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.