Pb, U and Th diffusion in natural zircon


Zircon (ZrSiO4) is one of the most widely used minerals for determining the age, origin and thermal history of rocks by U–Th–Pb geochronology. But the parameters describing the solid-state (volume) diffusion rates of these elements in natural zircon, which are themselves important for establishing the limits on the applicability of zircon for geochronological studies, have remained poorly quantified1. This is because of the measurement difficulties associated with the low (p.p.m.) concentrations and low diffusion rates of these elements in natural zircon, and the chemical and physical heterogeneity present in most crystals. Here we present direct measurements of the uranium, thorium and lead loss from a thermally treated gem-quality natural zircon and show that lead diffuses much faster than uranium or thorium. We find that the U–Th–Pb isotopic system in natural zircon will typically have a closure temperature greater than 900 °C, which explains why zircon is apparently such a robust geochrometer and is capable of remaining isotopically closed through extended periods of high-grade metamorphism and partial melting of the host rock.

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Figure 1: a, Typical Pb diffusion profile after heating at 1,100 °C.
Figure 2: Arrhenius diagram of Pb, U and Th diffusion coefficients.
Figure 3: Zircon Pb closure temperature plotted against effective radius of diffusion.


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We thank W. Compston and D. H. Green (Director, RSES) for supporting this project. We thank the following RSES technical personnel for assistance and advice: R. Willison for electron-beam welding; and R. Waterford, K. Massey, R. Willis and B. Hibberson for sample preparation. Numerous people provided guidance and assistance with equipment: J. Fitz Gerald (TEM), T. Senden and A. Hyde (AFM), R. Headey (SEM), S. Eggins (ICP-MS) and D. Tilley (XRD). We also thank E. B. Watson for advice and discussions about experimental procedures and Pb diffusion, H. O'Neill and S. Chakraborty for discussions about thermodynamics and diffusion, and A. Sinha for providing us with synthetic zircon crystals. This work was supported by the Research School of Earth Sciences and an Australian Research Council grant to D. Ellis for the experimental work carried out in the Geology Department.

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Correspondence to James K. W. Lee†.

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