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

Abstract

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 α-dose < 0.5 x 10¹⁸ events g^-1), tested by the measured FWHM of the ν₃ (SiO₄) Raman band < 5 cm^-1. An ID-TIMS weighted mean ²⁰⁶Pb/²³⁸U 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 μg g^-1) the in situ mean ¹⁷⁶Hf/¹⁷⁷Hf ratios of ten crystals overlap within uncertainty. We recommend a mean ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.282003 ± 0.000020 (2s) as preliminary working value for OZ domains. Two OZ megacrysts yield indistinguishable δ¹⁸O_VSMOW 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 δ¹⁸O and low εHf_i (-15.8 to -17.1) indicate a crustal contribution. The zircons probably grew from crustal-derived carbonate melts under high grade metamorphic conditions.