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Diffusion du plomb dans la monazite

Abstract : Proper knowledge of the diffusion rates of lead in monazite is necessary to understand the U-Th-Pb age anomalies of this mineral, which is one of the most used in geochronology after zircon.
Diffusion experiments were performed in NdPO4 monocrystals and in Nd0.66Ca0.17Th0.17PO4 polycrystals from Nd0.66Pb0.17Th0.17PO4 thin films to investigate Pb2+ + Th4+ 2 Nd3+ and Pb2+ Ca2+ exchanges. Diffusion annealings were run between 1200 and 1500°C, at room pressure, for durations ranging from one hour to one month. The diffusion profiles were analysed using TEM and RBS.
The diffusivities extracted for Pb2+ + Th4+ 2 Nd3+ exchange follow an Arrhenius law with parameters E = 509 ± 24 kJ mol-1 and log D0 (m2 s-1) = -3.41 ± 0.77. Preliminary data for Pb2+ Ca2+ exchange are in agreement with this result.
The extrapolation of our data to crustal temperatures yields very slow diffusivities. For instance, the time necessary for a 50 µm grain to lose all of its lead at 800°C is 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 lead diffusion, but rather to interactions with fluids.
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Submitted on : Tuesday, October 17, 2006 - 5:08:27 PM
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  • HAL Id : tel-00107246, version 1


Emmanuel Gardes. Diffusion du plomb dans la monazite. Minéralogie. Université Paul Sabatier - Toulouse III, 2006. Français. ⟨tel-00107246⟩



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