Abstract
New helium isotope data from hot springs on eastern Kyushu Island were obtained in order to elucidate the geographic distribution of 3He/4He ratios in the fore‐arc regions of southwest Japan. Systematic, regional variations in 3He/4He ratios are clearly distinguished across the fore arc from Kyushu Island (0.093–0.79 RA, where RA denotes the atmospheric 3He/4He ratio of 1.4 × 10−6), to Shikoku Island (0.17–3.6 RA) and to the Kii Peninsula (0.24–7.4 RA). The distribution of mantle helium relative to total helium reveals a close association with the geophysical evidence of a serpentinized fore‐arc mantle wedge including the occurrence of deep, nonvolcanic, low‐frequency tremors (LFTs), believed to be caused by upwardly mobile aqueous fluids derived from slab dehydration. Flux of mantle helium carried by aqueous fluids via a non serpentinized fore‐arc mantle wedge is very low on Kyushu Island where LFTs have not been detected and 3He/4He ratios are lower than the atmospheric ratio. On the other hand, hot springs above the regions where LFTs occur in the Kii Peninsula and northern Shikoku Island have high 3He/4He ratios, indicating a significant contribution of mantle helium carried by aqueous fluids via a serpentinized fore‐arc mantle wedge. The helium isotopes in hot and natural spring gases may be a useful geochemical indicator of a serpentinized fore‐arc mantle wedge, especially in areas with sparse seismic station coverage.
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