Zircon Megacrysts from the Kawisigamuwa Carbonatite, Sri Lanka - A Potential Reference Material for In Situ U- Pb and Hf Isotope Measurement

We characterise zircon megacrysts from the Kawisigamuwa carbonatite as a new potential reference material for laser ablation-inductively coupled plasma-mass spectrometry U-Pb and Hf isotope measurement. We studied ten 0.5-4 cm long, brown megacrysts that consist of oscillatory zoned (OZ) and nearly featureless cathodoluminescence-bright recrystallised (RX) zircon domains. The zircons have low to moderate radiation damage (total alpha-dose\ 0.5 x 10(18) events g(-1)), tested by the measured FWHM of the nu(3) (SiO4) Raman band\ 5 cm(-1). An ID-TIMS weighted mean Pb-206/U-238 age of 532.39 +/- 0.66 Ma (2s uncertainty) was determined for OZ zircon domains of three grains. While hafnium content varies within and between crystals (6630-9960 lg g(-1)) the in situ mean Hf-176/Hf-177 ratios of ten crystals overlap within uncertainty. We recommend a mean Hf-176/Hf-177 ratio of 0.282003 +/- 0.000020 (2s) as preliminary working value for OZ domains. Two OZ megacrysts yield indistinguishable delta(18)(OVSMOW) of 12.1 +/- 0.4% and 12.2 +/- 0.4% (2s). Elevated Th/U ([ 1.5), Zr/Hf (59-75) and low hafnium contents in OZ domains are similar to those of zircons from carbonatites of mantle origin, while high delta O-18 and low epsilon Hf-i (-15.8 to -17.1) indicate a crustal contribution. The zircons probably grew from crustal-derived carbonate melts under high grade metamorphic conditions.