Recent developments and applications of triple oxygen isotope measurements by secondary ion mass spectrometry

Oxygen isotopes are powerful proxies that are commonly used to decipher the formation of terrestrial and extraterrestrial rocks. Most of modern scientific approaches imply the determination of the oxygen isotopic composition at the mineral scale, thus requiring instruments enable to perform in situ, multi-collection, isotopic analyses in complex mineralogical assemblages and zoned minerals. Among them, large-geometry secondary ion mass spectrometer (LG-SIMS) is the most versatile with unique advantages such as (i) high spatial resolution (10–20 μm beam diameter and 1–2 μm depth); (ii) high sensitivity (detection limits below the ppm level for most elements) and (iii) high mass-resolution analysis allowing to remove most isobaric interferences (Villeneuve et al., 2019). Thanks to these capabilities, analytical uncertainties were significantly reduced for oxygen isotopes and reproducibilities much better that 1 ‰ on d¹⁷O and d¹⁸O are commonly obtained (e.g., Vacher et al. 2016; Marrocchi et al., 2018). Reaching such precisions is, however, linked to the use of 10¹¹ Ω Faraday Cups (FCs) that require minimum count rates of > 10⁶ cp/s for reaching permil precisions. This implies performing measurements with relatively large primary beam (i.e., 15-20 μm) that limits the minerals that can be targeted, especially in extraterrestrial samples (e.g., chondrule olivine crystals, Marrocchi et al., 2019).

Latest generation LG-SIMS instruments have been recently equipped with 10¹² Ω FCs that enable isotopic measurements to be performed at count rates significantly lower (i.e., 3 × 10⁵ cp/s) while maintaining good precision. This implies that high-precision oxygen isotopic measurements can be now performed with a less intense and smaller primary beam (~1 nA; 5 μm), In this contribution, we will report the specific characteristics of measurements using 10¹² Ω FCs and the reproducibilities obtained for oxygen isotope measurements. Few scientific examples where the use of 10¹² Ω FCs can represent a significant beakthrough will also be presented.

Marrocchi Y., Bekaert D.V. & Piani L. (2018). Origin and abundance of water in carbonaceous asteroids. Earth and Planetary Science Letters 482, 23-32.

Marrocchi Y., Euverte R., Villeneuve J., Batanova V., Welsch B., Ferrière L. & Jacquet E. (2019) Formation of CV chondrules by recycling of amoeboid olivine aggregate-like precursors. Geochimica et Cosmochimica Acta 247C, 121-141.

Villeneuve J., Chaussidon M., Marrocchi Y., Deng Z. & Watson B.E. (2019). High-precision silicon isotopic analyses by MC-SIMS in olivine and low-Ca pyroxene. Rapid Communication in Mass Spectrometry 33, 1589-1597.

Vacher L.G., Marrocchi Y., Verdier-Paoletti M., Villeneuve J. & Gounelle M. (2016) Inward radial mixing of interstellar water ices in the solar protoplanetary disk. The Astrophysical Journal Letters, 826, 1-6.