PB DIFFUSION IN MONAZITE

We have measured rates of Pb diffusion in both synthetic (CePO₄) and natural (eBay, 554) monazites run under dry, 1-atm conditions. Powdered mixtures of pre-reacted CePO₄ and PbZrO₃ were used as the source of Pb diffusant for "in-diffusion" experiments conducted in sealed Pt capsules for durations ranging from a few hours to several weeks. Following the diffusion anneals, Pb concentration profiles were measured by both Rutherford Backscattering Spectroscopy (RBS) and secondary ion mass spectrometry (SIMS). In order to evaluate potential compositional effects upon Pb diffusivity and simulate diffusional Pb loss that might occur in natural systems, we also conducted "out-diffusion" experiments on Pb-bearing natural monazites. In these experiments, monazite grains were surrounded by a synthetic zircon powder to act as a "sink". Monazites from these experiments were analyzed using SIMS. Over the temperature range 1100 to 1350°C, a linear Arrhenius relation was obtained yielding E = 592±39 kJ/mol and D_o = 0.94 m²/sec. Diffusivities for synthetic and natural monazites are quite similar, as are results of measurements made on the same samples using both RBS and SIMS. The activation energy for Pb diffusion of nearly 600 kJ/mol is more than three times that for a natural monazite reported previously, with Pb diffusivities at T<1200°C significantly lower than those reported earlier. Possible explanations for the disparity include surface dissolution during hydrothermal treatment and differences in diffusivity under 'wet' and 'dry' conditions. Our results indicate that Pb diffusion behavior in monazite is similar to that of zircon and consequently allows the possibility that the Pb closure behavior in natural monazite is higher than all previous estimates (e.g., a 10 micron-sized monazite grain would have a Pb closure temperature >900°C for a cooling rate of 10°C/Ma). If the extrapolation of our results to geological timescales is appropriate, then other factors must affect U-Th-Pb systematics in monazite including possible non-diffusive effects such as recrystallization mediated by fluids or triggered by metamorphic reactions with other REE-bearing minerals.