Re-Os and HSE in individual base metal sulfide grains: Evaluating micro-analytical procedures using a sulfide reference material

Two Fe-Ni sulfides, resembling the chemical composition of a typical mantle base metal sulfide (BMS), were synthesized and used to test micro-analytical procedures (sample digestion and chemical separation) to determine 187Os/188Os and highly siderophile element (HSE) mass fractions in single BMS grains. The bulk 187Os/188Os and HSE mass fractions of the synthetic sulfides were independently determined after high pressure asher (HPA) digestion and conventional HSE separation (Os solvent extraction and Ir, Ru, Pt, Pd, Re separation via anion exchange chromatography), while the homogenous distribution of HSE was assessed via LA-ICP-MS. Tests were performed following the protocol of Pearson et al. (1998), where sulfides are digested in H2SO4/CrO3 and Os is simultaneously extracted as OsO4. Additional tests were performed adding a pre-digestion step in HBr and/or HCl. The duration and the temperature of the pre-digestion and micro-distillation steps, as well as different chromatographic separations of the HSE were also evaluated. While the majority of the tests yielded 187Os/188Os in agreement with that obtained after HPA digestion, HSE mass fractions show large deviations from the reference content, depending on the used procedure. Such variations are interpreted as the result of incomplete sulfide digestion, Os spike loss, and the possible presence of undigested sub-micrometric platinum group minerals (PGM). Overall, the bulk sulfide HSE mass fractions obtained after HPA digestion are best reproduced (mean deviations ≤10%) using a pre-digestion step in HBr + HCl at 120 °C. This study highlights the need for matrix-matched sulfide reference materials for routine use in laboratories determining HSE mass fractions and Os isotope ratios on single BMS grains. Such an approach is fundamental for comparing and compiling Re-Os ages collected on BMS in many different laboratories. This study also demonstrates that there is need for further testing and for methodological developments/improvement of the analytical procedure(s) used for Re-Os dating and HSE mass fraction determinations in single BMS grains.