Detailed Distribution of the Helium Isotope Ratios in Northeastern Japan

The geographical distribution of helium isotope ratios (3He/4He ratios) is characterized by high values of 4 to 8RA (where RA is the atmospheric 3He/4He ratio of 1.39×10-6) along the volcanic front and in the back-arc region at Tohoku district, northeastern Japan. In contrast forearc region shows low values less than 1RA. On the other hand, there is no clear contrast of the 3He/4He ratios except at the central region (e.g., Sano and Wakita, 1985). We perform the helium isotope ratio analysis in northeastern Japan, and around the source region of the Niigataken Chuetsu-oki Earthquake in 2007 (M6.8) where 3He/4He ratios data were reported. We have collected 41 samples of gases from hot springs, mineral springs, and deep wells, distributing mainly in the forearc region at Tohoku district. In addition, we also collected 19 samples of gases from hot springs, volcanoes and natural gas fields around the source region of the Niigataken Chuetsu-oki Earthquake in 2007. We measured 3He/4He ratios by noble gas mass spectrometers (Helix and VG5400) of Ocean Research Institute (ORI), the University of Tokyo. The 4He/20Ne were measured by a quadruple mass spectrometer to evaluate air contamination in the samples. δ13C (CO2) values were measured by using a mass spectrometer (DELTA plus XP) of ORI. Main features of our results for Tohoku region are as follows: 1) The 3He/4He ratios in the forearc region are less than 1RA. 2) The 3He/4He ratios vary along the volcanic front. In Miyagi prefecture [38-39N], the ratios range from 2 to 5 RA. On the other hand, the ratios are less than 1RA in and around the southern boundary of Iwate and Akita prefectures [39-39.5N]. The distribution of 3He/4He ratios in Niigata plans to be discussed by comparing with the well-studied seismotectonics and the structure of the crust and upper mantle.