Sulfur-33 constraints on the origin of secondary pyrite in altered oceanic basement

Abstract

Low temperature alteration of oceanic basement rocks is characterized by net gain of sulfur, which commonly yields low δ34S values, suggesting involvement of microbial sulfate reduction. In order to test whether secondary sulfide minerals are consistent with a biogenic source, we apply high precision multiple sulfur isotope analysis to bulk rock sulfide and pyrite isolates from two contrasting types of altered oceanic basement rocks, namely serpentinized peridotites and altered basalts. Samples from two peridotite sites (Iberian Margin and Hess Deep) and from a basalt site on the eastern flank of the Juan de Fuca Ridge yield overlapping δ34S values ranging from 0‰ to −44‰. In contrast, sulfides in the basalt site are characterized by relatively low Δ33S values ranging from −0.06‰ to 0.04‰, compared to those from peridotite sites (0.00‰ to 0.16‰). The observed Δ33S signal is significant considering the analytical precision of 0.014‰ (2σ). We present a batch reaction model that uses observed δ34S and Δ33S relationships to quantify the effect of closed system processes and constrain the isotope enrichment factor intrinsic to sulfate reduction. The estimated enrichment factors as large as 61‰ and 53‰, for peridotite and basalt sites respectively, suggest the involvement of microbial sulfate reduction. The relatively high Δ33S values in the peridotite sites are due to sulfate reduction in a closed system environment, whereas negative Δ33S values in the basalt site reflect open system sulfate reduction. A larger extent of sulfate reduction during alteration of peridotite to serpentinite is consistent with its higher H2 production capacity compared to basalt alteration, and further supports in-situ microbial sulfate reduction coupled with H2 production during serpentinization reactions.