Grey Hill zircon – a natural high ¹⁷⁶Yb/¹⁷⁷Hf zircon reference material for LA-MC-ICP-MS Hf isotope measurement

Accurate measurements of Hf isotope ratios in zircon rely on adequate correction for isobaric interferences, which increase in complexity as the ratio of heavy rare earth elements (HREE) to Hf increases. Currently, synthetic high-HREE zircons are commonly used to bracket the highest naturally occurring HREE/Hf zircon grains during laser ablation-based measurement but these are in limited supply. We present results for Grey Hill zircon, a new high ¹⁷⁶Yb/¹⁷⁷Hf zircon with a weighted mean age of 482.97 ± 0.17 Ma (2s). We show that Hf is homogeneously distributed at the microscale in Grey Hill zircon, whereas Yb is heterogeneously distributed in oscillatory- and sector-zones following observed cathodoluminescence patterns. Atom probe tomography measurements show that Hf and Yb are homogeneously distributed at the nanoscale. Chemical abrasion solution multi-collector inductively coupled plasma-mass spectrometry (CA-S-MC-ICP-MS) yielded a mean ¹⁷⁶Hf/¹⁷⁷Hf of 0.282854 ± 0.000023 (2s, n = 15), with a correlation to ¹⁷⁶Lu/¹⁷⁷Hf that corresponds to radiogenic ingrowth since ca. 483 Ma. If analyses are back-calculated to the crystallisation age, the CA-S-MC-ICP-MS data yield a ¹⁷⁶Hf/¹⁷⁷Hf_(t) of 0.282805 ± 0.000010 (2s), which we recommend be used as the reference ratio for Grey Hill zircon. Non-abraded, in situ LA-MC-ICP-MS analyses yielded results consistent with the CA-S-MC-ICP-MS mean. Importantly, LA-MC-ICP-MS analyses do not show any correlation with HREE, and there is no apparent difference in measured ¹⁷⁶Hf/¹⁷⁷Hf between pristine and altered domains. The grains have ¹⁷⁶Yb/¹⁷⁷Hf (0.094–0.48) and ¹⁷⁶Lu/¹⁷⁷Hf (0.0028–0.014) ratios that are much higher than all commonly used natural reference materials. Thus, Grey Hill zircon is a useful natural reference material for LA-MC-ICP-MS Hf isotope measurement to guarantee accurate isobaric interference correction across the full spectrum of naturally occurring zircon grains. Grey Hill zircon concentrates can be requested from the authors.