di), following the model proposed in Ohba et al. (2008). For Na and Mn, Ci was extremely high compared with Di in the duration till Dec. 2019, but Ci was decreased to comparable with Di after Feb. 2020. For Ca and Si, Ci was usually low compared with Di. For K, Ci was lower than Di till Dec. 2019, but Ci was higher than Di after Feb. 2020. These temporal variations are attributed to a shift of cation sources for W4 hot spring. After Feb. 2020, the cation composition can be explained by complete dissolution of andesite interaction with the acidic water and removal of some cations by formation of secondary minerals such as alunite. Whereas, the cation composition is likely to be dominantly controlled by other source till Dec. 2019, and hydrothermal aquifer had been developed beneath the volcano is presumed as the possible source. We discuss this model in detail from data of temporal variation of minor cations (Li, Rb, Cs) composition.