Diffusion of Th was 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 1atm furnaces for times ranging from 3 weeks to a few hours, at 1350 to 1550C. The Th distributions in the monazite were profiled by Rutherford back-scattering spectrometry. The following Arrhenius relation was obtained for Th diffusion in monazite:

D(m2/s) = 67 exp[-740(kJ/mol)/RT]

The diffusivity of Th was found to be similar for monazites containing a single REE (Ce or Nd) and the mixed LREE phosphates.

D.J.Cherniak, J.M.Pyle: Chemical Geology, 2008, 256[1-2], 52-61