Comment: Hf-isotope heterogeneity in zircon 91500

A database of > 600 analyses of the zircon “standard” Harvard 91500 shows considerable heterogeneity in 176Hf/177Hf; the distribution is essentially bimodal with major peaks at 0.282284 ± 22 and 0.282330 ± 29 (2σ). Although the zircon shows a wide range of 176Yb/177Hf and 176Lu/177Hf, there is no correlation of 176Hf/177Hf with either parameter. This isotopic heterogeneity limits the degree to which 91500 can be used to evaluate the precision or accuracy of different treatments of mass bias and overlap corrections for in situ analysis of Hf-isotope compositions in zircons, or differences between solution and in situ data.