NanoSIMS imaging method of zircon U-Pb dating

We report an imaging method of zircon U-Pb dating with NanoSIMS 50L, which overcomes the significant U-Pb fractionation as the pit was sputtered deeper during conventional spot mode analysis and can be applied to irregular small grains or heterogeneous areas of zircon. The U-Pb and Pb-Pb ages can be acquired simultaneously for 2 mu mx2 mu m (for small grains) or 1 mu mx9 mu m (for zoned grains), together with Zr, Y and other trace elements distributions. Using zircon M257 as standard, the U-Pb ages of other zircon standards, including Qinghu, Plesovice, Temora and 91500, were measured to (2 sigma) as 158.8 +/- 0.8, 335.9 +/- 3.4, 412.0 +/- 12 and 1067 +/- 12 Ma, respectively, consistent with the recommended values within the analytical uncertainties. Tiny zircon grains in the impact melt breccia of the lunar meteorite SaU 169 were also measured in this study, with a Pb-Pb age of 3912 +/- 14 Ma and a U-Pb age of 3917 +/- 17 Ma, similar to previous results reported for the same meteorite. The imaging method was also applied to determine U-Pb age of the thin overgrowth rims of Longtan metamorphic zircon, with a Pb-Pb age of 1933 +/- 27 Ma and a U-Pb age of 1935 +/- 25 Ma, clearly distinct from the Pb-Pb age of 2098 +/- 61 Ma and the U-Pb age of 2054 +/- 40 Ma for detrital cores.