Zircon M257 ‐ a Homogeneous Natural Reference Material for the Ion Microprobe U‐Pb Analysis of Zircon

TLDR

The zircon’s oxygen isotope ratio of 13.9 %o VSMOW indicates a metamorphic origin in marble or calc‑silicate skarn. The study proposes zircon M257 as a future reference material for U‑Pb age determinations by secondary ion mass spectrometry. The specimen is a light‑brown, flawless, 5.14‑g (25.7‑carat) cut gemstone from Sri Lanka. Zircon M257 has concordant TIMS‑determined 206Pb/238U and 207Pb/235U ratios yielding a 561.3 ± 0.3 Ma age, contains ~840 µg g⁻¹ U (Th/U ≈ 0.27), shows negligible heterogeneity, moderate radiation damage consistent with its age and actinide content, and a (U+Th)/He age of 419 ± 9 Ma, confirming its suitability as a homogeneous reference material.

Abstract

We introduce and propose zircon M257 as a future reference material for the determination of zircon U‐Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone specimen from Sri Lanka weighed 5.14 g (25.7 carats). Zircon M257 has TIMS‐determined, mean isotopic ratios (2s uncertainties) of 0.09100 ± 0.00003 for 206 pb/ 238 U and 0.7392 ± 0.0003 for 207 pb/ 235 U. Its 206 pb/ 238 U age is 561.3 ± 0.3 Ma (unweighted mean, uncertainty quoted at the 95% confidence level); the U‐Pb system is concordant within uncertainty of decay constants. Zircon M257 contains ∼ 840 μg g −1 U (Th/U ∼ 0.27). The material exhibits remarkably low heterogeneity, with a virtual absence of any internal textures even in cathodoluminescence images. The uniform, moderate degree of radiation damage (estimated from the expansion of unit‐cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the “Sri Lankan trends”, with actinide concentrations, U‐Pb age, and the calculated alpha fluence of 1.66 × 10 18 g −1 . This, and a (U+Th)/He age of 419 ± 9 Ma (2s), enables us to exclude any unusual thermal history or heat treatment, which could potentially have affected the retention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9%o VSMOW suggesting a metamorphic genesis in a marble or calc‐silicate skarn.