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

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 similar to 450 pA with a diameter of similar to 480 nm was utilized, significantly smaller in size than the 1.7 mu m 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 x 10 mu m(2)) 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 Pb-207/Pb-206 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 mu m 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.

Automated summary

This paper presents a novel Pb-Pb and U-Pb analytical method at a sub-micron scale using a NanoSIMS with a new RF ion source. The technique allows for high spatial resolution but may introduce depth effects, requiring adjustments in the scan area to eliminate mass fractionation. Results from zircon and lunar samples are in good agreement with reference ages within analytical uncertainties.