Comment: Hf-isotope heterogeneity in zircon 91500

A database of > 600 analyses of the zircon standard Harvard 91500 shows considerable heterogeneity in Hf-176/Hf-177, the distribution is essentially bimodal with major peaks at 0.282284 +/- 22 and 0.282330 +/- 29 (2 sigma). Although the zircon shows a wide range of Yb-176/Hf-177 and Lu-176/Hf-177, there is no correlation of Hf-176/Hf-177 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. (c) 2006 Elsevier B.V. All rights reserved.