Highly siderophile element, triple oxygen-, ε¹⁸²W, and Re-Os isotopic composition of early Archean impact spherules from South Africa and the spherule–matrix complementarity

Journal
Meteoritics & planetary science  
Citation
Meteoritics & planetary science (in Press): (2026)
Publisher DOI
10.1111/maps.70124
Scopus ID
2-s2.0-105033272496
Publisher
University of Arkansa
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.
DDC Class
600: Technology