Examining three oxygen isotope mass-dependent fractionation in Uraninite

English Abstract:

On a plot of δ¹⁷O vs. δ¹⁸O values for terrestrial samples define a line with slope of ~0.52 because equilibrium isotope theory predicts that variations in δ¹⁸O caused by nearly any process on Earth (physical, chemical or biological) are approximately twice as large as variations in δ¹⁷O. This is known as mass-dependent fractionation. The ratios of logarithms of partition function ratios for oxygen compounds thus fall in a narrow range from about 0.520 to 0.530 over a range of temperatures. Kinetic processes can be examined in the same way as equilibrium processes to determine the proportionality between ¹⁸O/¹⁶O variations and ¹⁷O/¹⁶O variations for diffusion, in which the relative rates of isotopic species are inversely proportional to the square-root of the mass of the diffusing species. For example, the mass-dependent fractionation for oxygen atoms will define a line with a slope of 0.5232 whereas for SO₂ molecules the slope is 0.5087. It is evident that the fractionation ratio depends on the molecular weight of the diffusing species. Since the fractionation ratios both for equilibrium and for kinetic processes depend on the particular molecules, it is expected that terrestrial minerals, which form by various processes and reactions will yield a slope on a graph of δ¹⁷O vs. δ¹⁸O, which is an average of the many individual slopes for individual processes. A value of 0.520 is generally chosen for mass-dependent fractionation in terrestrial samples. Here we suggest that the slope of the line that defines mass-dependent fractionation in uraninite (UO₂) significantly deviates from a slope of 0.52 because U-O molecules are significantly heavier than Si-O or Al-O molecules. We will compare the calculated value for the slope for UO₂ (0.5019) to the measured slope defined by uraninite that was overprinted by meteoric water and a synthetic uraninite.