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 was evaluated as 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 13264±3226 μg g-1 (2SD), respectively. U-Pb analyzed with chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) yielded a Th-corrected weighted mean 206Pb/238U and 207Pb/206Pb ratios of 0.091052±0.000018 (2s, MSWD=2.4, n=6) and 0.058921±0.000023 (2s, MSWD=4.8, n=6). The Th-corrected weighted mean 206Pb/238U date of TGZ was 561.76±0.63 Ma (95% conf., n=6, MSWD=2.5), which is recommended as the reference date of 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, n=328), confirming the U-Pb isotopic homogeneity in TGZ. The 176Hf/177Hf ratio of TGZ zircon was 0.281915±0.000007 (2SD, n=18) from solution-based analysis. Extensive LA measurements from two independent laboratories yielded a mean 176Hf/177Hf ratios value of 0.281919±0.000033 (2SD, n=366), which showed good agreement with the solution-based analyzed value within the analytical uncertainty. The obtained δ¹⁸O values were 7.10 ± 0.14‰ (2SD, n=11) and 6.95 ± 0.33‰ (2SD, 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.