Analysis of Fe isotopes in sulfides and iron meteorites by laser ablation high-mass resolution multi-collector ICP mass spectrometry

Abstract

The isotopic composition of Fe in iron meteorites and iron sulfides was studied with a UV laser (213 nm wavelength) and a high-mass resolution multicollector-ICP-MS. The long term reproducibility of Fe isotopic measurements was 0.16 and 0.21‰ (2 standard deviations) for δ⁵⁶Fe and δ⁵⁷Fe, respectively, i.e., a factor of ∼2 worse than conventional solution analysis. At present the major limitations of precision and accuracy in the laser ablation technique are instrumental mass discrimination (2.0–4.9% per amu) and laser-induced fractionation (up to 4‰ during a single analysis) of Fe isotopes. Laser ablation of iron metal and iron sulfides produces two types of particles: (1) coarse (50–600 nm) spherical and crystalline particles that are isotopically heavier than (2) fine (2–7 nm) amorphous particles. Differential transport of the two types of particles to the ICP can compromise the accuracy of Fe isotope measurements by laser ablation MC-ICP-MS. Despite lower precision compared with solution MC-ICP-MS analysis, the laser ablation technique is capable of resolving Fe isotopic variations between seafloor and sub-seafloor hydrothermal sulfides, and between kamacite and taenite alloys that are commonly present as thin lamellae in iron meteorites. The most important applications of the technique will be for the analysis of small samples where the textural context of the analysed objects is important for the interpretation of Fe isotopic data.