14:15 〜 14:30
[BBC03-03] 14CH4および希ガス同位体を用いた白馬八方蛇紋岩温泉におけるメタンの起源の制約
キーワード：蛇紋岩、強アルカリ性温泉、メタン、放射性炭素、ヘリウム同位体
Ultramafic rock-water interaction systems attract more interest from the perspective of study for origin of life since the discovery of the hyperalkaline hydrothermal system in the Atlantic Ocean in 2001 (Kelley et al., 2001). These serpentinite-hosted systems are generally characterized by high concentrations of methane (CH4) and higher hydrocarbons (C2+) regardless of seafloor or on-land (e.g., Schrenk et al., 2013; Etiope and Sherwood-Lollar, 2013). Previous stable carbon and hydrogen isotopic studies have suggested that hydrocarbons could be formed abiotically via polymerization process (Proskurowski et al., 2008; Suda et al., 2017). However, the following is still poorly constrained; when and where hydrocarbons are produced. In this study, to constrain the origin of methane, we determined the radiocarbon (14C) content in CH4 and the helium isotope obtained from serpentinite-hosted hot spring in Hakuba Happo.
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. The hyperalkaline waters (pH>10), with temperatures of approximately 50°C, are pumped from two borehole wells (Happo #1 and Happo #3). N2, H2 and CH4 are main gas components (Homma and Tsukahara, 2008; Suda et al., 2014). Water chemistry and volatile component are controlled primarily by serpentinization reaction.
Sample collection was conducted in 2015-2018. Hot spring gas samples were collected by a displacement method in water from two borehole wells (Happo #1 and Happo #3). For radiocarbon (14C) analysis of methane, firstly, CH4 in gas samples was converted to CO2 by using the custom-built flow-through vacuum line system (similar to Pack et al., 2015) at JAMSTEC. Secondly, the purified CO2 gas was reduced to graphite by using the graphitization reactor with Fe powder and hydrogen gas, and then the 14C content was determined by using an accelerator mass spectrometry (AMS) at AORI, the University of Tokyo. Noble gas isotope abundances were measured with a noble gas mass spectrometer at GSJ, AIST. The concentrations of CO and CO2 in gas phase were determined by using a GC-methanizer-FID method, which was suitable for detection of trace amounts of CO and CO2 (Kaminski et al., 2003). The concentration of dissolved inorganic carbon (DIC) in spring water was measured by using GasBench/IRMS system at GSJ, AIST.
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 methane concentration (124-664 µmol/L; Suda et al., 2014). Methane is the most abundant carbon compound in Hakuba Happo hot spring. If methane 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. The 3He/4He ratios for Happo #1 and Happo #3 were 4.10 Ratm and 4.47 Ratm, respectively (where Ratm is the atmospheric 3He/4He ratio of 1.4×10-6). For Happo #1 site, relative contribution of air, mantle and crustal components are estimated to be 47%, 31% and 22%, respectively. For Happo #3 site, 69%, 24% and 8%, respectively. The result of AMS measurements on two CH4 samples from Hakuba Happo reveals that 14C contents are near the detectable limit, i.e., the 14C ages of CH4 is at least approximately 50,000 years before present. Radiocarbon evidence rules out a modern carbon compound (e.g., atmospheric CO2, organics in surface soil) as the carbon source of CH4 at Hakuba Happo hot spring system. The helium isotope and 14CH4 data could be consistent with deep-derived carbon source of the Hakuba Happo CH4. Therefore, it will be important to consider a relation between surface reaction system and deep mantle/crustal system.
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. The hyperalkaline waters (pH>10), with temperatures of approximately 50°C, are pumped from two borehole wells (Happo #1 and Happo #3). N2, H2 and CH4 are main gas components (Homma and Tsukahara, 2008; Suda et al., 2014). Water chemistry and volatile component are controlled primarily by serpentinization reaction.
Sample collection was conducted in 2015-2018. Hot spring gas samples were collected by a displacement method in water from two borehole wells (Happo #1 and Happo #3). For radiocarbon (14C) analysis of methane, firstly, CH4 in gas samples was converted to CO2 by using the custom-built flow-through vacuum line system (similar to Pack et al., 2015) at JAMSTEC. Secondly, the purified CO2 gas was reduced to graphite by using the graphitization reactor with Fe powder and hydrogen gas, and then the 14C content was determined by using an accelerator mass spectrometry (AMS) at AORI, the University of Tokyo. Noble gas isotope abundances were measured with a noble gas mass spectrometer at GSJ, AIST. The concentrations of CO and CO2 in gas phase were determined by using a GC-methanizer-FID method, which was suitable for detection of trace amounts of CO and CO2 (Kaminski et al., 2003). The concentration of dissolved inorganic carbon (DIC) in spring water was measured by using GasBench/IRMS system at GSJ, AIST.
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 methane concentration (124-664 µmol/L; Suda et al., 2014). Methane is the most abundant carbon compound in Hakuba Happo hot spring. If methane 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. The 3He/4He ratios for Happo #1 and Happo #3 were 4.10 Ratm and 4.47 Ratm, respectively (where Ratm is the atmospheric 3He/4He ratio of 1.4×10-6). For Happo #1 site, relative contribution of air, mantle and crustal components are estimated to be 47%, 31% and 22%, respectively. For Happo #3 site, 69%, 24% and 8%, respectively. The result of AMS measurements on two CH4 samples from Hakuba Happo reveals that 14C contents are near the detectable limit, i.e., the 14C ages of CH4 is at least approximately 50,000 years before present. Radiocarbon evidence rules out a modern carbon compound (e.g., atmospheric CO2, organics in surface soil) as the carbon source of CH4 at Hakuba Happo hot spring system. The helium isotope and 14CH4 data could be consistent with deep-derived carbon source of the Hakuba Happo CH4. Therefore, it will be important to consider a relation between surface reaction system and deep mantle/crustal system.