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High‐spatial‐resolution measurements of iron isotopes in pyrites by secondary ion mass spectrometry using the new Hyperion‐II radio‐frequency plasma source

Abstract : Iron isotopic signatures in pyrites are considered as a good proxy to reconstruct paleoenvironmental and local redox conditions. However, the investigation of micro-pyrites less than 20µm in size has been limited by the evaluable analytical techniques. The development of the new brighter radio-frequency plasma ion source (Hyperion-II source) enhances the spatial resolution by increasing the beam density 10 times compared with the Duoplasmatron source. Here we present high-spatial-resolution measurements of iron isotopes in pyrites using a 3 nA–3 μm primary 16O− beam on two Cameca IMS 1280-HR2 ion microprobe instruments equipped with Hyperion sources at CRPG-IPNT (France) and at SwissSIMS (Switzerland). We tested analytical effects, such as topography and crystal orientation, that could induce analytical biases perceptible through variations of the instrumental mass fractionation (IMF). Results: The δ56Fe reproducibility for the Balmat pyrite standard is ±0.25‰ (2 standard deviations) and the typical individual internal error is ±0.10‰(2 standard errors). The sensitivity on 56Fe+ was 1.2 × 107 cps/nA/ppm or better. Tests on Balmat pyrites revealed that neither the crystal orientation nor channeling effects seem to significantly influence the IMF. Different pyrite standards (Balmat and SpainCR) were used to test the accuracy of the measurements. Indium mounts must be carefully prepared with a sample topography less than 2 μm, which was checked using an interferometric microscope. Such a topography is negligible for introducing change in the IMF. This new source increases the spatial resolution while maintaining the high precision of analyses and the overall stability of the measurements compared with the previous Duoplasmatron source. Conclusions: A reliable method was developed for performing accurate and highresolution measurements of micrometric pyrites. The investigation of sedimentary micro-pyrites will improve our understanding of the processes and environmental conditions during pyrite precipitation, including the contribution of primary (microbial activities or abiotic reactions) and secondary (diagenesis and/or hydrothermal fluid circulation) signatures.
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https://hal.univ-lorraine.fr/hal-03442558
Contributor : Béatrice Luais Connect in order to contact the contributor
Submitted on : Tuesday, November 23, 2021 - 11:01:11 AM
Last modification on : Wednesday, November 24, 2021 - 6:20:49 PM

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Marie‐noëlle Decraene, Johanna Marin‐carbonne, Anne‐sophie Bouvier, Johan Villeneuve, Nordine Bouden, et al.. High‐spatial‐resolution measurements of iron isotopes in pyrites by secondary ion mass spectrometry using the new Hyperion‐II radio‐frequency plasma source. Rapid Communications in Mass Spectrometry, Wiley, 2021, 35 (3), ⟨10.1002/rcm.8986⟩. ⟨hal-03442558⟩

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