D H FRACTIONATION IN THE SYSTEM METHANE-HYDROGEN-WATER/

We report measurements of the equilibrium D/H fractionation factor (al pha) between methane and hydrogen in the temperature range 200-500 deg rees C, Isotopic equilibrium was achieved by recycling the gases over a Ni-Thoria catalyst, using an in-line sampling volume for sequesterin g aliquots of the gas mixture without contributions from adsorbed gase s on the catalyst, Equilibrium values of cr were approached from both sides by use of (1) enriched CH3D in the initial mixture and (2) pre-e quilibration of the gases at temperatures below that of the final equi librium mixture. The measured values of alpha are linear vs. 1/T-2 and fit the equation alpha = 0.8994 + 183,540/T-2, with a standard deviat ion sigma = +/-12.5%. The D/H fractionation factors for water vapor-hy drogen exchange measured by Suess (1949) and by Cerrai et al. (1954) a re also linear in alpha vs. 1/T-2 over the temperature range of the da ta: comparison with published D/H ratios in high-temperature (1127 deg rees C) volcanic gases at Surtsey volcano shows that the Suess (1949) data are much closer to the observed ratios in H-2 and H2O. The Suess (1949) measurements (80-200 degrees C) are also much closer to the the oretical values calculated by Bardo and Wolfsberg (1976), which fit th e observed Surtsey fractionations slightly better than the extrapolate d Suess (1949) results. We conclude that (1) the Suess (1949) measurem ents are the better set of experimental data, (2) the Surtsey gases ar e close to isotopic equilibrium at the vent temperatures, and (3) the Bardo and Wolfsberg (1976) theoretical equation gives the best represe ntation of the H2O-H-2 fractionation factors. This equation is combine d with the Horita and Wesolowski (1994) equation for H2O liquid-vapor fractionation factors and can be used with the CH4-H-2 alpha values to determine whether concordant temperatures are observed in the system CH4H2-H2O. Application to the D/H ratios in the East Pacific Rise hydr othermal vents measured by Welhan and Craig (1979) shows that concorda nt temperatures are obtained for both CH4-H-2 and H2O-H-2 data, and ar e close to the approximate vent temperatures (similar to 350 degrees C ). We note that fractionation equations in which alpha, rather than 1n alpha, is fit to powers of T are much more useful for geochemical stu dies because the precision estimate is uniform over the entire tempera ture range of the data..