SECULAR VARIATIONS OF CARBON AND HELIUM-ISOTOPES AT IZU-OSHIMA VOLCANO, JAPAN

Secular variations in C-13/C-12 ratios and chemical compositions of ga s samples from October 1986 to July 1992 are reported from a 92-95 deg rees C steam well located about 3 km north of Mt. Mihara, an active vo lcano on Izu-Oshima Island, Japan. The delta(13)C value steeply increa sed from -2.97 parts per thousand (relative to PDB carbonate) in Decem ber 1986 to -1.15 parts per thousand in March 1988 and then gradually decreased to -1.75 parts per thousand in July 1992. Over the same peri od, the CO2 content changed similarly with time, even though the exper imental error is relatively large. These variations are consistent wit h helium isotope changes. Initially rapid and then slow enhancements o f He-3/He-4 ratio, delta(13)C value and CO2 content are invoked by vio lent eruptions of Izu-Oshima volcano from 15 November to 18 December 1 986. After the eruptive activity, depletion of magmatic gas emission a nd subsequent mixing with crustal fluids in the hydrothermal system ma y produce the gradual decreases of He-3/He-4 ratio, delta(13)C value a nd CO2 content. Taking into account the rates of these decreases, we s uggest that helium and carbon isotope ratios will return to the situat ion of before the magmatic eruption within 15 years.