Time Variation in the Chemical and Isotopic Composition of Fumarolic Gasses at Kusatsu-Shirane Volcano, Japan
Time Variation in the Chemical and Isotopic Composition of Fumarolic Gasses at Kusatsu-Shirane Volcano, Japan
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DOI：
10.3389/feart.2019.00249
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发表时间：
2019-09-24
影响因子：
2.9
通讯作者：
Shingubara, Ryo
中科院分区：
文献类型：
--
作者：
Ohba, Takeshi;Yaguchi, Muga;Shingubara, Ryo
具有热液系统的火山在蒸汽喷发前可能发生轻微地震活动。为了预测任何蒸汽喷发，探测和了解这种浅层地震活动的性质是必不可少的。由于富马罗气体存在于热液系统中，它可以为阐明任何地震活动的性质提供见解，从而预测蒸汽喷发。在日本草津shirane火山，2014年和2018年发生了强烈的地震活动。为了研究地震活动性与气体化学的关系，对5次富马酸气体排放进行了反复分析。自2014年7月至2017年11月，位于火山峰顶以北的富氧气体中CO2/H2O、He/H2O和N-2/H2O比值单调下降，表明岩浆成分下降。相反，CH4/H2O比值在地震平静期显著升高，表明热液系统条件降低，有利于CH4的形成。静息期较高的N-2/He比值表明N-2的加入，可能来源于承载热液储层的地壳岩石。2018年的N-2/He比值明显低于2014年，表明岩浆是随着脱气过程而演化的。采用岩浆气体与大气源冷地下水混合产生气相、大气源冷地下水加入气相、近地表水汽部分凝结等过程模拟了富马酸气体排放的D（H2O）、O-18（H2O）值和CO2/H2O比值。上述模拟只需要一种岩浆气体。这些数据表明，草津shirane火山地震活动性的激活与火山气体中岩浆成分的增加是同步的。推测岩浆气的注入增加了储层流体压力，从而引发了地震活动。这次注入可能是由包围脱气岩浆的密封带破裂引起的。岩浆气体的注入可以通过监测富马醛气体的成分来检测，从而有可能预测任何未来的地震活动。
Minor seismicity may occur at volcanoes with hydrothermal system before a steam eruption. To forecast any steam eruption, it is indispensable to detect and understand the nature of this shallow seismicity. As the fumarolic gas resides in the hydrothermal system, it may provide insights for elucidating the nature of any seismicity and thus forecast steam eruptions. At Kusatsu-Shirane volcano Japan, intense seismic activity took place in 2014 and 2018. To investigate the relationship between the seismicity and gas chemistry, five fumarolic gas discharges have been repeatedly analyzed. Since July 2014 to November 2017 a monotonic decrease in CO2/H2O, He/H2O and N-2/H2O ratios was recorded in the fumarolic gasses located north of the summit of volcano, suggesting the decline of the magmatic component. On the contrary the CH4/H2O ratio significantly increased during the seismically quiet period, indicating that reduced conditions developed in the hydrothermal system, favoring the formation of CH4. The high N-2/He ratio in the quiet period indicates the addition of N-2, likely deriving from the crustal rocks hosting hydrothermal reservoir. The N-2/He ratio in 2018 was significantly lower than those recorded in 2014, indicating the evolution of magma with the progress of degassing. The delta D(H2O) and delta O-18(H2O) values and the CO2/H2O ratios of fumarolic gas discharges were modeled with the following processes: generation of vapor phase after the mixing between magmatic gas and a cold groundwater with meteoric origin, addition of vapor phase with meteoric origin, and partial condensation of water vapor near surface. Only a single magmatic gas is necessary for the above modeling. These data suggest that at Kusatsu-Shirane volcano the activation of seismicity was synchronized with the increase of the magmatic component in the fumarolic gas. It is postulated that the injection of magmatic gas increased the fluid pressure in the reservoir, which triggered seismicity. The injection would have been triggered by a break of the sealing zone surrounding the degassing magma. The injection of magmatic gas can be detected by monitoring the composition of the fumarolic gas, thus giving the possibility to forecast any future seismicity.