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

RATIONALE: Iron isotopic signatures in pyrites are considered as a good proxy for reconstructing paleoenvironmental and local redox conditions. However, the investigation of micro-pyrites less than 20 µm in size has been limited by the available analytical techniques. The development of a 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 3 nA-3 µm primary 16 O- 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 δ56 Fe 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 56 Fe+ 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 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 the contribution of primary (microbial activities or abiotic reactions) and secondary (diagenesis and/or hydrothermal fluid circulation) signatures.

Cellular remains in a ~3.42-billion-year-old subseafloor hydrothermal environment.

Subsurface habitats on Earth host an extensive extant biosphere and likely provided one of Earth's earliest microbial habitats. Although the site of life's emergence continues to be debated, evidence of early life provides insights into its early evolution and metabolic affinity. Here, we present the discovery of exceptionally well-preserved, ~3.42-billion-year-old putative filamentous microfossils that inhabited a paleo-subseafloor hydrothermal vein system of the Barberton greenstone belt in South Africa. The filaments colonized the walls of conduits created by low-temperature hydrothermal fluid. Combined with their morphological and chemical characteristics as investigated over a range of scales, they can be considered the oldest methanogens and/or methanotrophs that thrived in an ultramafic volcanic substrate.