SHRIMP U-Pb age of SoriZ93 zircon from the Sori Granodiorite, Northeast
Japan: a potential reference zircon of Late Cretaceous age
- SoriZ93 zircon was separated from residual mineral fraction after the preparation of the SORI93 biotite standard from the Sori Granodiorite in the Ashio Mountains, Northeast Japan, and analyzed for its U-Pb age using a sensitive high resolution ion microprobe (SHRIMP). The zircon grains of SoriZ93 are prismatic with pyramidal ends or broken prismatic fragments. Most zircons are 100-250m long and 50-150m wide. The zircons are clear crystals and colorless to pale yellow, although some grains are brown with optically low transparency. Cathodoluminescence (CL) imaging of the SoriZ93 zircons showed a fine oscillatory zoning, which is a typical characteristic of zircons in granitic rocks. A xenocrystic core was not present in the zircons. Although some mineral inclusions were present in the zircons, it is possible to select a typical analytical area with a dimension of 30m necessary for the microbeam technique. The analytical results of the colorless zircons provided a weighted mean Pb-207 corrected Pb-206/U-238 age of 93.9 +/- 0.6Ma (95%SoriZ93 zircon was separated from residual mineral fraction after the preparation of the SORI93 biotite standard from the Sori Granodiorite in the Ashio Mountains, Northeast Japan, and analyzed for its U-Pb age using a sensitive high resolution ion microprobe (SHRIMP). The zircon grains of SoriZ93 are prismatic with pyramidal ends or broken prismatic fragments. Most zircons are 100-250m long and 50-150m wide. The zircons are clear crystals and colorless to pale yellow, although some grains are brown with optically low transparency. Cathodoluminescence (CL) imaging of the SoriZ93 zircons showed a fine oscillatory zoning, which is a typical characteristic of zircons in granitic rocks. A xenocrystic core was not present in the zircons. Although some mineral inclusions were present in the zircons, it is possible to select a typical analytical area with a dimension of 30m necessary for the microbeam technique. The analytical results of the colorless zircons provided a weighted mean Pb-207 corrected Pb-206/U-238 age of 93.9 +/- 0.6Ma (95% confidence, MSWD=0.97). This Pb-206/U-238 age is 1.3m.y. older than the K-Ar age of the SORI93 biotite, indicating that the granodiorite cooled to a closure temperature of the K-Ar biotite system within a short time interval. Although some grains of the SoriZ93 zircons show high U concentration, a selection of colorless zircons provided the precise age to be used for the calibration and reference for zircons of the Late Cretaceous.…
