In situ micro-analytical characterisation of zircon U-Pb and Hf isotopic standards

Abstract Uranium-Pb and Hf isotopic analysis of zircon crystals provides a crystallisation age and origin of the mineral, critical information to investigate Earth’s history. Zircon geochemistry can vary over micrometre scales; therefore, natural reference materials that are used for zircon analyses need to be well characterised before valid measurements can be acquired for ‘unknowns’. This study aimed to assess U-Th-Pb isotope, Hf isotope and trace element heterogeneity of four widely used zircon reference materials: ‘91500’, ‘Mud Tank’, ‘Temora’ and ‘Plešovice’, as well as zircon crystals from the Mount Dromedary/Gulaga Igneous Complex (a locality used as a K-Ar standard). Multiple crystals of each reference material were characterised using optical microscopy, back-scattered electron (BSE) microscopy, cathodoluminescence (CL) imaging, micro-X-ray fluorescence (μXRF) imaging, electron probe micro-analysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Nanometre scale heterogeneity was also assessed with the worldfirst application of time-of-flight secondary ion mass spectrometry (TOF-SIMS) to zircon. High-resolution TOF-SIMS and LA-ICP-MS geochemical maps, and 1,378 LA-ICPMS spot analyses revealed that for each reference material, individual grains have heterogeneous U-Th-Pb isotope ratios and Hf isotope ratios, and in some cases heterogeneous trace element distribution. Many spot analyses are recommended to average intragrain heterogeneity when using these crystals as reference materials. Weighted mean U-Pb values are in agreement with previously published and accepted values for 91500, Temora, Plešovice and Mount Dromedary zircons, but not Mud Tank zircons. Weighted mean Hf isotope ratios only reflect the accepted value for 91500 and Plešovice zircons, despite minimal intracrystalline Hf isotope variability. The work presented here demonstrates that TOF-SIMS is an exciting new tool in the geochemical toolbox. It is capable of resolving isotopes of key trace elements (e.g., U, Th, Sc, Y, rare earth elements and Li) with an effective spatial resolution– 1000 μm wide. As the characterisation of commonly used zircon reference materials showed that zircon 91500 was relatively homogenous in trace elements, it is here considered to be the most suitable matrix-matched reference material for future TOF-SIMS analyses of ‘unknown’ zircon samples.