The Saga Plain is near Beppu-Shimabara graben. a region of potential active volcanism. In the graben, mantle He, which has a high He-3/He-4 ratio of 1.1 x 10(-5), escapes easily from the underlying subduction zone. In groundwater of the Saga Plain, except in the Shiroishi district, this ratio gradually increased as the dissolved He content increased, to a maximum of 5 x 10(-6). In central Shiroishi, however, the ratio reached a minimum of 8.7 x 10(-7) with increasing dissolved He content, suggesting that groundwater in central Shiroishi has selectively accumulated radiogenic He, which has a very low ratio of 1 x 10(-8), rather than reflecting the regional He, which is rich in mantle He. This can be explained if groundwater in Shiroishi has become mixed with fossil pore water drawn from impermeable marine clay aquitard layers. The withdrawal of pore water has also caused severe land subsidence in central Shiroishi.
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机译:佐贺平原就在别府岛原grab附近。潜在的活跃火山活动地区。在the斗中,He-3 / He-4比率高至1.1 x 10(-5)的地幔He容易从下面的俯冲带逸出。在佐贺平原的地下水中,除了白石地区以外,随着溶解的He含量的增加,该比率逐渐增加,最大为5 x 10(-6)。然而,在白石中心地区,随着溶解氦含量的增加,该比率最低达到8.7 x 10(-7),这表明白石中心地区的地下水选择性地积累了放射性He,其比率非常低,仅为1 x 10(-8)。 ),而不是反映出拥有丰富披风He的区域性He。如果Shiroishi的地下水已经与从不可渗透的海洋粘土阿基石层抽取的化石孔隙水混合在一起,则可以解释这一点。孔隙水的抽取也导致了白石中心地区严重的地面沉降。
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