Separation of U, Pb, Lu, and Hf from single zircons for combined U-Pb dating and Hf isotope measurements by TIMS and MC-ICPMS

The U-Pb system has long been used to precisely date zircons because the high U-to-common-Pb ratio in zircon produces very radiogenic Pb isotope compositions over time. In contrast to U/Pb, zircon has very low Lu/Hf and therefore unradiogenic Hf, making this mineral ideally suited for determining the initial Hf composition of its original host rock. A new chemical separation technique presented here enables the determination of both U-Pb age and initial Hf isotope composition of individual zircon grains. The acquisition of such complementary information for single detrital zircons is especially useful for provenance analyses and crustal growth studies. Zircons are spiked with mixed ¹⁷⁶ Lu- ¹⁸⁰ Hf and ²³³ U- ²⁰⁵ Pb tracers and then digested in HF-HNO ₃ . Lead, Lu, U, and Hf are sequentially separated from the zircon matrix on a single ion exchange column filled with Eichrom® Ln Spec resin. Using only ∼100 μl of resin for the separation keeps Pb blanks low (∼5 pg) while achieving better than 90% yields for each of the four elements. Hafnium isotope compositions and Lu concentrations are measured with multiple collector-inductively coupled plasma-mass spectrometry (MC-ICPMS), whereas U and Pb are analyzed by thermal ionization mass spectrometry (TIMS). The minimum grain size that can be processed is dictated by the amounts of Pb and Hf needed for an analysis. The smallest grains we currently analyze, as small as 50 μm (∼12pg of Pb and ∼3ng of Hf), can be analyzed with an external ¹⁷⁶ Hf/ ¹⁷⁷ Hf precision of ∼100 ppm (2 s.d.). The utility of this method is demonstrated with a population of detrital zircons from a Cambrian sediment of the Takaka Terrane, New Zealand. In addition, the technique has been used for 14 analyses of the standard zircon 91500, which yield a mean present-day ¹⁷⁶ Hf/ ¹⁷⁷ Hf of 0.282305 ± 12 (2 s.d., i.e., an external reproducibility of 43 ppm). The Hf isotopic compositions and U-Pb ages presented here are in good agreement with those of previous studies (e.g., Wiedenbeck, M., Alle, P., Corfu, F., Griffin, W.L., Meier, M., Oberli, F., von Quadt, A., Roddick, J.C., Spiegel, W., 1995. Three natural zircon standards for U-Th-Pb, Lu-Hf, Trace element and REE analyses. Geostandards Newsletter 19, 1-23.), but our data suggest that the 91500 zircon is heterogeneous with respect to Lu and Hf contents and Lu/Hf.