Abstract
Natural gas samples, including various types of gases from hot springs, mineral springs and water wells, were collected at nine sampling sites in the Tohoku district, northeastern Japan. The 3 He 4 He and 20 Ne 4 He ratios of these samples were measured using a mass spectrometer. The ranges of observed 3 He 4 He and 20 Ne 4 He ratios of gas samples are from 3.56·10 −7 to 9.01·10 −6 and from 2.9·10 −3 to 5.2·10 −1, respectively. The temporal variations in 3 He 4 He ratios observed for two sampling sites show little fluctuation over a period of 28 months. Samples collected in the forearc region have radiogenic He ratios, being about one-third of the atmospheric value, whereas in the volcanic arc region, the ratios are high with values 2.5–6.4 times the atmosphere. The 3 He 4 He ratios in the Tohoku district show good correlation to the subduction-related tectonic structure of the island-arc system. These observed variations in 3 He 4 He ratios may reflect the state of material flow associated with magma generation.
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