Zircon Hf model age: new insights on the 176 Lu/ 177 Hf crustal source estimation

The 176Lu/177Hf crustal composition remains one of the most controversial factors in the calculation of zircon hafnium two-stage model ages. The mafic, felsic and bulk crustal isotopic compositions currently employed in the literature show substantial differences in their values, which significantly influence the resulting model ages. In this study, we provide a dataset of Lu/Hf and 176Lu/177Hf whole-rock compositions covering a wide range of rock types and compare our results with previous studies. The analysis of these results denotes that Lu/Hf composition of rocks is primarily controlled by the lutetium and hafnium content of the minerals. We then analysed different rock proportion models for crustal layers and estimate their 176Lu/177Hf composition for zircon hafnium two-stage model age calculations: 0.0111 (upper crust), 0.0127 (middle crust), 0.0148 (lower crust), and 0.013 (bulk crust). The implications of the variations in depleted mantle evolution and 176Lu/177Hf crustal source composition on the two-stage model ages were assessed through a case study using coupled U–Pb and Lu–Hf zircon analyses. The comparisons show that both variables can produce significant differences in the two-stage model ages results. However, the 176Lu/177Hf recommended in this work show less variation than the commonly employed, resulting in smaller differences between model ages calculated using felsic/upper and mafic/lower crustal reservoirs. Although constraining zircon Hf model ages remains a challenging task, the new 176Lu/177Hf crustal values proposed here provide a more comprehensive framework for selecting and applying this parameter.