Tanz zircon megacrysts: a new zircon reference material for the microbeam determination of U–Pb ages and Zr–O isotopes

Abstract

We present detailed characterization of Tanz zircon megacrysts for the microbeam determination of U–Pb ages, Zr isotopes, Hf isotopes, O isotopes and trace element mass fractions. This batch of dark brown zircon megacrysts from Tanzania contains 269 ± 103 μg g⁻¹ U (Th/U ∼ 0.154), and its total weight is 9.06 kg. The largest Tanz zircon megacryst in this batch weighs 366 grams. The calculated average accumulated alpha-radiation dosage is 0.54 × 10¹⁸ events per g for the Tanz zircon, which correlates with a ‘well crystallized’ structure. Through a large number of laser ablation ICP-MS, laser ablation MC-ICP-MS and SIMS measurements, it is found that Tanz zircon megacrysts are quite uniform in U–Pb ages, O isotopes and Zr isotopes at scales of tens of microns. The Tanz zircon has chemical abrasion isotope dilution TIMS (CA-ID-TIMS) determined mean isotopic ratio values of 0.09180 ± 0.00013 (2SD, n = 47) for ²³⁰Th corrected ²⁰⁶Pb/²³⁸U, 0.7473 ± 0.0014 (2SD, n = 47) for ²⁰⁷Pb/²³⁵U and 0.059069 ± 0.000067 (2SD, n = 47) for ²³⁰Th corrected ²⁰⁷Pb/²⁰⁶Pb. The CA-ID-TIMS method yielded a mean ²³⁰Th corrected ²⁰⁶Pb/²³⁸U age of 566.16 ± 0.77 Ma (2SD, n = 47). The determined mean δ¹⁸O_VSMOW and δ^94/90Zr_SRM3169 values of the Tanz zircon megacrysts are 6.52 ± 0.28‰ (2SD, n = 29) and −0.01 ± 0.06‰ (2SD, n = 35) by gas source mass spectrometry and double spike TIMS, respectively. All of these values are in good agreement with the statistical mean values of microbeam measurements and are proposed as the best reference values for Tanz zircon megacrysts. These features make the Tanz zircon megacrysts suitable as primary U–Pb age and Zr–O isotope reference materials for microbeam analysis. However, our LA-MC-ICP-MS data suggest a relatively wide distribution of Hf isotope compositions in the Tanz zircons (0.281821 ± 0.000042, 2SD, n = 321), which makes Tanz zircons not an ideal Hf isotope reference material for microbeam analysis. We propose the Tanz zircon megacrysts as a potential reference material for the primary external calibration or monitoring of analytical validation for microbeam determination of U–Pb ages and Zr–O isotopes.