Characterisation of Five Natural Magnetite Reference Materials for In Situ Iron Isotope Measurement with Application to Magmatic Ni-Cu Sulfide Mineralisation

Abstract

Iron isotope ratios in magnetite have been widely used to reveal critical geological and biological processes. Laser ablation multi-collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) is ideally suited for measurement of Fe isotope ratios, however the lack of suitable reference materials poses a significant challenge for in situ Fe isotopic measurements in magnetites. In this study, five high-quality natural magnetite crystals were characterised for Fe isotope ratios using solution nebulisation (SN)-MC-ICP-MS and LA-MC-ICP-MS. The effects of LA-MC-ICP-MS analytical conditions were investigated to obtain precise and accurate Fe isotope ratios. The yielded intermediate measurement precisions for the δ⁵⁶Fe values in the five investigated magnetites were ± 0.05–0.06‰ (2s) using SN-MC-ICP-MS and ± 0.08–0.14‰ (2s) using LA-MC-ICP-MS. Magnetites with homogeneous Fe isotopic compositions in hand-specimen measurements and microanalysis can serve as potential reference materials for in situ Fe isotopic measurement. Furthermore, the Fe isotope ratios in the magnetites from the Jinchuan Ni-Cu-PGE sulfide deposit were measured using LA-MC-ICP-MS with natural magnetite as the bracketing calibrator. The increase in the Fe isotopic composition with magmatic sulfide evolution was primarily dominated by oxygen fugacity (fO₂) and hydrothermal fluids. This finding implies that the Fe isotopic composition of magnetite can serve as a potential geochemical indicator of magmatic Ni-Cu sulfide mineralisation.