Improved accuracy of LA-ICP-MS U-Pb ages of Cenozoic zircons by alpha dose correction

Abstract

While Laser Ablation Inductively-Coupled-Plasma Mass Spectrometry (LA-ICP-MS) is the method of choice for U-Pb dating of zircons in provenance analysis, its application to young (<100Ma) zircons is hindered by systematic analytical bias. In magmatic petrology, where the majority of studied units are young, this often places zircon dates at odds with established 40Ar/39Ar eruption ages or high-precision ID-TIMS crystallization ages. Zircon lattice properties, particularly the degree of lattice damage caused by the radioactive decay of U and Th, impart analytical bias by causing differential ablation rates and therefore differential fractionation of U and Pb throughout each analysis. Although it is possible to normalize the zircon lattice strengths to calibration reference zircons by thermal annealing to some extent, this may not entirely alleviate the problem. In this study, the effects of alpha decay dose (i.e., degree of radiation damage) on analytical biases in age determination are examined by analyzing a number of zircon reference materials under well-constrained analytical parameters. A regression-based, multi-standard correction method is demonstrated, which improves the accuracy of age data, particularly in young (Cenozoic) zircons. A novel data reduction scheme (Dose_Corrector.ipf) is introduced, which runs in conjunction with the widely-used Igor Pro/Iolite platform and performs a correction for alpha dose and Th disequilibrium. This scheme improves the accuracy of age data for unannealed zircons, and its utility is demonstrated by applying it to zircons from several well-studied units.