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

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 mu g g(-1) U (Th/U similar to 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 x 10(18) 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 Th-230 corrected Pb-206/U-238, 0.7473 +/- 0.0014 (2SD, n = 47) for Pb-207/U-235 and 0.059069 +/- 0.000067 (2SD, n = 47) for Th-230 corrected Pb-207/Pb-206. The CA-ID-TIMS method yielded a mean Th-230 corrected Pb-206/U-238 age of 566.16 +/- 0.77 Ma (2SD, n = 47). The determined mean delta O-18(VSMOW) and delta Zr-94/90(SRM3169) values of the Tanz zircon megacrysts are 6.52 +/- 0.28 parts per thousand (2SD, n = 29) and -0.01 +/- 0.06 parts per thousand (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.

Automated summary

The Tanz zircon megacrysts from Tanzania exhibit uniform U-Pb ages, O isotopes, and Zr isotopes, making them suitable as reference materials for microbeam analysis. However, they show a wide distribution of Hf isotope compositions, which hinders their potential as a reference material for microbeam analysis. These features suggest that the Tanz zircon megacrysts could be used as a primary reference material for U-Pb ages and Zr-O isotopes.