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High spatial resolution measurements of iron isotopes in pyrites by SIMS using the new Hyperion‐II Radio‐Frequency Plasma source

Decraene, M.N. ; Marin-Carbonne, J. ; Bouvier, A.S. ; Villeneuve, J. ; Bouden, N. ; Luais, B. ; Deloule, E., Rapid Comm. Mass Spectr.

High spatial resolution measurements of iron isotopes in pyrites by SIMS using the new Hyperion‐II Radio‐Frequency Plasma source

Decraene, M.N. ; Marin-Carbonne, J. ; Bouvier, A.S. ; Villeneuve, J. ; Bouden, N. ; Luais, B. ; Deloule, E.

Rapid Communications in Mass Spectrometry, 2020, e8986

Abstract :

Rationale

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. Methods

Here we present high spatial resolution measurements of iron isotopes in pyrites using a 3nA‐3μm 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‰ (2SD, standard deviation) and the typical individual internal error is ±0.10‰ (2SE, standard error). The sensitivity on 56Fe+ was 1.2x107cps/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 sample topography < 2μm, 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

We developed a reliable method to perform accurate and high‐resolution measurements of micrometric pyrites. The investigation of sedimentary micro‐pyrites will improve our understanding of the processes and environmental conditions during pyrite precipitation, including contribution of primary (microbial activities or abiotic reactions) and secondary signatures (diagenesis and/or hydrothermal fluid circulation).

Voir en ligne : https://doi.org/10.1002/rcm.8986




publié jeudi 26 novembre 2020