Pb diffusion in monazite: New constraints from the experimental study of Pb?Ca interdiffusion
Résumé
We performed diffusion annealing of Pb-bearing monazite thin films deposited on Ca-bearing monazite polycrystals to investigate Pb?Ca interdiffusion. Experiments were run at temperatures ranging from 1300 to 1500 °C with samples sealed in Pb-buffered capsules. The Focused Ion Beam (FIB) technique was used to cut TEM foils within grains, excluding boundaries, in order to measure volume diffusion profiles using EDX analysis. The diffusivities we determined are similar to those previously determined for Pb+Th?2Nd. The combination of both datasets yields an Arrhenius law for Pb diffusion in monazite with parameters: E=494±17kJ mol,logD0(ms)=-3.86±0.53. The good agreement between this experimental study and the two previous investigations demonstrates that Pb diffusion was measured in each case. It confirms that most of the perturbations in U Th Pb ages of monazite cannot be attributed to Pb diffusion.