Submicron spatial resolution Pb–Pb and U–Pb dating by using a NanoSIMS equipped with the new radio-frequency ion source

Abstract

This paper is the first to report a Pb–Pb and U–Pb analytical method at a sub-micron scale using a NanoSIMS equipped with the new radio-frequency (RF) ion source. The RF ion source can generate a high-density primary beam to improve spatial resolution. A focused Gaussian beam of ∼450 pA with a diameter of ∼480 nm was utilized, significantly smaller in size than the 1.7 μm beam with the same current generated by the conventional duoplasmatron source. However, the high-density primary beam causes a more significant depth effect. Thus, a larger raster area (10 × 10 μm²) is selected to eliminate the mass fractionation caused by the depth effect. Other settings and data processing were similar to previous methods with a Duo source. Zircon standard M257 and baddeleyite standard Phalaborwa yielded ²⁰⁷Pb/²⁰⁶Pb ages of 547 ± 30 Ma and 2058 ± 13.8 Ma, respectively. These results agree well with the reference age values within analytical uncertainties. Using zircon Qinghu as a standard, zircon M257 and 91500 yielded U–Pb ages of 567.7 ± 6.8 Ma and 1056 ± 15 Ma, respectively, consistent with the recommended values within the analytical uncertainties. Three 4–5 μm zirconolite grains from the lunar meteorite Northwest Africa 8127 (NWA 8127) gave Pb–Pb ages of 3201 ± 25 Ma, 3174 ± 6 Ma and 3170 ± 14 Ma. One grain shows a variation of Pb/Pb age corresponding to the sub-micron scale distribution of U and Pb, probably due to the heavy impact on the moon. This technique is the first reported submicron Pb–Pb and U–Pb dating, which can be widely applied for sub-micron Pb–Pb and U–Pb dating for the tiny Zr-rich minerals from extraterrestrial samples.