Th diffusion in monazite

Diffusion of Th has been characterized in synthetic monazite under dry conditions. The synthetic monazites (either pure CePO4, NdPO4, or a mixed LREE phosphate containing Ce, Nd, and Sm) were grown via a Na2CO3–MoO3 flux method. The sources of diffusant for the experiments were either synthesized ThSiO4 or CaTh(PO4)2 powders mixed with CePO4. Experiments were performed by placing source and monazite in Pt capsules and annealing capsules in 1-atm furnaces for times ranging from 3 weeks to a few hours, at temperatures from 1350 to 1550 °C. The Th distributions in the monazite were profiled by Rutherford Backscattering Spectrometry (RBS).The following Arrhenius relation was obtained for Th diffusion in monazite:D=6.7×101exp(−740±63kJmol−1/RT) m2s−1The diffusivity of Th was found to be similar for monazites containing a single REE (Ce or Nd) and the mixed LREE phosphates. Th diffusion was also similar for experiments run using the Th silicate and Ca–Th phosphate sources, suggesting that the substitutional mechanism for Th in monazite, i.e., Th+ 4 + Si+ 4 → REE+ 3 + P+ 5 for the ThSiO4 source, and Th+ 4 + Ca+ 2 → 2REE+ 3 for the CaTh(PO4)2 source, does not significantly affect Th diffusivities, and that Th is likely the rate-limiting species in diffusion.Th diffusion in monazite is about 3 orders of magnitude slower than Pb diffusion [Cherniak, D.J., Watson, E.B., Grove, M., Harrison, T.M., 2004a. Pb diffusion in monazite: a combined RBS/SIMS study. Geochimica et Cosmochimica Acta 68, 829–840.]. This contrasts with findings of Gardés et al. [Gardés, E., Jaoul, O., Montel, J.-M., Seydoux-Guillaume, A.-M., Wirth, R., 2006. Pb diffusion in monazite: An experimental study of Pb2+ + Th4+ → 2Nd3+ interdiffusion. Geochimica et Cosmochimica Acta 70, 2325–2336.] who determined that diffusivities measured in (Pb, Th) ↔ REE exchange (which would under most circumstances be rate-limited by the slowest-diffusing species) are comparable to those measured by Cherniak et al. [Cherniak, D.J., Watson, E.B., Grove, M., Harrison, T.M., 2004a. Pb diffusion in monazite: a combined RBS/SIMS study. Geochimica et Cosmochimica Acta 68, 829–840.] for Pb. Th diffusion in zircon [Cherniak, D.J., Hanchar, J.M., Watson, E.B., 1997. Diffusion of tetravalent cations in zircon. Contributions to Mineralogy and Petrology 127, 383–390.] is about an order of magnitude slower than in monazite, but with similar activation energy for diffusion. The lower diffusivities in zircon may be a consequence of the larger disparity in size between Th and the Zr site in zircon as compared with Th and the REE site in monazite. Nonetheless, Th is essentially immobile in monazite with respect to exchange by volume diffusion under most geologic conditions. These findings may also have implications for containment of high-level actinide-based nuclear waste in monazite ceramic waste forms, but it should be noted that mechanisms other than diffusive exchange (e.g., recrystallization, dissolution/reprecipitation) may be important to consider along with diffusion when evaluating the retentivity of monazite for Th and other actinide elements.