The effect of intra-crystal uranium zonation on apatite U-Pb thermochronology: A combined ID-TIMS and LA-MC-ICP-MS study

Abstract

Abstract Quantitative reconstruction of high-temperature thermal histories (>350 °C) has applications in metamorphic studies, processes which exhume deep crust, the evolution of active margins over hundreds of millions of years, and the study of extra-terrestrial rocks. Previous studies have demonstrated that apatite U-Pb data can be used to recover thermal history information at T > 350 °C from single crystal (bulk) dates. We present U-Pb bulk grain (ID-TIMS) and intra grain (LA-MC-ICP-MS) U-Pb data from apatites to evaluate the importance of parent isotope (U) distribution when extracting thermal history information from apatites that have experienced partial Pb loss. We find that parent U zonation can cause considerable scatter when single (bulk) grain dates are compared with their grain size, and therefore U zonation must be taken into account when inverting U-Pb dates and grain sizes to seek thermal history solutions. We show that accurate thermal history solutions can be obtained from apatites which have zoned concentrations when in-situ variations in U concentration and U-Pb dates are modelled. This observation validates the hypothesis that Pb is lost by thermally driven volume diffusion. In-situ analytical techniques yield significantly more accurate thermal history solutions than bulk grain ID-TIMS analyses when investigating rocks which contain compositionally zoned apatites.