Three natural zircon reference materials for in situ Hf isotope analysis covering a wide range of 176Yb/177Hf ratios

Abstract

The radiogenic Hf isotopic composition (¹⁷⁶Hf/¹⁷⁷Hf) of zircon is a critical tracer in isotope geochemistry. However, the accuracy of in situ Hf isotope analysis is often compromised by isobaric interference from ¹⁷⁶Yb on ¹⁷⁶Hf, particularly in zircons with high Yb/Hf ratios (¹⁷⁶Yb/¹⁷⁷Hf > 0.03). The limited availability of reference materials (RMs) with elevated Yb/Hf ratios for instrument optimisation and quality control remains an important challenge. In this study, three potential zircon RMs with a range of Yb/Hf ratios (ML-2, US-2, and MADA) were characterised for in situ Hf isotope analysis. These materials are demonstrated to be homogeneous at the micrometre scale in Hf isotopic composition by laser ablation MC-ICP-MS, with ¹⁷⁶Yb/¹⁷⁷Hf values clustered at <0.01, 0.03–0.11, and 0.08–0.16, respectively. The results show that high-Yb zircons can exhibit substantial bias and increased scatter in measured ¹⁷⁶Hf/¹⁷⁷Hf during in situ analysis, whereas commonly used low-Yb RMs (e.g., PLE, Mud Tank, and Tanz) yield accurate and consistent results. Introducing N₂ into the central gas flow expands the axial ion beam distribution, thereby stabilising mass bias and enabling accurate ¹⁷⁶Hf/¹⁷⁷Hf determination even for high-Yb/Hf zircons. It is therefore strongly recommended that RMs with comparable or higher Yb/Hf ratios than the unknown samples be used for instrument optimisation and quality control during in situ Hf isotope analysis of high-Yb zircons and that sufficient addition of N₂ be applied, as it has proven highly effective under such analytical conditions.