Rare gas isotopic compositions in natural gases of Japan |
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| Authors: | Keisuke Nagao Nobuo Takaoka Osamu Matsubayashi |
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| Institution: | 1. Department of Physics, Faculty of Science, Osaka University, Toyonaka, Osaka 560Japan;2. Geological Survey of Japan, Tsukuba, Ibaragi 305Japan |
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| Abstract: | Isotopic and elemental compositions of rare gases in various types of gas samples collected in the Japanese Islands were investigated. Excess3He was found in most samples. Many samples showed a regionally uniform high3He/4He ratio of about 7 times the atmospheric ratio. The He concentrations varied from 0.6 to 1800 ppm, and they were low in CO2-rich gases and high in N2-rich gases. Ne isotopic deviations from the atmospheric Ne were detected in most volcanic gases. The deviations and the elemental abundance patterns in volcanic gases can be explained by a mixing between two components, one is mass fractionated rare gases and the other is isotopically atmospheric and is enriched in heavy rare gas elements. Ar was a mixture of mass fractionated Ar, atmospheric Ar and radiogenic Ar, and the contribution of radiogenic40Ar was small in all samples. Except for He, elemental abundance patterns were progressively enriched in the heavier rare gases relative to the atmosphere. Several samples were highly enriched in Kr and Xe relative to the abundance pattern of dissolution equilibrium of atmospheric rare gases in water. The component which is highly enriched in heavy rare gases may be released from sedimentary materials in the crust. |
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