Uranium and manganese diffusion in apatite

Abstract

Diffusion of manganese and uranium has been characterized by Rutherford Backscattering Spectrometry (RBS) in fluorapatite. The following Arrhenius relation is obtained for Mn diffusion in natural Durango fluorapatite, for diffusion parallel to c: D Mn = 5.4 × 10 − 7 exp ( − 288 kJ mol − 1 / R ⁢ T ) m 2 s − 1 . Mn diffusion normal to c appears to be similar to diffusion parallel to c, and diffusivities in natural and synthetic (free of significant amounts of REE or other minor constituents) apatites are the same within experimental uncertainties. Experiments run in air, where Mn is in a more oxidized state, yield diffusivities similar to those for experiments run in sealed silica capsules, which the above Arrhenius relation represents. Uranium diffusion experiments over the temperature range 1000–1300 °C yield the following Arrhenius relation for diffusion parallel to c: D U = 1.3 × 10 − 6 exp ( − 394 kJ mol − 1 / R ⁢ T ) m 2 s − 1 . Diffusion coefficients for transport normal to c are quite similar to those parallel to c. U diffusion experiments run in air, where U is most likely hexavalent, yield diffusivities about an order of magnitude slower than the Ni–NiO buffered experiments. Diffusivities of Mn are comparable to those established previously for Sr and are slightly slower than Pb in apatite. The similar diffusion rates for Sr and Mn, despite their significant differences in cationic radii, suggest that cation size does not exert strong influence on diffusion of divalent cations in apatite, a finding consistent with that previously observed for the trivalent REE. In contrast, cation charge does seem to influence diffusivities strongly in apatite (although Mn appears perhaps something of an exception). U diffusion (buffered at NNO) is about 4 orders of magnitude slower than Mn diffusion, and about 2 orders of magnitude slower than REE diffusion. The differences between U diffusivities in experiments run with the NNO buffer, when U is more likely in the tetravalent state, and in air, where U is hexavalent, are also consistent with this observation.