University of Michigan, United States

Low temperature alteration of oceanic basement rocks is characterized by net gain of sulfur, which commonly yields low delta (super 34) S 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 delta (super 34) S values ranging from 0 ppm to -44 ppm. In contrast, sulfides in the basalt site are characterized by relatively low Delta (super 33) S values ranging from -0.06 ppm to 0.04 ppm, compared to those from peridotite sites (0.00 ppm to 0.16 ppm). The observed Delta (super 33) S signal is significant considering the analytical precision of 0.014 ppm (2sigma ). We present a batch reaction model that uses observed delta (super 34) S and Delta (super 33) S 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 ppm and 53 ppm, for peridotite and basalt sites respectively, suggest the involvement of microbial sulfate reduction. The relatively high Delta (super 33) S values in the peridotite sites are due to sulfate reduction in a closed system environment, whereas negative Delta (super 33) S 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 H (sub 2) production capacity compared to basalt alteration, and further supports in-situ microbial sulfate reduction coupled with H (sub 2) production during serpentinization reactions. Abstract Copyright (2012) Elsevier, B.V.