[MIS17-P01] Effect of crystallographic structure on hydrogen isotopic fractionation of ethane
Keywords:gas hydrate, ethane, hydrogen isotope, Lake Baikal
We put 0.7g of fine ice powder into a pressure cell (volume: 30mL), introduced methane and ethane mixed-gas, and formed a gas hydrate at 273.4K. Before the retrieval of gas hydrate sample, residual gas was sampled, and then gas hydrate was cooled at the temperature of liquid nitrogen and retrieved the hydrate-bound gas. We controlled the ethane composition of hydrate-bound gas from 2% to 98%, covering the area of methane-rich structure I, ethane-rich structure I, and their intermediate structure II. We measured compositions of methane and ethane by a gas chromatograph, and hydrogen isotopic composition (δD) of ethane using CF-IRMS. Crystallographic structure of gas hydrate was determined using a Raman spectrometer; the Raman spectra of C-C stretching mode of ethane in hydrate phase provides information of the crystallographic structure.
δD of hydrate-bound ethane was about 1-2‰ smaller than that of residual ethane in the case of the structure I, agreed with the previous report for pure methane and pure ethane hydrates by Hachikubo et al. (2007). On the contrary, these isotopic difference increased to 9-12‰ in the case of the structure II, those ethane compositions distributed around 15-34%. These results support the idea that the structure II hydrate at Lake Baikal formed by the dissociation of the structure I hydrate.
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
Hachikubo A, Khlystov O, Manakov A, Kida M, Krylov A, Sakagami H, Minami H, Takahashi N, Shoji H, Kalmychkov G, Poort J (2009) Model of formation of double structure gas hydrates in Lake Baikal based on isotopic data. Geophys Res Lett 36: L18504. doi:10.1029/2009GL039805