Keywords:gas hydrate, stable isotope, carbon dioxide, isotopic fractionation
Natural CO2 hydrate was discovered at the Okinawa Trough, and it is expected that CO2 hydrate exists in the polar ice caps of the Mars. Isotopic fractionation of CO2 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. We reported at the last JpGU conference (Kimura et al., 2019) that the isotopic difference between GH and residual gases distribute between between 1.2‰ and 1.5‰, agreed fairly well with the previous report. Ozeki et al. (2018) reported that the equilibrium pressure of CH3D is larger than that of CH4, causes isotopic fractionation in D between GH and residual gases as reported by Hachikubo et al. (2007). In this study, we measured the equilibrium pressures of 12CO2 and 13CO2 hydrates and checked the difference between them. CO2 hydrate samples were synthesized in small pressure cells (volume: 5 mL). Fine ice powder was put in a pressure cell and introduced guest 13CO2 gas. Hydrate crystals were formed by melting the ice powder at the temperature of the quadruple point. We also prepared normal CO2 (mixture 98.9% 12CO2 and 1.1% 13CO2) hydrate as a reference, using the same preparation method. These pressure cells were placed in a temperature-controlled liquid bath, and measured their equilibrium pressures from 270 K to 278 K. The equilibrium pressure of 13CO2 hydrate was about 0.01 MPa larger than that of normal CO2 (mainly 12CO2) hydrate, and it agreed with the previous results (Luzi et al., 2011; Ozeki et al., 2018).
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
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).
Ozeki T, Kikuchi Y, Takeya S, Hachikubo A (2018) Phase equilibrium of isotopologue methane hydrates enclathrated CH3D and CD4. J Chem Eng Data 63(6): 2266-2270, doi: 0.1021/acs.jced.8b00203