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

Matrix-matched calibration by natural zircon reference materials 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 it has a concordant U-Pb date with a weighted mean 206Pb/238U date of 337.33 ± 0.38 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 reference material 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 (2 SD). The age and Hf isotopic homogeneity of the Plešovice zircon together with its relatively high U and Pb contents makes 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.