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Sulfur isotope mass-independent fractionation in impact depositsof the 3.2 billion-year-old Mapepe Formation,Barberton Greenstone Belt, South Africa
Swedish Museum of Natural History, Department of Geology. (Nordsim)
2014 (English)In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 0016-1258, Vol. 142, 429-441 p.Article in journal (Refereed) Published
Abstract [en]

Theoretical and experimental studies have shown that atmospheric SO2 isotopologue self-shielding effects in the 190–220 nmregion of the solar spectrum are the likely cause for mass independent fractionation of sulfur isotopes (S-MIF). The main productsof this photochemical reaction – SO3 and S0 – typically define a compositional array of ca. D33S/d34S = 0.06–0.14. This is atodds with the generally observed trend in Archean sulfides, which broadly defines an array of ca. D33S/d34S = 0.9. Various explanationshave been proposed, including a diminution of d34S caused by chemical and biogenic mass-dependent fractionation ofsulfur isotopes (S-MDF), mixing with photolytic products produced during felsic volcanic events, or partial blocking of the lowwavelengthpart of the spectrum due to the presence of reduced atmospheric gases or an organic haze. Early in Earth history largemeteorite impacts would have ejected dust and gas clouds into the atmosphere that shielded solar radiation and affected globalclimate. It is thus likely that at certain time intervals of high meteorite flux the atmosphere was significantly perturbed, having aneffect on atmospheric photochemistry and possibly leaving anomalous sulfur isotopic signatures in the rock record. Here wedescribe the sulfur isotopic signatures in sulfides of spherule beds S2, S3 and S4 of the Barberton Greenstone Belt, South Africa.In particular, in spherule bed S3 – and to a lesser extent S4 – a trend of ca. D33S/d34S = 0.23 is observed that closely follows theexpected trend for SO2-photolysis in the 190–220 nm spectral range. This suggests that an impact dust cloud (deposited as spherulebeds), which sampled the higher region of the atmosphere, specifically incorporated products of SO2 photolysis in the 190–220 nm range, and blocked photochemical reactions at higher wavelengths (250–330 nm band). By implication, the generallyobserved Archean trend appears to be the result of mixing of different MIF-S sources arising from a variety of photochemicalreactions that took place in the lower part of the atmosphere.

Place, publisher, year, edition, pages
2014. Vol. 142, 429-441 p.
National Category
Geochemistry
Research subject
The changing Earth
Identifiers
URN: urn:nbn:se:nrm:diva-827DOI: 10.1016/j.gca.2014.07.018OAI: oai:DiVA.org:nrm-827DiVA: diva2:757191
Projects
Nordsim
Available from: 2014-10-21 Created: 2014-10-21 Last updated: 2014-10-21Bibliographically approved

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