Characterization of a new zircon reference material (TGZ) with substantial reserve for in situ U-Pb and Hf-O isotope analysis

Abstract

Zircon U-Pb ages and Hf-O isotopic compositions provide complementary constraints on crustal evolution and magmatic processes. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS) have become critical techniques for high-spatial-resolution U-Pb dating and Hf-O isotopic analyses. Matrix-matched reference materials are prerequisites for obtaining accurate and precise results during in situ zircon U-Pb and Hf-O isotopic analyses. Although numerous reference materials have been reported, those with substantial reserves and ready availability remain scarce. In this study, the new zircon megacrysts (TGZ) with a total weight of 1.5 kg were evaluated as a new reference material for in situ U-Pb and Hf-O isotope analyses. The mean U, Th, Pb, and Hf mass fractions for TGZ zircon were 98.9 ± 40.8 µg g⁻¹ (2SD), 24.4 ± 10.4 µg g⁻¹ (2SD), 36.0 ± 15.0 µg g⁻¹ (2SD), and 13 264 ± 3226 µg g⁻¹ (2SD), respectively. U-Pb, on being analyzed with chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS), yielded Th-corrected weighted mean ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²⁰⁶Pb ratios of 0.091052 ± 0.000018 (2s, MSWD = 2.4, and n = 6) and 0.058921 ± 0.000023 (2s, MSWD = 4.8, and n = 6), respectively. The Th-corrected weighted mean ²⁰⁶Pb/²³⁸U date of TGZ was 561.76 ± 0.63 Ma (95% conf., n = 6, and MSWD = 2.5), which is recommended as the reference date of the TGZ zircon. The results from three independent laser ablation laboratories yielded a weighted average ²⁰⁶Pb/²³⁸U date of 560.5 ± 0.6/11.2 Ma (2s, MSWD = 0.53, and n = 328), confirming the U-Pb isotopic homogeneity in TGZ. The ¹⁷⁶Hf/¹⁷⁷Hf ratio of the TGZ zircon was 0.281915 ± 0.000007 (2SD and n = 18) from solution-based analysis. Extensive LA measurements from two independent laboratories yielded a mean ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.281919 ± 0.000033 (2SD and n = 366) in good agreement with the solution-based value obtained within the analytical uncertainty. The obtained δ¹⁸O values were 7.10‰ ± 0.14‰ (2SD and n = 11) and 6.95‰ ± 0.33‰ (2SD and n = 39) from laser fluorination and SIMS analysis. We propose TGZ zircon as a new reference material for microbeam U-Th-Pb geochronology and Hf-O isotopic analyses, given its homogeneous U-Pb dates and Hf-O isotopic compositions, substantial reserve, and ready availability.