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[MIS21-02] Hydrogen isotopic fractionation of ethane encaged in different crystallographic structure
Keywords:gas hydrate, ethane, hydrogen isotope ratio
0.7 g of fine ice powder was introduced in a pressure cell (volume: 30 mL). Argon and ethane mixed-gas were introduced in the cell and gas hydrate sample formed at 274.2 K. 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 initial gas from 3% to 25%. We measured compositions of argon and ethane by a gas chromatograph, and hydrogen isotopic composition (δD) of ethane by a CF-IRMS. Crystallographic structure of gas hydrate was determined using a Raman spectrometer, because the Raman spectra (around 1000 cm-1) of C-C stretching mode of ethane in hydrate phase provide information of the crystallographic structure.
Because ethane was concentrated into hydrate phase rather than argon, ethane compositions of hydrate-bound gas distributed from 30% to 71%. In the case of low concentration of ethane, crystallographic structure II forms due to the effect of argon (the structure II former). δD of hydrate-bound ethane was about 4-7‰ smaller than that of residual ethane in the case of structure II, where ethane composition was 30-36% in the hydrate phase. On the other hand, in the case of high concentration of ethane (around 70% in the hydrate phase) δD of hydrate-bound ethane was around 2‰ larger than that of residual ethane. In the transition zone between the structures I and II the isotopic difference between hydrate and residual gas phases seemed to be an intermediate value (1-4‰). From these results we can conclude that light ethane (C2H6) prefers to be encaged into the structure II clathrate hydrate, compared to heavy ethane (C2H5D).
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