ABSTRACT

To investigate a fine structure of the ³He/⁴He ratio distribution in and around a single volcano, fumarolic gases, geothermal gases and waters were collected at 13 sites within the caldera of the Hakone volcano, Japan. The ³He/⁴He and ⁴He/²⁰Ne ratios vary from 6.7 × 10^-6 to 9.6 × 10^-6, and 0.85 to 26, respectively. The ³He/⁴He ratios, corrected for air contamination, of samples within the caldera are uniform and as high as those of magmatic fluids in the Japanese Islands. The ³He/⁴He ratios of two geothermal water samples outside the caldera are significantly lower than those of inside. The observed ³He/⁴He ratio distribution is interpreted as an indication of the presence of source magma with a high ³He/⁴He ratio beneath the volcanic region.