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Archean spherule layers, resulting from impacts by large extraterrestrial objects, to date represent the only remnants of the early meteorite, asteroid, and comet bombardment of the Earth. Only few Archean impact debris layers have been documented, all of them embedded in the 3.23–3.47 billion year old successions of the Barberton Greenstone Belt (BGB) in South Africa and the Pilbara Craton in Western Australia. Some of them might be correlated with each other. Given the scarcity of Archean spherule deposits, four spherule layer intersections from the recently recovered BARB5 drill core from the central Barberton Greenstone Belt, analyzed in this study, provide an opportunity to gain new insight into the early terrestrial impact bombardment. Despite being hydrothermally overprinted, siderophile element abundance signatures of spherule-rich samples from the BARB5 drill core, at least in part, retained a meteoritic fingerprint. The impact hypothesis for the generation of the BARB5 spherule layers is supported by correlations between the abundances of moderately (Cr, Co, Ni) and highly siderophile (Re, Os, Ir, Pt, Ru and Pd) elements, whose peak concentrations and interelement ratios are within the range of those for chondrites. Rhenium-Osmium isotope evidence further support the impact hypothesis. Collectively, this study provides evidence for extraterrestrial admixtures ranging between ∼40 and up to 100% to three of the four analyzed BARB5 spherule layers, and a scenario for their genesis involving (i) impact of a chondritic bolide into a sedimentary target, (ii) varying admixtures of meteoritic components to target materials, (iii) spherule formation via condensation in an impact vapor plume, (iv) transportation of the spherules and sedimentation under submarine conditions, followed by (v) moderate post-impact remobilization of transition metals and highly siderophile elements. [ABSTRACT FROM AUTHOR]

The first 2.5 billion years of the terrestrial impact history are not documented by any impact structures. Only a few spherule layers of impact origin are known, most of them of late Archean to early Proterozoic age. In the Barberton Greenstone Belt (South Africa), several spherule horizons (layers S1 to S4, possibly up to S8, with ages between ∼3.5 and ∼3.2 Ga) are amongst the oldest deposits from large bolide impacts onto Earth. Impact evidence is limited to (highly) elevated siderophile element contents and Cr isotopic compositions. Other isotope tools, such as the 187 Re- 188 Os radionuclide system in combination with high-precision concentration data for siderophile elements, might be useful to confirm the propositions regarding the presence of meteoritic components made so far. Two recently recovered drill cores from the central and northern Barberton area (CT3 and BARB5) with as many as 18 spherule layer intersections of Paleoarchean age (some of which may be due to tectonic duplication, some might correlate with the S2 to S4 layers) provide an outstanding opportunity to gain new insight into the early impact bombardment of Earth. We present new mineralogical, chemical, and 187 Re- 188 Os isotope data on CT3 and BARB5 drill core samples. Spherules in most layers exhibit undeformed shapes and include vesicles. Sulfides frequently are present in both matrix and spherules. Osmium data reveal a trend between the spherule-free horizons (intercalating the spherule layers) and spherule-matrix aggregates. Whereas the former typically exhibit elevated 187 Os/ 188 Os ratios of up to ∼1.2 and low Os and Ir concentrations below several hundred ppt, spherule-matrix aggregates tend to be less radiogenic (down to subchondritic present day 187 Os/ 188 Os ratios) with Os and Ir concentrations as high as in chondrites. Chromium-Ir correlations for CT3 and BARB5 samples mirror earlier results on S1 to S4 layers and can be interpreted in favor of an impact origin of the here investigated spherule horizons. [ABSTRACT FROM AUTHOR]