Publication
Zircon Megacrysts from the Kawisigamuwa Carbonatite, Sri Lanka – A Potential Reference Material for In Situ U-Pb and Hf Isotope Measurement
Publisher:
Wiley
Date:
15-09-2023
DOI:
10.1111/GGR.12522
Abstract: We characterise zircon megacrysts from Kawisigamuwa carbonatite as a new potential reference material for laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) U‐Pb and Hf isotope analysis. 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 .5 x 10 18 events/g), tested by the measured FWHM of the ν 3 (SiO 4 ) Raman band 5 cm ‐1 . An ID‐TIMS weighted mean 206 Pb/ 238 U 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) the in‐situ mean 176 Hf/ 177 Hf ratios of ten crystals overlap within uncertainty. We recommend a mean 176 Hf/ 177 Hf of 0.282003 ± 0.000020 (2s) as preliminary working value for OZ domains. Two OZ megacrysts yield indistinguishable δ 18 O VSMOW of 12.1 ± 0.4‰ and 12.2 ± 0.4‰ (2s). Elevated Th/U ( .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 δ 18 O 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.