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 ~ 0.55 μm depth resolution and ~ 6% 2σ 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 2σ uncertainty is ~ 2.5% for 206Pb/238U ages; 95% of samples and ~ 90% of integrations overlap with their accepted age at 2σ. 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 μm 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.