HF ISOTOPIC COMPOSITIONS OF THE STANDARD ZIRCONS AND BADDELEYITES USED IN U-PB GEOCHRONOLOGY

Abstract

Zircon and baddeleyite U-Pb geochronological dating is widely used in solid Earth sciences and the advent of rapid in-situ methods of analysis, such SIMS and ICP-MS, has re-emphasized the importance of having uniform standards. Recently, it has been shown that Hf isotopic data can provide important information on these minerals since they contain high concentrations of Hf, but have low Lu/Hf ratios, which results in negligible age correction. However, the complex internal structures that result from multiple thermal events, such as inherited cores and metamorphic overgrowths, require that the Hf isotopic data be measured with high spatial resolution. However, the isobaric interferences of 176Yb and 176Lu on 176Hf hamper the precise determination of the 176Hf/177Hf ratio during in-situ laser ablation MC-ICPMS analysis. It is shown here that mass biases of Yb (βYb) and Hf (βHf) change with time during analyses and behave differently for solutions and solid material. Therefore, it is suggested that the mean βYb value of the individual spot be used to obtain the precise isotopic composition for in-situ zircon and baddeleyite Hf isotopic analyses. For low Yb/Hf (176Yb/177Hf < 0.001) zircon and baddeleyite, the different methods used to obtain the βYb value have little effect on the accuracy of the Hf isotopic composition. However, the appropriate Yb isotopic abundance is necessary for high Yb/Hf (176Yb/177Hf > 0.001) zircons and baddeleyites, since the interference of 176Yb on 176Hf is significant. Using the mean βYb value of the individual spot and newly published Yb isotopic abundance data, six standard zircons and two standard baddeleyites, have been investigated using a Neptune MC-ICPMS, with 193 nm laser. For zircons, the obtained 176Hf/177Hf ratios are 0.282307 ± 31 (2SD) for 91500, 0.282680 ± 31 (2SD) for TEMORA, 0.281729 ± 21 (2SD) for CZ3, 282177 ± 17 (2SD) for CN92-1, 0.282983 ± 17 (2SD) for FM0411, and 281234 ± 11 (2SD) for Phalaborwa. The baddeleyites from Phalaborwa and SK10-2 have 176Hf/177Hf ratios of 0.281238 ± 12 and 0.282738 ± 13 (2SD). These results agree well with the values obtained by the solution method and indicate that these standards have different Hf isotopic compositions, in which the extremely low 176Lu/177Hf and 176Yb/177Hf values of CZ3 zircon and Phalaborwa baddeleyite make them excellent standards for machine calibration during in-situ zircon Hf isotopic measurement, with the other standards being more suitable for the development of the correction method. © 2006 Elsevier B.V. All rights reserved.