Diffusion annealing of Pb-bearing monazite thin films deposited onto Ca-bearing monazite polycrystals was used to investigate Pb2+ ↔ Ca2+ interdiffusion. Experiments were performed at 1300 to 1500C, using samples sealed in Pb-buffered capsules. The focused ion beam technique was used to cut transmission electron microscopy foils within grains, excluding boundaries, in order to measure volume diffusion profiles via X-ray diffraction analysis. The diffusivities which were determined were similar to those previously determined for Pb2++Th4+ ↔ 2Nd3+. The combined data yielded an Arrhenius law for Pb diffusion in monazite, with E = 494kJ/mol and log[Do] = -3.86m2/s. A good agreement between this experimental study and 2 previous investigations demonstrated that Pb diffusion was measured in each case. This confirmed that most of the perturbations in U–Th–Pb ages of monazite could not be attributed to Pb diffusion.

Pb Diffusion in Monazite - New Constraints from the Experimental Study of Pb2+↔Ca2+ Interdiffusion. E.Gardés, J.M.Montel, A.M.Seydoux-Guillaume, R.Wirth: Geochimica et Cosmochimica Acta, 2007, 71[16], 4036-43