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

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 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.

Atomic spectrometry update: review of advances in atomic spectrometry and related techniques

“…Kang et al 16 developed a related but slightly different approach. They immobilised Ln labelled (with 159 Tb, 165 Ho, 175 Lu) DNA strands on streptavidin-modied magnetic beads.…”

“…This resulted in v 56 Fe reproducibility of AE0.1& (2SD) on pyrite, manganite, hematite and chalcopyrite. Improvements in Fe isotope measurements by SIMS was the subject of a study by Decraene et al 175 They used a new RF plasma ion source, Hyperion-II, which increased the ion beam density by an order of magnitude compared to a conventional Duoplasmatron source. The aim of the study was to enable measurement of micro-pyrites, which are less than 20 mm, and results indicated that reproducibility for v 56 Fe was AE0.25& (2SD) on the Balmat pyrite standard.…”

Tianyu-Py pyrite: a new natural reference material for micro-beam determination of iron isotopic ratios

“…Therefore, the development of well-characterized natural standards is critical for accurate micro-beam analysis of Fe isotope analyses in pyrite. Before our study, one sintering pyrite material PAS-Py600 and several natural pyrite standards, such as Balmat, 35,64,65 JMC, 66 SpainCr, 55 Aa018, 59 have been used for micro-beam analyses of Fe isotopic composition (Table 1). It should be noted that some degree of natural variation is expected and observed in all these pyrite standards, which depends in part on the number/scale of characterization tests.…”

“…Laser ablation MC-ICP-MS (LA-MC-ICP-MS) and secondary ion mass spectrometry (SIMS) are powerful micro-beam techniques for in situ Fe isotopic analyses in pyrite. 34,35,[55][56][57][58][59][60] The micro-beam technique is capable of resolving Fe isotopic variations between natural minerals commonly present as thin lamellae in iron meteorites, micron scale phases in sedimentary, and zonation in hydrothermal deposits. However, both LA-MC-ICP-MS and SIMS analyses generate isotopic ratios that have been fractionated during the examinations, and this fractionation is matrixdependent (e.g., ref.…”

Mineralogical And Geochemical Characterization At A Micro-Scale Of A New Natural Chalcopyrite Reference Material For In-Situ Fe Isotopic Ratio Analysis

“…21,23 SIMS has been used for in-situ Fe isotope determination with high spatial resolution. 19,24 However, the significant effects of matrix and crystal orientation of some high-symmetry minerals, such as Fe isotope analysis of magnetite (Fe3O4), have limited its application. 25 In contrast, the effects of crystal orientation do not affect the LA-MC-ICP-MS analysis.…”