Paleoarchean sulfur cycling: multiple sulfur isotope constraints from the Barberton Greenstone Belt, South Africa

Mass-dependent and mass-independent sulfur isotope fractionation archived in volcanic and sedimentary rocks from the Barberton Greenstone Belt (3550–3215 Ma), South Africa, provide constraints for sulfur cycling on the early Earth. Four different samples suites were studied: komatiites and tholeiites, barite, massive and disseminated sulfide ores, and non-mineralized black shales. Previous multiple sulfur isotope centered either on specific areas and/or lithologies from the Barberton Greenstone Belt, while this study provides results for a much wider sample selection across the stratigraphic succession.
Variable but generally slightly positive δ34S values between -0.7 and +5.0‰, negative Δ33S values between -0.51 and -0.09‰, and a negative correlation between δ34S and Δ33S as well as between Δ33S and Δ36S for komatiites and tholeiites from the Komati Formation and from the Weltevreden Formation are outside the expected range of unfractionated juvenile sulfur. Instead, results indicate variable degrees of alteration through ambient seawater during serpentinisation of these rocks.
Barite from the Mapepe Formation displays positive δ34S values between 3.1 and 8.1‰ and negative Δ33S values between -0.77 and -0.34‰. Moreover, samples reveal a linear negative correlation between Δ33S and Δ36S with a slope of -0.7. The mass-independent sulfur isotope fractionation indicates an atmospheric sulfur source, whereas the positive δ34S values suggest bacterial sulfate reduction of the marine sulfate reservoir. The latter process is further discernible through a weak positive correlation between δ34S and δ18O of the barite.
Non-mineralized black shales from the presumed stratigraphic equivalent of the Mapepe Formation show positive δ34S values between 0.0 and 1.2‰ and positive Δ33S values between 0.59 and 2.48‰. These results are interpreted to result from the mixing between the two principal atmospheric sources, i.e. elemental sulfur, carrying a positive Δ33S signature and sulfate, carrying a negative Δ33S value, with its δ34S signature subsequently modified through bacterial sulfate reduction.
Positive δ34S values ranging from +0.5 to +3.4‰ and slightly negative Δ33S values between -0.17 and -0.13‰ characterize massive and disseminated sulfides from the Bien Venue Prospect. Results suggest a mixture between the unfractionated juvenile magmatic sulfur source and a contribution from recycled seawater sulfate indicative of submarine hydrothermal activity. Considering the isotope values, these three sets of samples show a common source of sulfur, characterized by negative Δ33S and positive Δ36S, represented by seawater sulfate.
Massive and disseminated sulfides from the M’hlati prospect are characterized by different values compared to massive and disseminated sulfide from the Bien Venue Prospect. They show negative δ34S values between -1.4 and -0.1‰ and positive Δ33S values between +2.66 and +3.17‰, thus, displaying a sizeable mass-independent sulfur isotopic fractionation. Again, these samples clearly exhibit the incorporation of an atmospheric MIF-S signal. The source of sulfur for these samples has positive Δ33S values, thus this is related to elemental sulfur.
In conclusion, the sulfur isotope inventories in the Paleoarchean rocks and hydrothermal precipitates from the Barberton Greenstone Belt are quite diverse and indicate the incorporation of at least two sources of sulfur. Komatiites and tholeiites, barite and massive and disseminated sulfides from Bien Venue show a common sulfur source, related to seawater sulfate, while massive and disseminated sulfides from M’hlati are related to elemental sulfur and not mineralized black shales with a mixing between these two sulfur sources.