¹Toni Schulz,¹Christian Koeberl,^1,2Olivier Heldwein,³Bo-Magnus Elfers,⁴Jonas Tusch,⁵Stefan T. M. Peters,⁶Andreas Pack,⁴Carsten Münker
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70124]
¹Department of Lithospheric Research, University of Vienna, Vienna, Austria
²Institute of Social Ecology, University of Natural Resources and Life Sciences, Vienna, Austria
³Technische Universität Hamburg, Zentrallabor Chemische Analytik, Hamburg, Germany
⁴Institut für Geologie und Mineralogie, Universität zu Köln, Köln, Germany
⁵Zentrum für Biodiversitätsmonitoring, Leibniz-Institut zur Analyse des Biodiversitätswandels, Hamburg, Germany
⁵Geowissenschaftliches Zentrum, Georg-August-Universität Göttingen, Göttingen, Germany
Published by arrangement with John Wiley & Sons

Archean impact spherule layers represent exceptional archives of extraterrestrial (ET) material, containing large amounts of ET highly siderophile elements (HSE) that dominate the bulk content of these elements. This enrichment makes them prime targets for testing additional impact tracers, such as ε182W and triple oxygen isotopes. We investigated samples from the Paleoarchean BARB5 drill core (Barberton Mountain Land, South Africa), which preserves four spherule layers with chondritic HSE contents and 187Os/188Os signatures. Tungsten isotope data from bulk spherule layer samples yield ε182W values indistinguishable from the bulk silicate Earth, most likely reflecting the limited sensitivity of the ε182W composition to detect meteoritic admixture. If present, such a component must lie within analytical uncertainties, limiting contributions to ≤6% for a chondritic endmember or ≤3% for an iron-meteorite endmember, unless a larger signal was erased by postimpact hydrothermal overprint. In addition, bulk triple oxygen data fall within Archean shale fields and do not show resolvable ET signatures, consistent with a chondritic contribution of at most ~5% given analytical uncertainties; elevated 18O values most likely reflect seawater alteration of glass spherules. Thus, despite clear HSE–Os isotope evidence for admixture of ET components, ε182W and oxygen isotopes yield no such information. This can be explained by plume condensation models predicting temporally separated fallout of refractory and volatile element carriers. To test this, we separated spherules, matrix, and mixed fractions from one of the four BARB5 beds. While the matrix hosts the highest HSE contents and least radiogenic 187Os/188Os, spherules have the lowest HSE contents and slightly more radiogenic 187Os/188Os signatures, with mixed fractions being intermediate. Together with highly siderophile interelement trends, these results most likely highlight stepwise condensation followed by early syn-depositional to diagenetic alteration, establishing Archean spherule beds as unique probes of early plume dynamics and impact processes.