Reassessment of relative oxide formation rates and molecular interferences on in situ lutetium–hafnium analysis with laser ablation MC-ICP-MS

Abstract

A series of Hf and rare earth element (REE) solutions and glass beads have been produced in order to assess the influence of isobaric and molecular interferences on LA-MC-ICP-MS analysis of Lu–Hf isotopes in zircon. We demonstrate the capability to accurately correct for isobaric interferences of ¹⁷⁶Yb on ¹⁷⁶Hf at levels up to ¹⁷⁶Yb/¹⁷⁷Hf = 0.6 in solution mode. When using laser ablation sample introduction and REE–Hf doped glass beads we are able to accurately correct for ¹⁷⁶Yb interferences up to ¹⁷⁶Yb/¹⁷⁷Hf = 0.15. These thresholds exceed the ¹⁷⁶Yb/¹⁷⁷Hf ratios in most natural zircon and demonstrate the general robustness of the method. Unlike solution analysis, at extreme Yb interference levels (¹⁷⁶Yb/¹⁷⁷Hf ≈ 0.8) there appears to be a slight over-correction of Yb interferences. We demonstrate using theoretical calculations that for high-REE zircons, even modest oxide formation rates can lead to inaccurate ¹⁷⁶Hf/¹⁷⁷Hf ratios. This finding is confirmed by data collected on REE-doped glass beads and a natural zircon sample. Importantly, Gd oxides dominate over Dy oxides as a source of molecular interferences on Hf isotope data because Gd is more prone to oxide formation. Oxide formation rates vary depending upon sample introduction, instrument tuning and N₂ addition (in laser mode). Correction for molecular interferences is possible using a dynamic analysis routine but requires measurement of the relative Gd and Hf oxide formation rates for the analysis session. Hence the daily monitoring of Gd and Hf oxide formation rates will improve the accuracy of Lu–Hf LA-MC-ICP-MS results for high-REE/Hf zircons.