Isotope fractionation by diffusion in molten oxides

Abstract

Laboratory diffusion experiments specifically designed to resolve small differences in the relative mobility of isotopes were used to measure the mobility of 76Ge vs. 70Ge in molten GeO 2 (1400°C, 0.5 GPa) and 48Ca vs. 40Ca in molten CaO–SiO 2–Al 2O 3 (1500°C, 1.0 GPa). When the mass ( m) dependence of the diffusion coefficient ( D) of isotopes 1,2 is written in the form ( D 1/ D 2) = ( m 2/ m 1) β, we find β ≤ 0.025 for Ge isotopes and β = 0.05 to 0.1 for Ca isotopes. The degree of isotope fractionation by diffusion between melts of the same initial isotopic composition but different elemental abundance depends not only on β but also on the ratio of the elemental concentrations in the two melts. Given our measured value of β for Ca isotopes, we predict that diffusion between melts with a factor of 5 to 10 difference in Ca content will produce variations in 44Ca/ 40Ca ratio of 2 to 5‰. To the extent that Ge is a good analogue for Si, we would not expect significant fractionation of Si isotopes. But even if the value of β for Si isotopes were as large as that for Ca, we would still not expect measurable diffusive isotopic fractionations of Si because the contrast in Si content between most natural melts is less than a factor of 2.