2:45 PM - 3:00 PM
[MIS17-05] Isotopic fractionation of methane during the formation of hydrate crystals near the ice point
Keywords:gas hydrate, stable isotope, methane, isotopic fractionation
The methane hydrate samples were experimentally prepared in a pressure cell, and the temperature was controlled by a liquid bath. Isotopic compositions of both residual and hydrate-bound methane were measured by an IRMS. Samples were formed in the temperature range from 274.2K to 283.2K, corresponded to the temperature of natural near-surface gas hydrates in the world. The hydrogen isotopic difference between hydrate-bound and residual methane were 3.5-5.5‰, agreed with the previous report above the ice point. Kikuchi et al. (2018) reported that the equilibrium pressure of CH3D hydrate is about 0.07MPa larger than that of CH4 hydrate in the temperature range from 273.2K to 277.2K. Therefore, we can explain that the difference in equilibrium pressure between CH3D and CH4 hydrates causes such isotopic difference in δD. Hachikubo et al. (2007) reported that the difference in δD between hydrate-bound and residual methane is around 9‰ below the ice point, suggesting that the difference in equilibrium pressure below 273.2K becomes larger than above 273.2K.
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
Kikuchi Y, Ozeki T, Hachikubo A, Takeya S (2018) Equilibrium pressure of clathrate hydrates composed of isotopologue methane. JpGU Meeting 2018, ibid