Discrete multi-pulse laser ablation depth profiling with a single-collector ICP-MS: Sub-micron U-Pb geochronology of zircon and the effect of radiation damage on depth-dependent fractionation

A discrete multi-pulse method for single-collector ICP-MS laser ablation systems is presented that interrogates isotopic variation as a function of sample depth. The fidelity of the method is assessed with a 183-sample U-Pb analysis session of zircons with known age. By using bursts of 5 laser pulses the method resolves integration-level ages with similar to 0.55 mu m depth resolution and similar to 6% 2 sigma age uncertainty. To avoid signal aliasing, isotopic ratios are calculated using total ion counts for each integration, instead of on a cycle-by-cycle basis. Fractionation correction is achieved by constructing a continuous-function, non-parametric 3D surface from which discrete values for any time and sample depth can be calculated. At the sample level (15 integrations for this study), average 2s uncertainty is similar to 2.5% for Pb-206/U-238 ages; 95% of samples and similar to 90% of integrations overlap with their accepted age at 2 sigma. The data reduction software developed here is designed to be flexible and a discussion of the effects of varying method parameters is provided. Total ablation depth is measured using white light interferometry, ranges between 7 and 10 mu m and is found to vary as a function of parent radionuclide concentration, measures of crystal lattice disorganization from Raman spectroscopy, and metrics of radiation damage (alpha dose). These data indicate that radiation damage exerts a fundamental control on laser ablation efficiency, although the exact physical process is unknown at present. Consequently, fractionation correction factors derived for a reference material may not be appropriate for unknowns with vastly different crystal structure. (C) 2014 Elsevier B.V. All rights reserved.