5:15 PM - 6:30 PM
[MIS09-P15] Isotopic fractionation of ethane at the formation of sII gas hydrate composed of methane and ethane
Keywords:gas hydrate, stable isotope, ethane, Lake Baikal
In this study, synthetic mixed-gas (methane and ethane) hydrates were formed and checked isotopic fractionation between phases of hydrate and residual gas. We made a hydrate sample from methane and ethane mixed-gas (85% C1; 15% C2) in a pressure chamber (volume: 120 mL). Before the retrieval of gas hydrate sample, residual gas was also sampled. We measured isotopic compositions (13C and D) of methane and 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 indicated that the sample belonged to structure II. Delta D of hydrate-bound ethane was several permil smaller than that of residual ethane, similar to the behavior of methane delta D in the structure I. Although the mechanism of ethane fractionation at the formation process of the structure II is not fully understood, the results agree with the observation at the Kukuy K-2 mud volcano reported by Hachikubo et al. (2009).
Hachikubo et al. (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
Subramanian et al. (2000a) Evidence of structure II hydrate formation from methane + ethane mixtures. Chem Eng Sci 55: 1981-1999. doi:10.1016/S0009-2509(99)00389-9
Subramanian et al. (2000b) Structural transitions in methane + ethane gas hydrates - part I: upper transition point and applications.
Chem Eng Sci 55: 5763-5771. doi:10.1016/S0009-2509(00)00162-7