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

We report a new method of Pb-Pb dating for zircon and baddeleyite with a lateral resolution of <2 mm and U-Pb dating for zircon with a lateral resolution of <5 mu m using CAMECA NanoSIMS 50L. The O- primary beam was optimized to a current of similar to 500 pA with a diameter of 1.7 mm. The zircon standard M257 and baddeleyite standard Phalaborwa were dated by the Pb-Pb method, yielding Pb-207/Pb-206 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 x 3 mu m(2) areas in order to eliminate pit depth-dependent U-Pb fractionation. The weighted average Pb-206/U-238 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 mu m, and U-Pb ages of zircon can be measured with a lateral resolution of <5 mu m 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.