Thermal-gradient-induced non-mass-dependent isotope fractionation.

Isotope fractionation resulting from gas diffusion along a thermal gradient has always been considered entirely mass-dependent. A previous report, however, showed that non-mass-dependent (17)O anomalies can be generated simply by subjecting O(2) gas in an enclosure to a thermal gradient. To explore the underlying mechanism for the anomalies, we tested the effect of gas pressure, duration of experiment, and geometry of the apparatus on the (17)O anomalies for O(2) as well as on the (33)S or (36)S anomalies for SF(6) gas. The results are consistent with our proposal that a previously ignored nuclear spin effect on gas diffusion coefficient may be largely responsible for generating the observed anomalies. This discovery provides clues to some of the puzzling non-mass-dependent isotope signatures encountered in experiments and in nature, including the triple oxygen or quadruple sulfur isotope heterogeneity in the solar system.