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Archean spherule layers represent the only currently known remnants of the early impact record on Earth. Based on the lunar cratering record, the small number of spherule layers identified so far contrasts to the high impact flux that can be expected for the Earth at that time. The recent discovery of several Paleoarchean spherule layers in the BARB5 and CT3 drill cores from the Barberton area, South Africa, drastically increases the number of known Archean impact spherule layers and may provide a unique opportunity to improve our knowledge of the impact record on the early Earth. This study is focused on the spherule layers in the CT3 drill core from the northeastern Barberton Greenstone Belt. We present highly siderophile element (HSE: Re, Os, Ir, Pt, Ru, and Pd) concentrations and Re-Os isotope signatures for spherule layer samples and their host rocks in order to unravel the potential presence of extraterrestrial fingerprints within them. Most spherule layer samples exhibit extreme enrichments in HSE concentrations of up to superchondritic abundances in conjunction with, in some cases, subchondritic present-day Os-187/Os-188 isotope ratios. This indicates a significant meteoritic contribution to the spherule layers. In contrast to some of the data reported earlier for other Archean spherule layers from the Barberton area, the CT3 core is significantly overprinted by secondary events. However, HSE and Re-Os isotope signatures presented in this study indicate chondritic admixtures of up to (and even above) 100% chondrite component in some of the analyzed spherule layers. There is no significant correlation between HSE abundances and respective spherule contents. Although strongly supporting the impact origin of these layers and the presence of significant meteoritic admixtures, peak HSE concentrations are difficult to explain without postdepositional enrichment processes.

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 develo, SCOPUS: ar.j, info:eu-repo/semantics/published