Spatial distribution and temporal variation of ³He/⁴He in hot spring gas released from Unzen volcanic area, Japan

Following the first phreatic explosion on 17 November 1990, hot spring gases were collected periodically over the next 10 years for ³He/⁴He isotopic ratio and chemical analyses from three hot springs (Obanma, Unzen and Shimabara) located around Unzen volcano, Japan. The ³He/⁴He ratios, although showing some scatter at each site, show an increase from west to east (Obama < Unzen < Shimabara). This trend corresponds to a decrease in crustal gas contribution as shown by the CH₄ content. The ³He/⁴He ratios at Shimabara and Unzen after 1990 were significantly higher than the single values for the 1983 sample (Marty et al., 1989), possibly due to an additional supply of magmatic helium related to the recent post-1990 volcanic activity. The ³He/⁴He ratio at Shimabara hot spring increased slightly after November 1990, reaching a maximum value in July 1992, and decreasing later. This suggests that magmatic helium with relatively high ³He/⁴He ratios took about one year to travel 5 km from beneath Fugendake volcanic cone to Shimabara hot spring site, because the magma effusion rate (and magma degassing rate) reached a maximum in June 1991.