[BBC03-P01] Radiocarbon and noble gas isotope study to constrain origin of methane in Hakuba Happo serpentinite-hosted hot spring
キーワード：蛇紋岩、強アルカリ性温泉、メタン、放射性炭素、希ガス
Serpentinite-hosted systems attract more interest from the perspective of studying the origin of life. High concentrations of methane (CH4) and higher hydrocarbons (C2+) are commonly observed in serpentinite-hosted systems (e.g., Schrenk et al., 2013; Etiope and Sherwood-Lollar, 2013). Previous studies have suggested that stable hydrogen and carbon isotopic compositions of hydrocarbons are compatible with abiotic formation via polymerization process (Proskurowski et al., 2008; Suda et al., 2017). However, little is known about when and where the hydrocarbon synthesis occurs. Our purpose is to constrain both the time and place for methane production by analyzing radiocarbon isotope (14C) and noble gas isotopes (3He, 4He, 20Ne) in the serpentinite-hosted hot spring in Hakuba Happo, Japan.
Hakuba Happo hot spring lies on a serpentinized ultramafic rock body in the Shiroumadake area, which belongs to the Hida Marginal Tectonic Belt in central Japan. Hyperalkaline hot spring water (ca. 50°C, pH = 10.6) rich in H2 and CH4 gas is pumped from two borehole wells; Happo #1 and Happo #3 (Homma and Tsukahara, 2008; Suda et al., 2014). Sample collection was conducted between 2015 and 2018. In the same manner as Suda et al. (2017), gas and water samples were obtained from two borehole wells. Pipe-scale carbonate samples of the Happo #1 borehole well were collected at the timing of when the old water-pumping pipes were replaced with new pipes in 2016. For the 14C measurements of CH4, a sample preparation step of CH4 combustion to CO2 was conducted at JAMSTEC by using the custom-built flow-through vacuum line system, which was based on Pack et al. (2015). Radiocarbon contents of purified CO2 samples were determined at the University of Tokyo by using the graphitization system (Yokoyama et al., 2007) and the Accelerator Mass Spectrometry (AMS). The sample preparation and 14C-AMS measurement for carbonate samples were conducted at the University of Tokyo by using an automated sample preparation system, which is composed of an elemental analyzer and a glass vacuum line based on Kato et al. (2014). Noble gas isotope abundances were measured with a noble gas mass spectrometer at GSJ, AIST. The concentration of dissolved inorganic carbon (DIC) in spring water was measured by using GasBench/IRMS system at GSJ, AIST.
We found that 14C contents in CH4 were close to the detectable limit, whereas pipe-scale carbonates which precipitated from hot spring water contained the detectable 14C. Radiocarbon evidence rules out a modern carbon (e.g., atmospheric CO2, organics in surface soil) as the carbon source for CH4, suggesting that Hakuba Happo CH4 is derived from old carbon source in deep crust or mantle. In Hakuba Happo hot spring, CO and CO2 were below detection limit (<0.0005 vol.%). DIC concentration was <28 µmol/L (upper values because of suspicion of air contamination during sampling), which was lower than dissolved CH4 concentration (124-664 µmol/L; Suda et al., 2014). If CH4 production occurs under on-site condition, the inorganic carbon compounds (CO, CO2 and DIC) are not likely to be a carbon source for CH4. The high helium isotope ratio (3He/4He) was observed in both well sites, indicating that a major portion of He is mantle derived. In Shiroumadake area, the 3He/4He ratios tend to decrease with distance from Norikura Volcanic Chain, suggesting volcanic gases are dissolved in groundwater and then brought to surrounding hot spring. The CH4/3He ratios of Hakuba Happo samples were one order of magnitude higher than those of other hot springs which are not directly related to serpentinite rocks. Exceptionally high CH4/3He ratio can not be explained by simple influx of mantle-derived CH4. Another CH4 source different from mantle-CH4 may be required. Based on the 14C and noble gas results, we suggest that a major portion of CH4 at Hakuba Happo is produced in deep crust rather than shallow and modern groundwater.
Hakuba Happo hot spring lies on a serpentinized ultramafic rock body in the Shiroumadake area, which belongs to the Hida Marginal Tectonic Belt in central Japan. Hyperalkaline hot spring water (ca. 50°C, pH = 10.6) rich in H2 and CH4 gas is pumped from two borehole wells; Happo #1 and Happo #3 (Homma and Tsukahara, 2008; Suda et al., 2014). Sample collection was conducted between 2015 and 2018. In the same manner as Suda et al. (2017), gas and water samples were obtained from two borehole wells. Pipe-scale carbonate samples of the Happo #1 borehole well were collected at the timing of when the old water-pumping pipes were replaced with new pipes in 2016. For the 14C measurements of CH4, a sample preparation step of CH4 combustion to CO2 was conducted at JAMSTEC by using the custom-built flow-through vacuum line system, which was based on Pack et al. (2015). Radiocarbon contents of purified CO2 samples were determined at the University of Tokyo by using the graphitization system (Yokoyama et al., 2007) and the Accelerator Mass Spectrometry (AMS). The sample preparation and 14C-AMS measurement for carbonate samples were conducted at the University of Tokyo by using an automated sample preparation system, which is composed of an elemental analyzer and a glass vacuum line based on Kato et al. (2014). Noble gas isotope abundances were measured with a noble gas mass spectrometer at GSJ, AIST. The concentration of dissolved inorganic carbon (DIC) in spring water was measured by using GasBench/IRMS system at GSJ, AIST.
We found that 14C contents in CH4 were close to the detectable limit, whereas pipe-scale carbonates which precipitated from hot spring water contained the detectable 14C. Radiocarbon evidence rules out a modern carbon (e.g., atmospheric CO2, organics in surface soil) as the carbon source for CH4, suggesting that Hakuba Happo CH4 is derived from old carbon source in deep crust or mantle. In Hakuba Happo hot spring, CO and CO2 were below detection limit (<0.0005 vol.%). DIC concentration was <28 µmol/L (upper values because of suspicion of air contamination during sampling), which was lower than dissolved CH4 concentration (124-664 µmol/L; Suda et al., 2014). If CH4 production occurs under on-site condition, the inorganic carbon compounds (CO, CO2 and DIC) are not likely to be a carbon source for CH4. The high helium isotope ratio (3He/4He) was observed in both well sites, indicating that a major portion of He is mantle derived. In Shiroumadake area, the 3He/4He ratios tend to decrease with distance from Norikura Volcanic Chain, suggesting volcanic gases are dissolved in groundwater and then brought to surrounding hot spring. The CH4/3He ratios of Hakuba Happo samples were one order of magnitude higher than those of other hot springs which are not directly related to serpentinite rocks. Exceptionally high CH4/3He ratio can not be explained by simple influx of mantle-derived CH4. Another CH4 source different from mantle-CH4 may be required. Based on the 14C and noble gas results, we suggest that a major portion of CH4 at Hakuba Happo is produced in deep crust rather than shallow and modern groundwater.