High Precision And Resolution Chlorine Isotopic Analysis Of Apatite Using NanoSIMS

Abstract

This paper reports a high analytical precision method for measuring the Cl 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 ~400 pA (~Φ 700 nm), 1 nA (~Φ 1μm), and 2 nA (~Φ 1.5 μm) were used with different raster sizes (4 × 4 μm2, 8 × 8 μm2, and 10 × 10 μm2, 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 ~10 μm, the external precision of each session varied from 0.06 to 0.48 ‰ (1SD) 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 ‰. Using 1 nA and 0.4 nA with spatial resolutions of ~8 μm and 5 μm, the external precision for standards with Cl contents >1 % ranged from 0.12 to 0.2 ‰ and 0.10 to 0.33 ‰, 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.