Quadruple sulfur isotope records of quartz-richsandstones of the 3.22 Ga Moodies Group (Barberton Greenstone Belt, South Africa)

International audience; Sulfur isotope is a strong proxy to decipher atmospheric and biological processes on Earthand have provided important constraints onthe atmospheric chemistry inthe Archean. The 3.22 GaMoodies Group, the stratigraphycally upper partof the Barberton Greenstone Belt(BGB),South Africa,is interpreted as the oldest terrestrialto marinetransitionalsuccession(e.g., [1]).The lower Moodies Group consistsmainlyof quartzitic sandstoneswith cross beddingsandflat and wavy laminas(putative fossilized microbial mats,[2]),deposited in shallow water settings. The uniquepetrology and depositionalenvironnement of the Moodies made this group an interesting target for better understanding the Paleoarchean(3.6-3.2Ga)sulfurcycle.The sulfur isotopeandother geochemicalanalyses were conducted on46samples collected froma350 m thick Moodies succession(upper MdQ1)in the Saddleback Synclinein the centralBGB.The δ34S values of sulfide in the rocksvary between –8 and +2‰, whereas the ∆33S values range from +0.05 to +0.25‰.The ∆33S values are small but show significant mass independent fractionation of sulfur isotopes(S-MIF).The present Moodies sulfur records can largely be explained by the mixing between a primary atmospheric sulfur and a secondary sulfur derived from a hydrothermal fluid after deposition.The frequent occurrence of microquartz and muscovite/sericite cements in the rocks suggestspostdepositionalmetasomatic fluid circulationandsupportsthe mixture with juvenilesulfur.The Moodies data are not on the conventional Archean reference array(ARA)on the δ34S-∆33S cross plot and imply that the Paleoarcheansulfur cycle is different from the Neoarchean (2.8-2.5 Ga) cycle. OtherPaleoarchean records supportthis apparent evolutionof the sulfur cycleduring the Archean time.