Evaluation of fission-track and U-Pb double dating method for identical zircon grains:

概要

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Using the double-dating approach, we have conducted fission-track (FT) and U-Pb age dating on the same zircon grain to evaluate the reliability of the resulting age data. The zircon grains (OD-3) used for the investigation were collected from the Mihara Body of the Kawamoto Granodiorite, Shimane Prefecture, Japan. The zircon crystals yielded a FT age of 33 Ma, with a spontaneous FT density of 106–107 cm2. The U-Pb age data were obtained using inductively coupled plasma-mass spectrometry (ICP-MS) combined with a frequency quintupled Nd-YAG (λ= 213 nm) laser ablation (LA) sample introduction system. The U-Pb age determinations on the zircon samples were performed after chemical leaching using 47% HF for 20 hrs at room temperature, or after FT etching using a KOH–NaOH eutectic solution for 20 hrs at 225 °C. The resulting U-Pb age of 33 Ma was concordant with the FT age data, suggesting that there was no significant loss of Pb during etching or chemical preparation, thus indicating the feasibility of FT and U-Pb double dating of single zircon grains. The results indicate that the zircon grains (OD-3) from the Kawamoto Granodiorite (Mihara Body) can become a new working standard for precise U-Pb age determinations of young zircons.