Plešovice zircon - A new natural reference material for U-Pb and Hf isotopic microanalysis

Matrix-matched calibration by natural zircon standards and analysis of natural materials as a reference are the principle methods for achieving accurate results in microbeam U-Pb dating and Hf isotopic analysis. We describe a new potential zircon reference material for laser ablation ICP-MS that was extracted from a potassic granulite facies rock collected in the southern part of the Bohemian Massif (Plešovice, Czech Republic). Data from different techniques (ID-TIMS, SIMS and LA ICP-MS) and several laboratories suggest that this zircon has a concordant U-Pb age with a weighted mean ²⁰⁶Pb/²³⁸U date of 337.13 ± 0.37 Ma (ID-TIMS, 95% confidence limits, including tracer calibration uncertainty) and U-Pb age homogeneity on the scale used in LA ICP-MS dating. Inhomogeneities in trace element composition due to primary growth zoning prevent its use as a calibration standard for trace element analysis. The content of U varies from 465 ppm in pristine parts of the grains to ~ 3000 ppm in actinide-rich sectors that correspond to pyramidal faces with a high degree of metamictization (present in ca. 30% of the grains). These domains are easily recognized from high intensities on BSE images and should be avoided during the analysis. Hf isotopic composition of the Plešovice zircon (> 0.9 wt.% Hf) is homogenous within and between the grains with a mean ¹⁷⁶Hf/¹⁷⁷Hf value of 0.282482 ± 0.000013 (2SD). The age and Hf isotopic homogeneity of the Plešovice zircon together with its relatively high U and Pb contents make it an ideal calibration and reference material for laser ablation ICP-MS measurements, especially when using low laser energies and/or small diameters of laser beam required for improved spatial resolution.