[MIS21-P07] CO2クラスレートハイドレート生成時の炭素安定同位体分別
キーワード：ガスハイドレート、安定同位体、CO2、同位体分別
Hydrogen Isotope fractionation in methane during the formation of clathrate hydrate was reported by Hachikubo et al. (2007) that δD of hydrate-bound methane is 4.8±0.4‰ lower than that of residual methane at 274.2K. Although natural methane hydrates distribute widely in the world, natural CO2 hydrate was only reported at the Okinawa Trough (Sakai et al., 1990). It is also possible that CO2 hydrate exists in Mars. Isotopic fractionation of guest gas may provide useful information to understand formation processes of gas hydrate. Luzi et al. (2011) revealed that CO2 δ13C in hydrate-bound gas is 0.9‰ lower than that of residual gas, suggesting that light CO2 molecules prefer to be encaged into clathrate cages. In this study, we checked the temperature effect on the isotopic difference between residual and hydrate-bound CO2.
Samples of CO2 hydrate (weight: 0.7 g) were experimentally prepared in a pressure cell (volume: 30 mL), and the temperature was controlled by a liquid bath (258-274 K) and cold rooms (226-254 K). Isotopic compositions of both residual and hydrate-bound CO2 were measured by an isotope ratio mass spectrometer (IRMS). Samples were formed in the temperature range from 226 K to 274 K. The carbon isotopic differences between hydrate-bound and residual CO2 distributed between 1.2‰ and 1.5‰, agreed fairly well with the previous report (Luzi et al., 2011). The difference seemed to be large (1.5-2.0 ‰) at 226 K, indicating that the equilibrium pressure of 13CO2 hydrate is slightly larger than that of 12CO2 hydrate, and the difference between them becomes large in lower temperature. We will show these equilibrium data for 13CO2 and 12CO2 hydrates and compare with the results in stable isotope.
Reference
Hachikubo A, Kosaka T, Kida M, Krylov A, Sakagami H, Minami H, Takahashi N, Shoji H (2007) Isotopic fractionation of methane and ethane hydrates between gas and hydrate phases. Geophys Res Lett 34: L21502. doi:10.1029/2007GL030557
Sakai H, Gamo T, Kim ES, Tsutsumi M, Tanaka T, Ishibashi J, Wakita H, Yamano M, Oomori T (1990) Venting of carbon dioxide-rich fluid and hydrate formation in mid-okinawa trough backarc basin. Science: 248, 4959, 1093-1096.
Luzi M, Schicks JM, Erzinger J (2011) Carbon isotopic fractionation of synthetic methane and carbon dioxide hydrates. Proc. 7th International Conference on Gas Hydrates (ICGH2011).
Samples of CO2 hydrate (weight: 0.7 g) were experimentally prepared in a pressure cell (volume: 30 mL), and the temperature was controlled by a liquid bath (258-274 K) and cold rooms (226-254 K). Isotopic compositions of both residual and hydrate-bound CO2 were measured by an isotope ratio mass spectrometer (IRMS). Samples were formed in the temperature range from 226 K to 274 K. The carbon isotopic differences between hydrate-bound and residual CO2 distributed between 1.2‰ and 1.5‰, agreed fairly well with the previous report (Luzi et al., 2011). The difference seemed to be large (1.5-2.0 ‰) at 226 K, indicating that the equilibrium pressure of 13CO2 hydrate is slightly larger than that of 12CO2 hydrate, and the difference between them becomes large in lower temperature. We will show these equilibrium data for 13CO2 and 12CO2 hydrates and compare with the results in stable isotope.
Reference
Hachikubo A, Kosaka T, Kida M, Krylov A, Sakagami H, Minami H, Takahashi N, Shoji H (2007) Isotopic fractionation of methane and ethane hydrates between gas and hydrate phases. Geophys Res Lett 34: L21502. doi:10.1029/2007GL030557
Sakai H, Gamo T, Kim ES, Tsutsumi M, Tanaka T, Ishibashi J, Wakita H, Yamano M, Oomori T (1990) Venting of carbon dioxide-rich fluid and hydrate formation in mid-okinawa trough backarc basin. Science: 248, 4959, 1093-1096.
Luzi M, Schicks JM, Erzinger J (2011) Carbon isotopic fractionation of synthetic methane and carbon dioxide hydrates. Proc. 7th International Conference on Gas Hydrates (ICGH2011).