Zircon Megacrysts from the Kawisigamuwa Carbonatite, Sri Lanka – A Potential Reference Material for In SituU‐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 δ18OVSMOW 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 εHfi (‐15.8 to ‐17.1) indicate a crustal contribution. The zircons probably grew from crustal‐derived carbonate melts under high grade metamorphic conditions.