High Precision and Resolution Chlorine Isotopic Analysis of Apatite Using NanoSIMS

This paper reports a high analytical precision method for measuring the CI isotopic composition of apatite using a CAMECA NanoSIMS 50L with Faraday cup detectors (FC). Six apatite standard samples were used for calibration. Three sets of focused Cs+ beams with currents of similar to 400 pA (similar to Phi 700 nm), 1 nA (similar to Phi 1 mu m), and 2 nA (similar to Phi 1.5 mu m) were used with different raster sizes (4 x 4 mu m(2), 8 x 8 mu m(2), and 10 x 10 mu m(2), respectively). We performed depth effect experiments with three primary beam settings to optimize the instrument configuration. While using a primary beam of 2 nA with a raster size of similar to 10 mu m, the external precision of each session varied from 0.06 to 0.48 parts per thousand (1 SD) depending on the Cl concentration of the measured apatite. For standards with a Cl content higher than 0.5%, the external precision was mostly higher than 0.2 parts per thousand. Using 1 nA and 0.4 nA with spatial resolutions of similar to 8 mu m and 5 mu m, the external precision for standards with Cl contents > 1% ranged from 0.12 to 0.2 parts per thousand and 0.10 to 0.33 parts per thousand, respectively. This technique enables high-precision measurements of chlorine isotopes with high spatial resolutions and can be applied to small apatite minerals in extraterrestrial samples.

Automated summary

This paper presents a high analytical precision method for measuring the CI isotopic composition of apatite using a CAMECA NanoSIMS 50L instrument with Faraday cup detectors. By optimizing the instrument configuration and adjusting the primary beam settings, high-precision measurements of chlorine isotopes with high spatial resolutions can be achieved, which can be applied to small apatite minerals in extraterrestrial samples.