Isotope fractionation by diffusion in molten oxides

Laboratory diffusion experiments specifically designed to resolve small differences in the relative mobility of isotopes were used to measure the mobility of Ge-76 vs. Ge-70 in molten GeO2 (1400 degreesC, 0.5 GPa) and Ca-48 vs. Ca-40 in molten CaO-SiO2-Al2O3 (1500 degrees 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))(beta), we find beta less than or equal to 0.025 for Ge isotopes and beta = 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 beta but also on the ratio of the elemental concentrations in the two melts. Given our measured value of beta for Ca isotopes, we predict that diffusion between melts with a factor of 5 to 10 difference in Cacontent will produce variations in Ca-44/Ca-40 ratio of 2to 5 parts per thousand. 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 beta 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. (C) 1999 El sevier Science Ltd.