In-situ measurements of iron isotopes by SIMS: MC-ICP-MS intercalibration and application to a magnetite crystal from the Gunflint chert

An analytical procedure is described for in situ measurements of iron isotope compositions of different sulfides and oxides with a spatial resolution of 15–20 μm using a Cameca IMS 1270 ion microprobe (SIMS). The 54 Fe and 56 Fe isotopes were measured on two Faraday cups. The isobaric 54 Cr interference on 54 Fe was corrected by measuring 53 Cr and 52 Cr intensities. For magnetite crystals, which contain some chromium, this correction is consistent at 0.11 ± 0.06‰. A SIMS instrumental mass fractionation was observed between oxides and sulfides, and this instrumental mass fractionation decreases with the oxidation state. The iron isotope composition of various reference materials, including hematite, pyrite, siderite and magnetite, were intercalibrated with those obtained by MC-ICP-MS. The SIMS δ 56 Fe reproducibility obtained on these various reference materials is typically 0.2–0.3‰ at 2σ; the MC-ICP-MS δ 56 Fe reproducibility is about 0.1‰ (2σ). This analytical procedure was applied to a Precambrian magnetite grain from the Gunflint Iron formation (1.88 Ga, Canada). This magnetite shows a large range of iron isotope compositions of about 4‰ between the rim and the core. We have also measured the oxygen isotope composition of this magnetite with the new IMS 1280 HR2 (CRPG–CNRS, Nancy), and observed no relation between δ 56 Fe and δ 18 O. The magnetite rim exhibits some extreme δ 56 Fe values of + 5.2‰ never reported before. This result emphasizes the crucial interest of in situ-measurements for studying the processes of Fe isotopic fractionation in depositional environments.