A New Appraisal of Sri Lankan BB Zircon as a Reference Materialfor LA-ICP-MS U-Pb Geochronology and Lu-Hf IsotopeTracing

A potential zircon reference material (BB zircon) for laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb geochronology and Hf isotope geochemistry is described. A batch of twenty zircon megacrysts (0.5-1.5cm(3)) from Sri Lanka was studied. Within-grain rare earth element (REE) compositions are largely homogeneous, albeit with some variation seen between fractured and homogeneous domains. Excluding fractured cathodoluminescence bright domains, the variation in U content for all analysed crystals ranged from 227 to 368 mu g g(-1) and the average Th/U ratios were between 0.20 and 0.47. The Hf isotope composition (0.56-0.84 g/100 g Hf) is homogeneous within and between the grains - mean Hf-176/Hf-177 of 0.281674 +/- 0.000018 (2s). The calculated alpha dose of 0.59 x 10(18) g(-1) for a number of BB grains falls within the trend of previously studied, untreated zircon samples from Sri Lanka. Aliquots of the same crystal (analysed by ID-TIMS in four different laboratories) gave consistent U-Pb ages with excellent measurement reproducibility (0.1-0.4% RSD). Interlaboratory assessment (by LA-ICP-MS) from individual crystals returned results that are within uncertainty equivalent to the TIMS ages. Finally, we report on within- and between-grain homogeneity of the oxygen isotope systematic of four BB crystals (13.16 parts per thousand VSMOW).