Diffusivity Fractionations of H2(16)O/H2(17)O and H2(16)O/H2(18)O in Air and Their Implications for Isotope Hydrology

We have determined the isotope effects of (17)O and (18)O substitution of (16)O in H(2)O on molecular diffusivities of water vapor in air by the use of evaporation experiments. The derived diffusion fractionation coefficients (17)alpha(diff) and (18)alpha(diff) are 1.0146 +/- 0.0002 and 1.0283 +/- 0.0003, respectively. We also determined, for the first time, the ratio ln((17)alpha(diff))/ln((18)alpha(diff)) as 0.5185 +/- 0.0002. This ratio, which is in excellent agreement with the theoretical value of 0.5184, is significantly smaller than the ratio in vapor-liquid equilibrium (0.529). We show how this new experimental information gives rise to (17)O excess in meteoric water, and how it can be applied in isotope hydrology.