Anomalous fractionation of sulfur isotopes during heterogeneous reactions

Abstract Applying square adsorption-well and Morse potential models for the adsorption of a sulfur specie on a solid surface, we have evaluated the fractionation of four sulfur isotopes ( 32 S, 33 S, 34 S and 36 S) during chemisorption reactions. Heterogeneous reactions between solid and aqueous (or gaseous) species are found to produce anomalous fractionations of sulfur isotopes when the adsorption energy is small ( 33 S and 33 θ ’ (≈  δ 33 S /  δ 34 S) values of a sample fall outside of 0 ± 0.2‰ and 0.51 ± 0.01‰, respectively, and/or when the ∆ 36 S and 36 θ ’ (≈  δ 36 S/ δ 34 S) values of a sample fall outside of 0 ± 0.4‰ and 1.9 ± 0.1‰, respectively. The magnitude of anomalous sulfur isotope effects during a heterogeneous reaction increases with increasing temperature. Depending on the values of chemisorption energy and bond strength, the δ 33 S, δ 34 S, δ 36 S, Δ 33 S and Δ 36 S values of chemisorption products may be as variable as those observed in Archean sedimentary rocks. Ab initio calculations for SO 2 adsorption on a kerogen surface indicate the possibility of creating anomalous isotope signatures for the adsorbed species, such as: δ 33 S/ δ 34 S ≈ 1.08, δ 36 S/ δ 34 S ≈ 0.84, Δ 33 S = 7.0 to 13.6‰, and Δ 36 S = − 13.0 to − 25.2‰ in a temperature range of 0 to 200 °C; the magnitude of Δ 33 S and Δ 36 S increase with increasing temperature. These data, together with various geochemical data (e.g., organic C and sulfide S contents; hydrothermal alteration effects) on Archean sedimentary rocks with anomalous sulfur isotope fractionations, suggest that the anomalous sulfur isotope signatures in such rocks may have been created by heterogeneous reactions between organic matter (+ minerals) and sulfur-bearing aqueous solutions under hydrothermal conditions, rather than by UV photolysis of volcanic SO 2 in an O 2 -poor atmosphere.