Huang, Hui-Qing1; Guillong, Marcel2; Hu, Yi3; Spandler, Carl1
1Economic Geology Research Center, Division of Tropical Environments and Societies, James Cook University, Townsville, QLD 4811, Australia. Email: firstname.lastname@example.org, 2Department of Earth Sciences, ETH Zurich, 8092 Zurich, Switzerland, 3Advanced Analytical Centre, James Cook University, Townsville, QLD 4811, Australia
Spatial resolution and precision of U-Pb isotope analysis by LA-ICP-MS has been greatly improved in the last two decades. However, reproducibility of this most widely used in situ technique is still relatively poor, and error sources remain challenging to determine. Factors such as matrix composition, air, laser fluence have been systematically examined. Here we evaluate a previously underappreciated source of error on U-Pb isotope determination associated with laser focus. Using two different LA-ICP-MS systems but similar ablation parameters (a circular spot of ~20 µm and a final depth of ablation of ~10 µm only with a laser fluence of 2 J/cm2, an ablation time of 30 seconds and a repetition rate of 4 and 5 Hz), we show that sole variation of laser focus by 30 µm can lead to a systematic offset in 206Pb/238U of ~4 – 6% for zircons. Focus position variation led to change of laser irradiance on sample surface and shape of ablation craters. The degree of age offset is controlled by the final depth of ablation and crater aspect ratios (depth to diameter). We demonstrate further that the impact of focus variation on U-Pb isotope fractionation is matrix dependent. Using same conditions, defocus of laser beam by 30 µm can cause an offset in 206Pb/238U by <1% for NIST610 glass, ~3% for titanite, and up to 12% for rutile. The uncertainty related to laser focus appear random. We suggest that enhanced repeatability of laser focusing is required for improved uncertainty and better reproducibility in the determination of elements and particularly high precision U-Pb isotopes by LA-ICP-MS.
Huiqing did his PhD at Curtin University. His research involves using geochemistry and igneous and experimental petrology to understand continental crust formation and evolution. He currently works for James Cook University as a laboratory specialist with a focus on method development using LA-ICP-MS.