18:15 〜 19:30
★ [HTT35-P14] 蔵王山の火山湖と温泉における129I / 127Iを用いた火山活動モニタリングの試み
キーワード：蔵王山, 火山活動, 火山湖, 温泉, 129I / 127I, 加速器質量分析
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.
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.