Nickel isotopes in olivine from magmatic Ni-Cu deposits

Description

Nickel isotopic compositions in olivine, bulk sulfide, and sulfide minerals from the East Tianshan magmatic Ni-Cu sulfide deposits

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The major and trace element concentrations of the Huangshannan olivine were determined using EPMA and LA-ICP-MS at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, whereas that of the Huangshanxi and Huangshannan olivine were sourced from Mao et al. (2022). The detailed analytical conditions are similar to those in Mao et al. (2022). The Ni isotopic measurement was performed at the Isotope Geochemistry Laboratory of China University of Geosciences, Beijing. Around 50–200 mg separated grains (depending on the Ni concentration) were digested in Savillex screw-top beakers using a mixture of distilled concentrated HF + HNO3 (3:1) and put on a hotplate at 150°C for 24 hr. Subsequently, the solutions were evaporated until dry. The dissolution-evaporation process was repeated to remove fluoride. The solutions were then dried at 130°C and redissolved in 2 ml of distilled 6M HCl. Aliquots of sample solutions containing 500–800 ng Ni were spiked with a 61Ni-62Ni double spike to reach an optimal sample-spike ratio of 36:64. The mixtures were heated on a hotplate to ensure sample-spike equilibration before column chemistry. Nickel separation for sulfide and olivine samples was achieved using a three-stage exchange chromatography procedure using Bio-Rad AG 50W-X8 and AG1-X8 resins that have been described in Wang et al., (2023). Nickel isotopic ratios were determined on a Thermo Fisher Neptune MC-ICP-MS coupled with an Aridus II introduction system. The Neptune MC-ICP-MS was operated under a medium mass resolution mode to separate polyatomic interferences on 58Ni. The four Ni isotopes (62Ni, 61Ni, 60Ni, 58Ni) together with 57Fe were measured simultaneously on separate Faraday cups (H2, H1, Axial, L1, and L3). Samples were bracketed with double-spiked NIST SRM 986 with similar Ni concentration. The measured 57Fe was used to correct the 58Fe interference. Double-spike equations were applied to correct for isotope fractionation caused by instrument-induced mass bias and imperfect column yields. Each sample solution was measured at least three times in a sequence to yield a reliable average value. The Ni isotopic ratio is presented in the delta (δ) notation as per mil deviation (‰) relative to SRM 986. The analytical result of the bracketed standard SRM 986 and two in-house pure Ni solutions (WSJ-1 and WSJ-2) during this study is consistent with their reference values, and the precisions of the analyzed reference materials in this study are better than 0.04‰ (2SD). Reference: Mao, Y.-J., Schoneveld, L., Barnes, S.J., Williams, M.J., Su, B.-X., Ruprecht, P., Evans, N.J., Qin, K.-Z., 2022b. Coupled Li-P Zoning and Trace Elements of Olivine from Magmatic Ni-Cu Deposits: Implications for Postcumulus Re-Equilibration in Olivine. J Petrol 63(3), 1-22.

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