ISOTOPIC ANOMALIES OF RARE-GASES IN THE NIGORIKAWA GEOTHERMAL AREA, HOKKAIDO, JAPAN

Isotopic and elemental compositions of rare gases were determined for gas samples enriched in CO2 collected in the Nigorikawa geothermal area, Hokkaido, Japan. A high isotopic ratio of 3He/4He = (9.95 ± 0.25) × 10-6 was obtained for a sample collected from bubbles naturally seeping out. This sample has a rare gas elemental abundance pattern attributable to low-temperature solubility of atmospheric rare gases into water. Soil gas samples yield evidence for mass fractionation of atmospheric rare gases, in addition to the originally high 3He/4He ratio. The highest 20Ne/22Ne ratio found, 10.33 ± 0.10, was accompanied by the highest 36Ar/38Ar ratio, 5.60 ± 0.08, and a systematic enrichment of the light isotopes of Kr and Xe. The enrichment of the light isotopes in each gas in the soil gas samples was also accompanied by a systematic elemental enrichment of the light gases. Migration of rare gas in the crust is responsible for this isotopic and elemental fractionation. Primordial He from the mantle is extensively present in CO2-rich gases in this area. © 1979.