Spherule layers from South Africa as windows into the Paleoarchean meteorite bombardment of the Earth-Evidences from coupled Os-W isotope systematics and siderophile elements

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Much of the Archean impact record remains shrouded in mystery. Large asteroid fragments have undoubtedly hit the Earth during and in the wake of the late heavy bombardment, but the scars of these impact events have long since been obliterated. However, spherules, ejected during the impact event, provide information about an impact event even when the source crater cannot be found. Based on their Cr isotope signatures three spherule layers of Paleoarchean age (exclusively preserved in the Barberton Greenstone Belt, South Africa) unambiguously represent the oldest impact remnants identified so far. Two recently recovered drill cores from the Barberton area with possibly up to 21 new spherule layers of Paleoarchean age provide an outstanding opportunity to gain new insights into the meteorite bombardment of the Early Earth. This proposal aims to (i) assess possible correlations between the closely spaced spherule beds from both drill cores, (ii) determine the magnitude and spatial distribution of meteoritic admixtures within them, (iii) characterize impactor materials involved, and (iv) assess the number of cratering events they represent. These objectives will be achieved by a combination of in-situ and whole rock geochemical and petrological approaches, including detailed mineralogical investigations, the first high precision Os and W isotope analysis and isotope dilution generated concentrations of highly siderophile elements. Moreover, this study intends to analyze pure spherule separates instead of spherule-matrix aggregates as commonly utilized. The combination of these approaches opens a promising new tool-box for the investigation of Archean impact deposits, leading ultimately to new insights into the element delivery to the Archean crust, possibly global tectonic reorganizations resulting from large impacts and the characterization of conditions on Earth that prevailed in an era from which the first signs of life were discovered.