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The 3He/4He and 4He/20Ne 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 3He/4He and 4He/20Ne ratios range from 7.47×10−8 to 9.65×10 −6 and from 0.26 to 1100, respectively. The 3He/4He ratios reflect well the geotectonic structure of the Japanese Islands. In northeastern (NE) Japan there is a clear geographical difference in the 3He/4He 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 3He/4He 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 3He/4He ratios. Some samples in the frontal arc region show quite high 3He/4He ratios. The tendency of the 3He/4He ratios agrees with the distribution of terrestrial heat flow data and reflects geotectonic structures different from NE Japan. The high 3He/4He 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.

The potential impact of increases in atmospheric CO2 is a topic of considerable controversy. Even though volcanic emission of CO2 may be very small as compared to anthropogenic emissions, evaluation of natural degassing of CO2 is important for any model of the geochemical C cycle and evolution of the Earth's atmosphere. We report here the mantle C flux in subduction zones based on He and C isotopes and CO2/³ He ratios of high‐temperature volcanic gases and medium‐ and low‐temperature fumaroles in circum‐Pacific volcanic regions. The calculated volcanic C flux of 3.1 × 1012 mol/a from subduction zones is larger than the flux of 1.5 × 1012 mol/a from mid‐ocean ridges, while contributions from the mantle in subduction zone is only 0.30 × 1012 mol/a, equivalent to about 20% of the C flux in mid‐ocean ridges. Since the estimated mantle C flux in hot spot regions is insignificant, 0.029 × 1012 mol/a, we propose that the global mantle C flux is 1.8 × 1012 mol/a in total. The flux, if accumulated over 4.5 billion year of geological time, amounts to 8.3 × 1021 mol which agrees well with 9 × 1021 mol of the present inventory of C at the Earth's surface. This may support a continuous degassing model of C or the idea that subducted C is recycled into the lower mantle.

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Radon concentration in ground water increased for several months before the 1995 southern Hyogo Prefecture (Kobe) earthquake on 17 January 1995. From late October 1994, the beginning of the observation, to the end of December 1994, radon concentration increased about fourfold. On 8 January, 9 days before the earthquake, the radon concentration reached a peak of more than 10 times that at the beginning of the observation, before starting to decrease. These radon changes are likely to be precursory phenomena of the disastrous earthquake.