Timely sample acquisition of Archean spherule layers from the 2011 ICDP drilling in the Barberton Mountain Land: Early impact record on Earth
Final Report Abstract
This project dealt with the characterization of two new drill cores from the Barberton Greenstone Belt (BGB). The ICDP core BARB 5 intersected not less than 5, ca. 4 cm wide spherule layer (SL) beds and 3 further spherule showings, and core CT 3 retrieved 17 SL deposits. The BARB 5 SLs occur at the transition from Fig Tree Group to Onverwacht Group (Barberton Greenstone Belt), and the CT 3 occurrences are likely part of the uppermost Fig Tree Group section or also occur at the transition to the Onverwacht Group. All SLs were petrographically studied and microchemically investigated. As in most other SL occurrences in the BGB, the new SLs and some of the hosting strata have a very large range of Ir concentrations, with many samples displaying super-chondritic values. In this study, the application of a combination of instrumental neutron activation and microchemical techniques resolved the cause of such anomalous enrichments. The work was based on scanning electron microscopy–energy dispersive X-ray spectrometry at high spatial resolution identified local areas of platinum group element (PGE) enrichment, invariably in the vicinity of Ni-rich chromium spinel (Ni-Cr spinel) clusters. These areas are invariably present in subsamples for which bulk analysis has previously demonstrated enrichment of the Extraterrestrial Component (ETC). By applying high-resolution BSE-EDS scanning microscopy and dedicated feature analysis techniques it was then possible to identify µm to sub-µm sized platinum groups metals (PGMs) and PGE sulfarsenides as carriers of the ETC. The PGMs are considered primary impact-related/generated particles, whereas the sulfarsenides represent the product of secondary alteration with S-rich solutions. Step three in this innovative approach was then to apply TEM techniques to study the textures/structures of the PGMs and obtain sub-µm to nanometer scale chemical compositions. The variability of PGM types indicates that most likely several processes (sublimation from the impact vapor plume, crystallization from impact melt, etc) were responsible for the formation of these particles. Having demonstrated that the excessively high contents of ETC are the result of a PGM micro-nugget effect, the previous hypothesis that measured PGE abundances could be directly converted to sizes of Archean impact projectiles has been shown as not valid. Furthermore, the µ-XRF technique was successfully employed to identify structures and semi-quantitative chemical information on bulk drill core of SL and host rock material. Further dating efforts and stratigraphic refinement of SL positions are required to obtain conclusive evidence on whether these new SL intersections should be linked to previously identified SLs at the transition from Fig Tree to Onverwacht groups.
Publications
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2015. Discovery of extraterrestrial component carrier phases in Archean spherule layers: Implications for estimation of Archean bolide sizes. Geology 43:299-302
Mohr-Westheide, T., Reimold, W.U., Fritz, J., Koeberl, C., Salge, T., Hofmann, A., Schmitt, R.T.
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2015. Remnants of Early Archean impact deposits on Earth: search for a meteoritic component in the BARB5 and CT3 drill cores (Barberton Greenstone Belt, South Africa). Expedia Engineering 103: 310-317
Koeberl, C., Schulz, T., Reimold, W.U.
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2016. Nondestructive spectroscopic and petrochemical investigations of Paleoarchean spherule layers from the ICDP drill core BARB5, Barberton Mountain Land, South Africa. Meteoritics & Planetary Science 51: 2441-2458
Fritz, J., Tagle, R., Ashowrth, L., Schmitt, R.T., Hofmann, A., Luais, B., Harris, P.D., Hoehnel, D., Oezdemir, S., Mohr-Westheide, T., Koeberl, C.
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2017. Early Archean spherule layers from the Barberton greenstone belt, South Africa: mineralogy and geochemistry of the spherule beds in the CT3 drill core. Meteoritics & Planetary Science 52:2586-2631
Oezdemir, S., Schulz, T., Koeberl, C., Reimold, W.U., Mohr-Westheide, T., Hoehnel, D., Schmitt, R.T.
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2017. New constraints on the Paleoarchean meteorite bombardment of the Earth - Geochemistry and Re-Os isotope signatures of spherule layers in the BARB5 ICDP drill core from the Barberton Greenstone Belt, South Africa. Geochimica et Cosmochimica Acta 211:322-340
Schulz, T., Koeberl, C., Luguet, A., Van Acken, D., Mohr-Westheide, T., Oezdemir, S., Reimold, W.U.
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2018. Petrographic and micro-XRF analysis of multiple Archean impact-derived spherule layers in drill core CT3 from the northern Barberton Greenstone Belt (South Africa). Journal of African Earth Sciences 138: 264-288
Hoehnel, D., Reimold, W.U., Altenberger, U., Hofmann, A., Mohr-Westheide, T., Oezdemir, S., Koeberl, C.
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2018. Transmission electron microscopy of impact-generated platinum-group-element alloys from Barberton spherule layers: New clues to their formation. Meteoritics & Planetary Science 53: 1516-1536
Mohr-Westheide, T., Greshake, A., Wirth, R., Reimold, W.U.
