Carbon Isotope Fractionation during the Formation of CO Hydrate and Equilibrium Pressures of CO and CO Hydrates.

Knowledge of carbon isotope fractionation is needed in order to discuss the formation and dissociation of naturally occurring CO hydrates. We investigated carbon isotope fractionation during CO hydrate formation and measured the three-phase equilibria of CO-HO and CO-HO systems. From a crystal structure viewpoint, the difference in the Raman spectra of hydrate-bound CO and CO was revealed, although their unit cell size was similar. The δC of hydrate-bound CO was lower than that of the residual CO (1.0-1.5‰) in a formation temperature ranging between 226 K and 278 K. The results show that the small difference between equilibrium pressures of ~0.01 MPa in CO and CO hydrates causes carbon isotope fractionation of ~1‰. However, the difference between equilibrium pressures in the CO-HO and CO-HO systems was smaller than the standard uncertainties of measurement; more accurate pressure measurement is required for quantitative discussion.