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

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

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.