Toni Schulza, Christian Koeberla,b, Ambre Luguetc, David Van Ackenc,d, Tanja Mohr-Westheidee,f, Seda Ozdemira, Wolf Uwe Reimolde,g,h
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.040]
aDepartment of Lithospheric Research, University Vienna, Althanstrasse 14, 1090 Vienna, Austria
bNatural History Museum, Burgring 7, A-1010 Vienna, Austria
cSteinmann-Institut of Geology, Mineralogy and Palaeontology, University of Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany
dIrish Centre for Research in Applied Geosciences (iCRAG), University College Dublin, Belfield, Dublin 4, Ireland
eMuseum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Invalidenstrasse 43, 10115 Berlin, Germany
fFreie Universität Berlin (FU Berlin), Institut für Geologische Wissenschaften, Malteserstrasse 74-100, 12249 Berlin, Germany
gHumboldt Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany
hGeochronology Laboratory, University of Brasília, Brazil
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 to 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.