Direct measurement of Fe isotope compositions in iron-dominated minerals without column chromatography using MC-ICP-MS

Abstract

This study presents a practical method for high-precision Fe isotope determination without column chromatography purification for iron-dominated mineral samples using multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). A series of comparable experiments (with or without column chromatography) were performed to evaluate whether non-deviated Fe isotope ratios can be obtained directly by MC-ICP-MS using standard-sample bracketing (SSB) without column chromatography purification. Forty iron-dominated material/mineral samples (pure iron wire, pyrite, chalcopyrite, magnetite, hematite, siderite, goethite, pyrrhotite, arsenopyrite, specularite, ilmenite, limonite, lollingite, ilvaite, and Fe–Mn nodule) were investigated, and the results showed little bias in δ⁵⁶Fe values compared with those of mineral materials via column chromatography purification, with Δδ⁵⁶Fe_{without–with} ranging from −0.06‰ to +0.06‰. This means that Fe isotope ratios in Fe-dominated minerals can be achieved without column chromatography, due to the simple matrix and the stability of the machine by using the SSB method in high resolution wet plasma mode. The concentration effect and nitric acidity mismatch effects, as well as the matrix effect, were strictly assessed by MC-ICP-MS in high resolution wet plasma mode. Finally, a long-term reproducibility of better than ± 0.05‰ (n = 123, 2 sd) was achieved by repeatedly measuring pyrite without column chromatography over 6 months. Compared with the column chromatography purification method, the without column chromatography method significantly simplifies the chemical procedure and improves the efficiency of iron-dominated mineral Fe isotope analysis.