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 Caméca IMS 1270 ion microprobe (SIMS). The 54Fe and 56Fe isotopes were measured on two Faraday cups. The isobaric 54Cr interference on 54Fe was corrected by measuring 53Cr and 52Cr 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 δ56Fe reproducibility obtained on these various reference materials is typically 0.2–0.3‰ at 2σ; the MC-ICP-MS δ56Fe 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 δ56Fe and δ18O. The magnetite rim exhibits some extreme δ56Fe 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.

Research highlights
► Development of in situ iron isotopes measurements by ion microprobe.
► Analyses of various reference materials; oxide, sulfides and iron carbonate.
► high reproducibility of 0.2‰ to 0.3‰ (2 s) obtained on reference material.
► Intercalibration with MC-ICP-MS.
► Application to a magnetite from the Gunflint Iron Formation.