Precise micrometre-sized Pb-Pb and U-Pb dating with NanoSIMS

Abstract

We report a new method of Pb-Pb dating for zircon and baddeleyite with a lateral resolution of <2 μm and U-Pb dating for zircon with a lateral resolution of <5 μm using CAMECA NanoSIMS 50L. The O⁻ primary beam was optimized to a current of ∼500 pA with a diameter of 1.7 μm. The zircon standard M257 and baddeleyite standard Phalaborwa were dated by the Pb-Pb method, yielding ²⁰⁷Pb/²⁰⁶Pb ages of 563 ± 14 Ma and 2058 ± 6 Ma, respectively. These results agree well with the recommended ages within analytical uncertainties. Four zircon standards, including Qinghu, Plesovice, Temora, and 91500 were dated by the U-Pb method. The samples were measured in a scanning mode by rastering 3 × 3 μm² areas in order to eliminate pit depth-dependent U-Pb fractionation. The weighted average ²⁰⁶Pb/²³⁸U ages obtained by this method for the four zircon standards are 158 ± 3 Ma, 337 ± 7 Ma, 427 ± 10 Ma, and 1076 ± 14 Ma, respectively. These results are consistent with the reported ID-TIMS ages of these samples within error. Our study demonstrates that Pb-Pb ages of zircon and baddeleyite can be determined with a lateral resolution of <2 μm, and U-Pb ages of zircon can be measured with a lateral resolution of <5 μm by CAMECA NanoSIMS. This technique will have important applications to high lateral resolution dating, such as thin layers of zoned zircons, small grains of lunar zircon, and micron-sized baddeleyites in Martian meteorites and in various achondrites from differentiated asteroids.