Noble gas isotopic composition of deep underground water in Osaka plain, central Japan: Evidence for mantle He and model for new volcanism

Abstract

Abstract Elemental and isotopic compositions of noble gases extracted from the bore hole water in Osaka plain, central Japan were examined. The water samples were collected from four shallow bore holes (180‐450 m) and seven deep bore holes (600‐1370 m) which have been used for an urban resort hot spring zone. The water temperatures of the deep bore holes were 22‐50°C and that of the shallow bore holes, 13‐23°C. The elemental abundance patterns show the progressive enrichment of the heavier noble gases compared with the atmospheric noble gas composition except for He, which is heavily enriched in deep bore hole water samples. 3He/4He ratios from the bore holes reaching the Ryoke granitic basement were higher than the atmospheric value (1.4 × 10−6), indicating a release of mantle He through the basement. The highest value of 8.2 × 10−6 is in the range of arc volcanism. On the other hand, the bore holes in sedimentary rocks overlying the basement release He enriched in radiogenic 4He, resulted in a low 3He/4He ratio of 0.5 × 10−6. 4He/20Ne and 40Ar/36Ar ratios indicate that the air contamination is generally larger in shallow bore holes than in deep ones from each site. The helium enriched in mantle He is compatible with the previous work which suggested up‐rising magma in ‘Kinki Spot’, the area of Osaka and western Wakayama, in spite of no volcanic activity in the area. A model to explain an initiation of magma generation beneath this area is presented.