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

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 x indicating a release of mantle He through the basement. The highest value of 8.2 x 10 ~ 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 x 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.

Relationship between geological structure and helium isotopes in deep ground-water from the Osaka Basin: Application to deep groundwater hydrology

“…Sixteen out of 43 samples show even lower values than the atmospheric value (1.4 × 10 -6 ), which indicates crustal 4 He accumulation is dominant. The data of Okada et al (1994), which is also from the Osaka Basin, are similar to our data. In the following section, we will discuss the modification of the 3 He/ 4 He ratio in the near surface of the shallow crust and its implication to deep groundwater hydrology.…”

“…Some samples have relatively low 4 He/ 20 Ne ratios indicating a large con- Benson and Krause (1980) and Smith and Kennedy (1983). Open circles are data from Nagao et al (1981), Sano and Wakita (1985), Okada et al (1994) and Matsumoto et al (2003). tribution of the atmospheric component (Fig.…”

“…We employ the procedure of (after Geological Survey of Japan/AIST, 2005). The values with asterisks (*) are data from Nagao et al (1981), Sano and Wakita (1985), Okada et al (1994) and Matsumoto et al (2003). Craig et al (1978).…”

“…The 3 He/ 4 He ratios of hot spring gases in this region are exceptionally high, sometimes up to the MORB value, in spite of a fore-arc region with no Quaternary volcanism (e.g., Nagao et al, 1981;Sano and Wakita, 1985). Although many noble gas studies have been undertaken for hot spring gases in the vicinity of the Osaka Basin, they are focused on discussing the mechanism of mantle He release from fore-arc regions and their relation with the tectonics of the Kii Peninsula (Wakita et al, 1987;Okada et al, 1994;Matsumoto et al, 2003;Umeda et al, 2006). In this paper, we present the noble gas data dissolved in deep groundwaters from the Osaka Basin and discuss the distribution of the 3 He/ 4 He ratio in the shallower region with emphasis on the hydrological application of noble gases.…”

Arima hot spring waters as a deep-seated brine from subducting slab

“…Despite its presence in non-volcanic regions in the forearc domain, in which Quaternary volcanoes do not appear (Figure 1), the oxygen and hydrogen isotope compositions have affinities to magmatic/metamorphic fluids; thus, the presence of a deep brine distinct from near-surface water, including meteoric water and seawater, has been long argued for the origin of Arima hot springs (e.g., Matsubaya et al 1973;Tanaka et al 1984;Masuda et al 1985 and the references therein; overview presented in Sakai and Matsuhisa 1996). Although a precise definition of Arima-type brine has not been presented, nonvolcanic hot springs with high chlorine (approximately 40,000 ppm) and other solute contents, as well as the distinct oxygen and hydrogen isotopic ratios, occur in the Osaka to Kii areas, southwestern Japan, along large fault zones (Matsubaya 1981;Nishimura 2000), where high He of gases occur associated with spring waters (Nagao et al 1981;Sano and Wakita 1985;Okada et al 1994;Matsumoto et al 2003;Umeda et al 2006;Morikawa et al 2008).…”