New Potential Sulfide Reference Materials for Microbeam S-Fe-Cu Isotope Measurements

Abstract

S-Fe-Cu isotope systems are powerful tracers for revealing geochemical processes. However, the microanalysis of S-Fe-Cu isotopes is critically limited by the lack of suitable reference materials. Herein, we present three potential reference materials Ll-Cpy (chalcopyrite), Ll-Po (pyrrhotite) and Ll-Sp (sphalerite) for in situ S-Fe-Cu isotope measurements. Numerous in situ S-Fe-Cu isotope measurements were performed over two years to assess isotopic homogeneity. The bulk S isotopic compositions were determined independently in seven laboratories by isotope ratio mass spectrometry (IRMS); the preferred δ³⁴S_V-CDT for Ll-Cpy, Ll-Po, Ll-Sp are 6.13 ± 0.37‰ (2s), 6.42 ± 0.37‰ (2s) and 6.28 ± 0.38‰ (2s), respectively. The bulk Fe isotope ratios in Fe-bearing Ll-Cpy and Ll-Po were determined using solution nebulisation multi-collector inductively coupled plasma-mass spectrometry, and the obtained δ⁵⁶Fe_IRMM-014 values are 0.57 ± 0.07‰ (2s) and -0.62 ± 0.07‰ (2s), respectively. The mean bulk δ⁶⁵Cu_{NIST SRM 976} value of Ll-Cpy is 0.57 ± 0.06‰ (2s). All the bulk values are in good agreement with the long-term statistical results of laser ablation-MC-ICP-MS and proposed as the recommended values. These sulfides are well characterised and isotopically homogeneous (at 30–40 μm spatial resolution), and can be used as potential calibration materials for in situ S-Fe-Cu isotope measurements.