The five stable isotope compositions of Fig Tree barites: Implications on sulfur cycle in ca. 3.2 Ga oceans

Abstract

The discovery of 33S anomalies in Archean sedimentary rocks has established that the early Earth before ∼2.2 Ga (billion years ago) had a very different sulfur cycle than today. The origin of the anomalies and the nature of early sulfur cycle are, however, poorly known and debated. In this study, we analyzed the total sulfur and oxygen isotope compositions, the δ 18O, Δ 17O, δ 34S, Δ 33S, and Δ 36S, for the >3.2 Ga Fig Tree barite deposits from the Barberton Greenstone Belt, South Africa. The goal is to address two questions: (1) was Archean barite sulfate a mixture of 33S-anomalous sulfate of photolysis origin and 33S-normal sulfate of other origins? (2) did the underlying photochemical reactions that generated the observed 33S anomalies for sulfide and sulfate also generate 17O anomalies for sulfate? We developed a new method in which pure barite sulfate is extracted for oxygen and sulfur isotope measurements from a mixture of barite sands, cherts, and other oxygen-bearing silicates. The isotope data reveal that (1) there is no distinct 17O anomaly for Fig Tree barite, with an average Δ 17O value the same as that of the bulk Earth (−0.02 ± 0.07‰, N = 49); and (2) the average δ 18O value is +10.6 ± 1.1‰, close to that of the modern seawater sulfate value (+9.3‰). Evidence from petrography and from the δ 18O of barites and co-existing cherts suggest minimum overprinting of later metamorphism on the sulfate’s oxygen isotope composition. Assuming no other processes (e.g., biological) independently induced oxygen isotope exchange between sulfate and water, the lack of reasonable correlation between the δ 18O and Δ 33S or between the δ 34S and Δ 33S suggests two mutually exclusive scenarios: (1) An overwhelming majority of the sulfate in the Archean ocean was of photolysis origin, or (2) The early Archean sulfate was a mixture of 33S-normal sulfates and a small portion (<5%?) of 33S-anomalous sulfate of photolysis origin from the atmosphere. Scenario 1 requires that sulfate of photolysis origin must have had only small 33S or 36S anomalies and no 17O anomaly. Scenario 2 requires that the photolysis sulfate have had highly negative δ 34S and Δ 33S values, recommending future theoretical and experimental work to look into photochemical processes that generate sulfate in Quadrant I and sulfide in Quadrant III in a δ 34S ( X)–Δ 33S ( Y) Cartesian plane. A total sulfur and oxygen isotope analysis has provided constraints on the underlying chemical reactions that produced the observed sulfate isotope signature as well as the accompanying atmospheric, oceanic, and biological conditions.