Lead diffusion in apatite and zircon using ion implantation and Rutherford Backscattering techniques

Abstract

Ion implantation was used to introduce Pb into the minerals zircon and apatite. Diffusion profiles were obtained using Rutherford Backscattering, and the results were fitted with a model to determine the diffusion coefficients. This approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times. Results for apatite over the temperature range 600–900°C are in good agreement with earlier results obtained for higher temperatures ( WATSON et al., 1985) and are described by the following Arrhenius Law: D = 1.27 × 10 −4 cm 2 s −1 exp( −54.6 ± 1.7 Kcl mol −1 RT . This suggests that radiation damage induced by ion implantation has little effect on Pb diffusion in this case due to rapid annealing of induced damage at low temperatures. The activation energy stated above is somewhat smaller than that previously determined (70 kcal mol −1), reflecting the larger range in 1/T sampled by the present results. Closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data. Diffusion coefficients for Pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of Pb diffusion based on geochronologic data and U/ Pb zoning in natural zircons. The differing results for zircon and apatite obtained in this study appear to be related to each mineral's ability to anneal the radiation damage induced by ion implantation.