Pb diffusion in monazite: An experimental study of Pb+Th?2Nd interdiffusion
Résumé
We have measured Pb diffusivity in synthetic NdPO4 crystals from an epitaxial Nd0.66Pb0.17Th0.17PO4 thin film deposited on (?1 0 1) faces. Annealings were performed at room pressure at temperatures ranging from 1200 to 1500 °C for 1 h to 1 month. Samples were analysed using transmission electron microscopy (TEM) and Rutherford backscattering spectrometry (RBS). TEM demonstrates that Pb and Th diffuse together from the thin film towards the crystal and Nd in the opposite direction, according to the interdiffusion exchange Pb+Th?2Nd. RBS was used to perform depth profiling of all samples. Measured diffusivities follow the Arrhenius law: logD0(ms)=-3.41±0.77, E=509±24kJmol. The extrapolation of these data to crustal temperatures yield very low diffusivities. For instance, the time necessary for a 50 ?m grain to lose all of its Pb at 800 °C is calculated to be greater than the age of the Earth. From these results and other evidence from the literature, we conclude that most of the perturbations in U Th Pb ages of monazite cannot be attributed to Pb diffusion, but rather to interactions with fluids.