Variations in the He/He ratios of hot springs on Shikoku Island, Southwest Japan

西南日本,四国地方における温泉ガスのヘリウム同位体比

梅田 浩司; 金沢 淳; 角田 地文*; 浅森 浩一; 及川 輝樹

Umeda, Koji; Kanazawa, Sunao; Kakuta, Chifumi*; Asamori, Koichi; Oikawa, Teruki

西南日本の前弧域における温泉ガスのヘリウム同位体比の分布を明らかにするため、これまでデータが得られていない四国地方を対象に温泉ガスの採取,分析を行った。その結果、東北日本の前弧域の温泉に比べて高いヘリウム同位体比が四国全域で認められた。沈み込み帯の前弧域におけるヘリウム同位体比の異常については、スラブから脱水した流体によって運ばれるマントル起源のHeの影響が指摘されている。しかしながら、四国下の地殻内地震の活動が極めて乏しいこと、震源メカニズムが圧縮応力場を示すこと等を考慮すると、深部流体によるHeの寄与は小さいと考えられ、ヘリウム同位体比の分布は、中新世の火成岩体,地殻及び大気起源のヘリウムの混合によって説明できる。

New helium isotopic data from hot springs on Shikoku Island with much denser sampling than previous studies were obtained, in order to elucidate the geographical distribution of He/He ratios in the fore-arc regions of Southwest Japan. Systematic, regional variations in He/He ratios were observed on both Sikoku Island and the Kii Peninsula. Most of the hot springs having He/He ratios in the range 1.0 RA to 2.0 RA were determined throughout Shikoku Island. On the other hand, the majority of hot springs in the Kii Peninsula have He/He ratios higher than 2.0 RA. It has been suggested by some authors that the anomalously high He/He ratios in the fore-arc region of Southwest Japan are related to the migration of mantle-derived helium with aqueous fluids generated by dehydration of the subducting PHS and mixing with terrestrial waters [e.g., Matsumoto et al., 2003]. According to recent seismological observations of crustal microearthquakes and their focal mechanisms, the geotectonic environment beneath Shikoku Island is subject to a stress field without normal faulting and the extremely low crustal seismicity, which does not favor migration of aqueous fluids from the subcrustal lithosphere, resulting in a relatively small influence of MORB-type helium at surface. As a consequence, the regional variation in He/He ratios of hot springs on Shikoku Island are interpreted as a three-component mixture of mantle-derived helium associated with magmatism of Middle Miocene of age ( 3.4 RA), crustal, radiogenic helium and atmospheric helium.