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 size has been limited so far by 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 to the Duoplasmatron source.Methods: Here we present high spatial resolution measurements of iron isotopes in pyrites using a 3nA-3µm primary 16 Obeam on two ion microprobes Cameca IMS 1280-HR2 at CRPG-IPNT (France) and at SwissSIMS (Switzerland) equipped with Hyperion sources. 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 δ 56 Fe 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 56 Fe + was 1.2x10 7 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 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 to the 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 (diagenesis and/or hydrothermal fluid circulation) signatures.

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