Hf isotopic compositions of the standard zircons for U-Pb dating

Abstract

Using the newly published Yb isotopic abundances and the mass bias relationship between Yb and Hf, we carried out an analysis of Hf isotopes in the standard zircon 91500 by means of 193 nm laser attached to Neptune multi-collector ICP-MS (LA-MC-ICPMS). The obtained Hf isotopic data, in either in situ or line scan modes, are not only identical for different spot sizes, but also are consistent with previously published results obtained on TIMS or other MC-ICPMS machines within errors. This indicates that it is possible to obtain reliable 176Hf/177Hf isotopic ratios for zircon in either in situ or line scan conditions on LA-MC-ICPMS machine, and the applied procedures in our study for elemental interfering correction are appropriate for the purpose of acquiring satisfactory accuracy for Hf isotope analyses. The Hf isotopic compositions of four zircon standards in high spatial resolution U-Pb dating, 91500, CZ3, CN92-1 and TEMORA, are measured, respectively. The obtained 176Hf/177Hf ratios are 0.282316±4 (n = 34, 2~for 91500, 0.281704±6 (n = 16, 2~for CZ3, 0.282200±6 (n = 20, 20") for CN92-1 and 0.282684±14 (n = 24, 20) for TEMORA, respectively, with 176Lu/177Hf ratios of -0.00031, 0.000036, 0.00083 and 0.00127. Zircons 91500 and CZ3 show narrower variations in 176Hf/77Hf and 176Lu/I77Hf ratios than those of zircons CN92-1 and TEMORA, and thus are appropriate standards for the Hf isotope analysis.Zircon, a connnon accessory mineral in various kinds of rocks, is an important subject for V-Pb isotopic dating. With requirements for high spatial resolution with high precision and accuracy, SHRIMP (Sensitive High Resolution Ion Micro Probe) and LA-ICPMS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) now serve as the important tools in zircon V-Pb dating. Since both methods need standard zircon for external age calibration, an ideal standard zircon is critical for obtaining accurate and precise age for zircon analyzed. In addition, zircon is also an ideal mineral for Hf isotope analysis since it (ZrSi0 4 ) normally contains 0.5%-2% (weight percent) Hf. Among the 6 isotopes of Hf element, 176Hf is a radiogenic isotope yielded by beta decay of 176Lu. Due 1642 to extremely low Lu/Hf ratio in zircon (normally 176Lu/ 177 Hf ratio < 0.002), the 17~f almost has remained constant since the zircon crystallized. Therefore, the 176Hf/ 177 Hf ratio of zircon can be regarded as its initial value when the zircon formed, providing important information for deciphering its origination [I].However, the high electrical potential hampers the analysis of Hf isotope. Generally, the Hf isotope measurements can be carried out by N-TIMS (Negative Thermal Ionization Mass Spectrometry) through dissolving the bulk grain and separating the pure Hf element from other interfering elements and matrix elements. But this purification is laborious and time-consuming, and the special skills are needed as well. Moreover, any spatial information of Hf isotope variation in zircon cannot be achieved because the whole grain of zircon is destroyed....

Geochronological and geochemical features of the Cathaysia block (South China): New evidence for the Neoproterozoic breakup of Rodinia

“…This machine is a double focusing multi-collector ICPMS and has the capability of high mass resolution measurements in multiple collector modes. The analytical techniques are similar to those described in detail by Xu et al (2004) and Wu et al (2006 ( Nowell et al, 1998, Vervoort and Blichert-Toft, 1999and Griffin et al, 2004. A two-stage Hf model age (T DM2 ) is calculated based on assuming 176 Lu/ 177 Hf of average crust is 0.015 ( Griffin et al, 2004 and; for zircons with negative ɛ Hf (t) value, the model age T DM2 is more reasonable than the model age T DM1 .…”

Hf isotopes of zircon megacrysts from the Cenozoic basalts in eastern China

“…[27]. Before analysis, the zircons were cut into two pieces with one of them being finely polished for cathodoluminescence imaging, electron microprobe and trace element analysis.…”

U–Pb ages and Hf isotopic composition of zircons and whole rock geochemistry of volcanic rocks from the Fangniugou area: Implications for early–middle Paleozoic tectonic evolution in Jilin Province, NE China

“…Helium was used as the carrier gas to transport the ablated sample from the laser-ablation cell to the MC-ICP-MS torch and was mixed with argon. The analytical procedures are similar to those described by Xu et al (2004) (Griffin et al, 2000). Two-stage model ages (T DM2 ) were calculated using an assumed 176 Lu / 177 Hf ratio of 0.015 (Rudnick and Gao, 2003) for the average continental crust.…”