[H-TT35_1PO1] Developments and applications of AMS techniques for earth and human environmental research

★ [HTT35-P14] Study on monitoring of volcanic activity using ¹²⁹I / ¹²⁷I ratios in crater lake and hot spring at Zao volcano

*Tetsuya MATSUNAKA¹, Kimikazu SASA¹, Keisuke SUEKI¹, Nao SHIBAYAMA¹, Tsutomu TAKAHASHI¹, Masumi MATSUMURA¹, Yukihiko SATOU¹, Hiroyuki MATSUZAKI², Akio GOTO³, Takahiro WATANABE³, Noriyoshi TSUCHIYA³, Nobuo HIRANO³, Akihisa KIZAKI³ (1.University of Tsukuba, 2.The University of Tokyo, 3.Tohoku University)

Keywords:Zao volcano, volcanic activity, crator lake, hot spring, ¹²⁹I / ¹²⁷I, AMS

Volcanic tremors and mountain gradient changes have been detected at Zao volcano in Miyagi and Yamagata since January 2013, volcanic activity began to intensify although Zao volcano will not erupt immediately^[1]. Since the water quality of crater lake are correlating with volcanism changes^[2][3], basic water quality of crater lake and hot spring at Zao volcano have been studied by the group of Tohoku University from September 2013. As a part of this project, we are trying to monitor the volcanic activity using ¹²⁹I / ¹²⁷I ratios (atomic ratio of radioiodine and stable iodine) in crater lake and hot spring of Zao volcano. Natural ¹²⁹I (half-life: 15.7 million year) are produced by nuclear spallation reaction of ¹²⁹Xe with cosmic ray in the atmosphere and spontaneous fission of ²³⁸U in the geological layer. In the ocean, steady-state ¹²⁹I / ¹²⁷I ratio of the seawater is estimated to be 1.5 × 10^-12[4]. Sunken iodine by the ocean plate having lower ¹²⁹I / ¹²⁷I ratio (older ¹²⁹I age) compared to the steady-state ratio of seawater, are supplied to the atmosphere mainly via magmatic activity. In general, ¹²⁹I / ¹²⁷I ratio in hot spring water and brine water are used as indicator of origin and behavior of iodine in the water^[5][6]. ¹²⁹I / ¹²⁷I ratio of hydrothermal at Zao volcano are considered to become lower by the supply of chronologically-old iodine in terms of global iodine cycle. In September 2013, water samples of 2 L were collected from the surface of crater lake (Okama, diameter: 350 m, maximum depth: 35 m) located at 1,560 m in elevation and hot spring (Kamoshika Hot Spring) located at 1,230 m in elevation in the eastern side of Zao volcano. Water temperature and pH were measured on site. After water samples were filtered by 0.2 μm filter, ¹²⁹I / ¹²⁷I ratio were measured for the isotopic diluted water samples by adding carrier (¹²⁷I standard) at MALT, The University of Tokyo. ¹²⁷I concentrations were measured by ICP-MS, and original ¹²⁹I / ¹²⁷I ratio of water samples were estimated. Water temperature and pH were 10.2℃ and 3.3 at Okama; 40.0℃ and 3.3 - 4.0 at Kamoshika Hot Spring. ¹²⁹I / ¹²⁷I ratios of Okama and Kamoshika Hot Spring were respectively, estimated to be (1.5 ± 0.4) × 10^-9 and (0.78 ± 0.2) × 10^-9, 500 - 1000 times higher than the steady-state ratio of sea water (1.5 × 10^-12)^[4]. Since ¹²⁹I / ¹²⁷I ratio of anthropogenic metric water were over 9.0 × 10^-12[7], surface water of Okama and Kamoshika Hot Spring water were very likely to be strong affected by the meteoric water including anthropogenic ¹²⁹I. For the monitoring of volcanic activity using ¹²⁹I / ¹²⁷I ratio, it is necessary to decide the site as few anthropogenic ¹²⁹I as possible through the measuring of ¹²⁹I / ¹²⁷I ratio of the Okama bottom water and some hot spring around Zao volcano. Continuous water quality survey of 1 - 2 times for Okama and 1 time per 1 - 2 months for hot springs are planned from June to November of this year.[1] Japan Meteorological Agency (2013) Monthly Volcanic Activity Report.[2] Ohba et al. (2000) Journal of Volcanology and Geothermal Research, 97, 329-346.[3] Ohba et al. (2008) Journal of Volcanology and Geothermal Research, 178, 131-144.[4] Moran et al. (1998) Chemical Geology, 152, 193-203.[5] Snyder and Fehn (2002) Geochimica et Cosmochimica Acta, 66, 3827-3838.[6] Muramatsu et al. (2001) Earth and Planetary Science Letter, 192, 583-593.[7] Tomaru et al. (2007) Applied Geochemisty, 22, 676-691.