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

Abstract

a b s t r a c tMass-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 sample suites were studied: komatiites and tholeiites, barite, massive and disseminated sulfide ores, and non-mineralized black shales.Variable but generally slightly positive ı 34 S values between −0.7 and +5.2‰, negative 33 S values between −0.50 and −0.09‰, and a negative correlation between ı 34 S and 33 S as well as between 33 S and 36 S for komatiites and tholeiites from the Komati Formation and from the Weltevreden Formation are outside the expected range of unfractionated juvenile sulfur. Instead, results suggest alteration of oceanic crustal rock sulfur through interactions with fluids that most likely derived their sulfur from seawater.Barite from the Mapepe Formation displays positive ı 34 S values between +3.1 and +8.1‰ and negative 33 S values between −0.77 and −0.34‰. The mass-independent sulfur isotope fractionation indicates an atmospheric sulfur source, notably photolytic sulfate, whereas the positive ı 34 S values suggest bacterial sulfate reduction of the marine sulfate reservoir.Non-mineralized black shale samples from the presumed stratigraphic equivalent of the Mapepe Formation show positive ı 34 S values between 0.0 and +1.3‰ and positive 33 S values between +0.59 and +2.45‰. These results are interpreted to result from the reduction of photolytic elemental sulfur, carrying a positive 33 S signature.Positive ı 34 S values ranging from +0.7 to +3.5‰ and slightly negative 33 S values between −0.17 and −0.12‰ characterize massive and disseminated sulfides from the Bien Venue Prospect. Results suggest unfractionated juvenile magmatic sulfur source as the primary sulfur source, but a contribution from recycled seawater sulfate, which would be indicative of submarine hydrothermal activity, cannot be ruled out.Massive and disseminated sulfides from the M'hlati prospect are distinctly different from massive and disseminated sulfide from the Bien Venue Prospect. They show negative ı 34 S values between −1.2 and −0.1‰ and positive 33 S values between +2.66 and +3.17‰, thus, displaying a sizeable mass-independent A. Montinaro et al. / Precambrian Research 267 (2015) 311-322 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 33 S values, suggesting a connection with photolytic elemental sulfur. In conclusion, the sulfur isotope signatures in Paleoarchean rocks from the Barberton Greenstone Belt are diverse and indicate the incorporation of different sources of sulfur. For komatiites and tholeiites, barite and massive and possibly also disseminated sulfides from Bien Venue, multiple sulfur isotopes are related to ambient seawater sulfate and its photolytic origin, while massive and disseminated sulfides from M'hlati and non-...

In Situ Fe and S isotope analyses in pyrite from the 3.2 Ga Mendon Formation (Barberton Greenstone Belt, South Africa): Evidence for early microbial iron reduction

“…Pyrites in the Mendon Formation show large isotope variability in δ 56 Fe, δ 34 S, and Δ 33 S, which is not related to the stratigraphic position within the core (Figure ). δ 34 S and Δ 33 S values range from −2.66 to +6.22‰ and −0.39 to +4.25‰, respectively (Figures and a), consistent with previous reports on the same formation (Galić et al, ; Montinaro et al, ; Roerdink, Mason, Whitehouse, & Reimer, ). The δ 56 Fe values vary from −4.02 to +2.54‰, covering almost the total terrestrial range reported so far (Dauphas, John, & Rouxel, ).…”

“…About 100 m of the uppermost Mendon Formation was recovered. The Mendon Formation was deposited between 3.34 and 3.26 Gyr and has been studied in detail (Busigny et al, ; Galić et al, ; Hofmann & Bolhar, ; Lowe & Byerly, ; Montinaro et al, ; Philippot et al, ; Philippot, Zuilen, & Rollion‐Bard, ; Tice et al, ; Trower & Lowe, ). The sequence consists of five volcanic cycles, termed Mv1 through Mv5, which are each capped by thin chert sequences, termed Mc1 through Mc5 (Byerly et al, ).…”

Multiple sulfur-isotope signatures in Archean sulfates and their implications for the chemistry and dynamics of the early atmosphere

“…Hence, a key issue for elucidating the early Archean sulfur cycle concerns the significance of the isotopic composition of Archean barites. The main deposits in Australia and South Africa define a narrow range of both δ 34 S values between +3‰ and +8‰ and Δ 33 S anomalies between −0.1‰ and −1.2‰ (1,(5)(6)(7)(8)(9)(10)(11). In contrast to the δ 34 S values that are relatively constant in all barite deposits, the Δ 33 S values define a potential trend increasing from 3.5 (∼−1.5‰) to 3.2 Ga (∼−0.5‰; Fig.…”

Multiple sulfur isotope constraints on microbial sulfate reduction below an Archean seafloor hydrothermal system

“…Montinaro et al. () have reported multiple sulfur isotopes of komatiite and tholeiite from the Barberton Greenstone Belt, South Africa, and showed limited variation in δ 34 S values. A multiple sulfur isotope study of amphibolites from the Isua supracrustal belt, which is believed to be the oldest oceanic crust, also shows a near‐zero δ 34 S signature (Siedenberg, Strauss, & Hoffmann, ).…”

“…The 34 S of the sulfide inclusions within the putative microbial textures is heavily depleted, and varies widely from −40‰ to −10‰ (McLoughlin, Grosch, Kilburn, & Wacey, 2012); the heterogeneity and magnitude of the sulfur isotope fractionation in the pillow lava support a microbial origin of sulfide, though the putative microbial textures may have been produced on the surface. Montinaro et al (2015) have reported multiple sulfur isotopes of komatiite and tholeiite from the Barberton Greenstone Belt, South Africa, and showed limited variation in δ 34 S values. A multiple sulfur isotope study of amphibolites from the Isua supracrustal belt, which is believed to be the oldest oceanic crust, also shows a near-zero δ 34 S signature (Siedenberg, Strauss, & Hoffmann, 2016).…”