In-run measuring 177Hf16O/177Hf as a routine technique for in-situ Hf isotopic compositions analysis in zirconium-bearing minerals by laser ablation MC-ICP-MS

Abstract

Problems with REE oxide interferences during in-situ Hf isotope analysis in zirconium-bearing minerals can give rise to inaccuracies in the obtained dataset. In this study, we present an in-run 177Hf16O/177Hf measurement protocol pertaining to in-situ Hf isotope analysis by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) as a routine method, allowing for a robust real-time method of monitoring the oxide formation rates and offering a method to monitor REE oxide interferences (e.g., Gd and Dy). For this purpose, we set up a static data collection mode to measure Hf isotopes and 177Hf16O signals concurrently in-run. By analyzing zirconium-bearing minerals characterized with a broad range of Dy/Hf and Gd/Hf ratios (e.g., zircon: typically, 0.0001–0.022 for Dy/Hf and 0.00002–0.013 for Gd/Hf; zirconolite: typically, >0.2 for Dy/Hf and > 0.4 for Gd/Hf; eudialyte: typically, >0.1 for Dy/Hf and > 0.1 for Gd/Hf), influences of oxide formation rate on 176Hf/177Hf ratios are investigated via the present in-run method. The level of oxide formation during the analysis is found to impact the measurement accuracy, especially for samples with higher REE/Hf ratios. Therefore, it is imperative that oxide ratios should be routinely monitored for daily in-situ Hf isotope analysis along with oxide formation rates. In addition, based on results of the present work, a higher REE/Hf zircon, like SA02 zircon, is strongly recommended as the primary refence material for routine in-situ analysis of Hf isotopic compositions in zircons to carefully test the accuracy of analysis for zircons.