PB, U AND TH DIFFUSION IN NATURAL ZIRCON

Zircon (ZrSiO4) is one of the most widely used minerals for determinin g the age, origin and thermal history of rocks by U-Th-Pb geochronolog y. But the parameters describing the solid-state (volume) diffusion ra tes of these elements in natural zircon, which are themselves importan t for establishing the limits on the applicability of zircon for geoch ronological studies, have remained poorly quantified(1). This is becau se of the measurement difficulties associated with the low (p.p.m.) co ncentrations and low diffusion rates of these elements in natural zirc on, and the chemical and physical heterogeneity present in most crysta ls. Here we present direct measurements of the uranium, thorium and le ad loss from a thermally treated gem-quality natural zircon and show t hat lead diffuses much faster than uranium or thorium. We find that th e U-Th-Pb isotopic system in natural zircon will typically have a clos ure temperature greater than 900 degrees C, which explains why zircon is apparently such a robust geochrometer and is capable of remaining i sotopically closed through extended periods of high-grade metamorphism and partial melting of the host rock.