Geographical distribution of ³He/⁴He ratios in Japan: Implications for arc tectonics and incipient magmatism

The 3 He/ 4 He and 4 He/ 20 Ne ratios of 115 natural gas samples, including various types of gases from volcanoes, hot springs, mineral springs, water wells, petroleum fields, and natural gas fields, were measured using a mass spectrometer. The observed 3 He/ 4 He and 4 He/ 20 Ne ratios range from 7.47×10 −8 to 9.65×10 −6 and from 0.26 to 1100, respectively. The 3 He/ 4 He ratios reflect well the geotectonic structure of the Japanese Islands. In northeastern (NE) Japan there is a clear geographical difference in the 3 He/ 4 He ratios between the frontal arc (forearc) and volcanic arc (back arc) regions. Lower ratios were found in the trench side region and higher ratios in the back arc side. This result suggests that the mantle‐derived helium in the volcanic arc region is associated with the diapiric uprise of a magma. Lower 3 He/ 4 He ratios in the frontal arc region may be due to radiogenic He produced by radioactive decay of U and Th in the crustal and sedimentary rocks. In southwestern (SW) Japan there is no clear geographical contrast in the 3 He/ 4 He ratios. Some samples in the frontal arc region show quite high 3 He/ 4 He ratios. The tendency of the 3 He/ 4 He ratios agrees with the distribution of terrestrial heat flow data and reflects geotectonic structures different from NE Japan. The high 3 He/ 4 He ratios observed in the frontal arc region in SW Japan may be indicative of renewed or incipient magmatism due to a descending young and warm slab.