High-Spatial-Resolution Measurement Of Water Content In Olivine Using NanoSIMS 50L

Abstract

Olivine is the most abundant mineral in the planetary mantle. Its water content provides critical constraints on the processes and dynamics of the planetary interior. Olivine usually develops zonings with typical widths of 5-20 μm, which requires high spatial resolution. For secondary ion mass spectrometry (SIMS) measurements, primary beams with low currents were utilized to achieve high spatial resolution. However, this strategy also resulted in high background, which cannot be applied to nominally anhydrous minerals, e.g., olivine. Therefore, achieving high spatial resolution with low background is essential but challenging. For example, even though the NanoSIMS is designed for high-spatial-resolution measurements, the spatial resolution remained at > 10 μm for water content analysis in order to maintain a low water background of < 10 ppm. In this study, we optimized the primary beam settings and raster size for water content analysis of olivine using a CAMECA NanoSIMS 50L to improve the spatial resolution and the background simultaneously. Olivine standard samples (KLB-1, ICH-30, Mongok) with a water content ranging from 11.2 ppm to 70.6 ppm were measured for water content calibration with 1 H -/ 16 Oratio. San Carlos olivine with a water content of 1.42 ppm was used for background monitoring. The results showed that a spatial resolution of ~6 μm (primary beam size + raster size) with a background of 6 ± 2 ppm could be achieved by applying a Cs + beam current of 2 nA with a diameter of ~2 μm, rastering an area of 4 × 4 μm 2 . The analytical reproducibility of this method is better than 13% for standard samples with a water content of > 10 ppm. Overall, this method improved the spatial resolution for measuring water content by a factor of ~2 (in comparison to previous studies) and could be applied to olivine grains with complex zoning.

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“…Compared with the detected using Faraday Cup, the internal precision of the isotope measurement employing EMs is limited to shot noise (Poisson error) and independent of thermal/JN noise (Hao et al, 2022). Therefore, when the isotopic ratio is stable without drift, improving the signal statistics is key to reducing the shot noise (Poisson error) and internal presicon.…”