Secular variations in 3He 4He and 4He 20Ne ratios are reported for October 1986 to April 1991 in gas samples from a 90°C steam well located about 3 km north of Mt. Mihara, an active volcano in Izu-Oshima Island, Japan. The 3He 4He and 4He 20Ne ratios increased anomalously from 4.18 Ratm to 4.59 Ratm and from 0.93 to 1.08, respectively, about 3 months prior to the explosive eruption of the volcano on 4 October 1990. The 3He 4He enhancement may be related to the concurrent decrease in height of volcanic smoke at the central cone. The gas pressure in the conduit may have significantly increased before the eruption owing to solidified cap lava or ash in the cone, which may have strengthened the transfer of magmatic helium from the source to the observation well through fissures or permeable channels in the volcanic edifice.
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