Magmatic He distribution around Unzen volcano inferred from intensive investigation of helium isotopes in groundwater
Magmatic He distribution around Unzen volcano inferred from intensive investigation of helium isotopes in groundwater
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DOI：
10.1016/j.jvolgeores.2008.03.038
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发表时间：
2008-07-30
影响因子：
2.9
通讯作者：
Nakama, A.
中科院分区：
文献类型：
--
作者：
Morikawa, N.;Kazahaya, K.;Nakama, A.
通过密集采样和测量地下水(包括热水和气体)中的氦浓度和同位素比值，研究了温泉火山周围上升的岩浆成分。在温泉地块东部，即使是浅层地下水也含有大量溶解的岩浆氦，氦-3/氦-4比值高，约为大气值的7倍。这一数值与该地区(岛原温泉)热水中的数值几乎相同。相比之下，没有大量的岩浆被并入其他地区的浅层地下水中。地堑内热水中He-3/He-4的分布没有遵循在许多活火山中观察到的随着距离管道距离的增加而减小的趋势。火山周围He-3/He-4比值的分布反映了各地区热水储集层滞留时间的差异和/或岩浆气体组分通量的差异。考虑水库内He-4浓度和He-3/He-4比值演化的模型计算结果表明，奥巴马温泉(地堑西侧)和温泉温泉(中部)的停留时间分别约为2.8万年和1.1万年，显著高于岛原温泉(东部)。这些值随地壳氦通量、含水层厚度和孔隙度等几个参数的变化而变化很大，应视为一级近似。然而，观测到的He-3/He-4比值反映了三个地区热水停留时间和岩浆氦通量大小的质的差异。岛原温泉热水库流出的岩浆氦-3通量大约是奥巴马温泉和温泉温泉的4倍。所有这些结果都与岩浆气体通过云顶地块东部密集存在的东西向断裂以及一条输导上升的概念并不矛盾。(C)2008爱思唯尔B.V.保留所有权利。
The ascending magmatic component around Unzen volcano was investigated by intensive sampling and measurements of helium concentration and isotopic ratios in groundwaters including thermal waters (and gases). In the eastern part of the Unzen graben, even shallow groundwaters contain large amount of dissolved magmatic helium with high He-3/He-4 ratio, about 7 times higher than the atmospheric value. This value is almost identical as those in thermal water of this region (Shimabara Spa). In contrast, no significant amount of magmatic He was incorporated into the shallow groundwaters in other regions. The He-3/He-4 distribution among thermal waters inside the graben does not follow the trend observed in many active volcanoes of decreasing the ratio with increasing the distance from a conduit. The distribution of He-3/He-4 ratios around Unzen volcano reflects the difference in residence time in thermal water reservoir and/or difference in the fluxes of magmatic gas components in each region. The results of model calculation, which considered the evolution of He-4 concentration and He-3/He-4 ratio in the reservoir, indicated that the residence times of Obama Spa (western side of the graben) and Unzen Spa (central part) were about 28,000 and 11,000 years, respectively, which are significantly higher than those of Shimabara Spa (eastern part). These values are highly variable according to several parameters, such as crustal He flux, aquifer thickness and porosity etc, and should be taken as a first order approximation. Observed He-3/He-4 ratios, however, reflect the qualitative difference of thermal water residence time as well as the magnitude of magmatic He fluxes among three regions. The magmatic He-3 flux out from the thermal water reservoir of Shimabara Spa is about four times higher than those of Obama and Unzen Spas. All of these results are not inconsistent with the concept that magmatic gas ascends through the densely existing EW faults in the eastern part of the Unzen graben as well as a conduit. (C) 2008 Elsevier B.V. All rights reserved.