LEAD DIFFUSION IN TITANITE AND PRELIMINARY RESULTS ON THE EFFECTS OF RADIATION DAMAGE ON PB TRANSPORT

Abstract

Lead diffusion has been measured in natural titanite using two different methods to introduce the diffusant: (1) implantation of Pb ions; and (2) immersion of the crystals in a PbS powder reservoir. In both sets of experiments, Rutherford backscattering spectrometry (RBS) was used to obtain concentration profiles, which were then fit with appropriate solutions to the diffusion equation.Experiments using the PbS powder source, run over the temperature range 650–1027°C, define the Arrhenius relationship:for diffusion parallel to the (100) plane. Diffusivities are not significantly affected by orientation or differences in preannealing treatment.The ion implantation experiments, in contrast, deviate from simple linear Arrhenius behavior, with the high-temperature data (850–950°C) displaying good agreement with the above relation but the lower-temperature data (600–800°C) exhibiting enhanced diffusivities and a lowered activation energy (19 kcal mol−1). These results suggest that radiation damage induced by the ion implantation is repaired at high temperatures, thus leading to agreement of the two data sets. At lower temperatures, lattice repair proceeds much more slowly in relation to the duration of the diffusion anneal, resulting in elevated diffusion coefficients typical of materials that have become amorphous.A comparison with earlier work on apatite and zircon places the rate of repair of implantation-induced damage in titanite between these minerals, with apatite exhibiting a more rapid rate of repair and zircon a much slower rate for the same implant dose and similar annealing conditions. This difference is largely a function of structural and compositional characteristics that affect the rate of accumulation and repair of damage in each mineral. The results presented here may have important implications for Pb transport and interpretation of isotope ratios in naturally radiation-damaged minerals, as ion implantation produces damage comparable to that resulting from α-recoil.