Pb diffusion in monazite: New constraints from the experimental study of Pb2+ <=> Ca2+ interdiffusion

Emmanuel Gardès (1) , Jean-Marc Montel (1) , Anne-Magali Seydoux-Guillaume (1) , Richard Wirth (2)

1 LMTG - Laboratoire des Mécanismes et Transfert en Géologie
2 GFZ - German Research Centre for Geosciences - Helmholtz-Centre Potsdam

We performed diffusion annealing of Pb-bearing monazite thin films deposited on Ca-bearing monazite polycrystals to investigate Pb2+ <=> Ca2+ 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 Pb2+ + Th4+ <=> 2Nd3+. The combination of both datasets yields an Arrhenius law for Pb diffusion in monazite with parameters: E = 494 +/- 17 kJ/mol log D0 (m2/s)= -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.

Domaines: Minéralogie

Emmanuel Gardès, Jean-Marc Montel, Anne-Magali Seydoux-Guillaume, Richard Wirth. Pb diffusion in monazite: New constraints from the experimental study of Pb2+ <=> Ca2+ interdiffusion. Geochimica et Cosmochimica Acta, 2007, pp.4036-4043. ⟨hal-00607326⟩