Measurements of 3He/4He and 20Ne/4He ratios in 12 CH4-rich natural gas samples were made with a magnetic deflection mass spectrometer equipped with a metallic high-vacuum purification line. In order to check the resolving power, sensitivity and stability of the mass spectrometer, atmospheric air in Tokyo was measured repeatedly. The mean 3He/4He ratio in Tokyo air was (1.43 ± 0.03) × 10-6. CH4-rich gases with significantly high 3He/4He ratios were first observed in this study. The 3He/4He ratios for 5 natural gas samples collected from hot springs and mineral springs in inland basins were as high as (1.7–7.3) × 10-6, probably due to the large contribution of the upper mantle-derived He. In comparison, the 3He/4He ratios for 2 samples from water wells distributed in coastal areas facing the Pacific Ocean and 5 samples from gas fields in the southern Kanto district were as low as ∼10-7. Most of the He in these gases is inferred to be radiogenic and of crustal origin.
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Sano et al. (1982) studied this question.