Matrix effects and Hf isotope analysis of zircon by laser ablation MC-ICP-MS

In the last decade there have been significant advances made in the in situ measurement of Hf isotopes in zircon and the application of the Lu-Hf isotopic system to help constrain the sources that contributed to zircon formation. The information obtained from the in situ Hf analysis of zircon has been used to monitor magmatic processes, to identify inherited and juvenile components in magmatic sources, and to evaluate models of crustal evolution. Despite the success of the technique, its application is still limited by the contribution to the total uncertainty by corrections for isobaric interferences, especially for zircons with high REE/Hf ratios. A significant effort has been devoted to demonstrating the robustness of correction methods for isobaric interferences of (super 176) Yb and (super 176) Lu on (super 176) Hf but little attention has been paid to the contribution of molecular interferences. In this study the accuracy and precision of isobaric corrections and the contribution of REE-oxides on Hf isotopes are evaluated using a suite of synthetic glass beads doped with the JMC475 Hf standard and varying concentrations of Yb, Gd and Dy. The results demonstrate the robustness of the isobaric interference correction procedures for (super 176) Yb/ (super 177) Hf ratios greater than 0.8 (i.e. approx. three times that of (super 176) Hf/ (super 177) Hf ratio). Theoretical calculations and measurement of Gd- and Dy-doped glass beads demonstrate that REE-oxides are also able to bias Hf-isotope data. Gd oxides can produce interferences on a number of Yb isotopes and Dy oxides overlap several Hf isotopes. The significance of these interferences depends on the oxide production of the mass spectrometer as well as the slope of the REE patterns (Gd /Yb) and REE/Hf ratios (Dy/Hf). The results can potentially explain a number of observed correlations between REE content of zircon and measured Hf isotope ratios. These 'matrix-effects' are most apparent in high REE zircons and in zircons with high Gd/Yb ratios and leads us to recommend monitoring of REE levels and oxide interference corrections.