Unprecedented 34S-enrichment of pyrite formed following microbial sulfate reduction in fractured crystalline rocks


Drake, Henrik
Whitehouse, Martin J.
Heim, Christine
Reiners, Peter W.
Tillberg, Mikael; Hogmalm, K. Johan Dopson; Mark, Broman; Curt, Åström; Mats E.

In the deep biosphere, microbial sulfate reduction (MSR) is exploited for energy. Here, we show hat, in fractured continental crystalline bedrock in three areas in Sweden, this process produced sulfide that reacted with iron to form pyrite extremely enriched in 34S relative to 32S. As documented by secondary ion mass spectrometry (SIMS) microanalyses, the δ34Spyrite values are up to +132‰V-CDT and with a total range of 186‰. The lightest δ34Spyrite values (−54‰) suggest very large fractionation during MSR from an initial sulfate with δ34S values (δ34Ssulfate,0) of +14 to +28‰. Fractionation of this magnitude requires a slow MSR rate, a feature we attribute to nutrient and electron donor shortage as well as initial sulfate abundance. The superheavy δ34Spyrite values were produced by Rayleigh fractionation effects in a diminishing sulfate pool. Large volumes of pyrite with superheavy values (+120 ± 15‰) within single fracture intercepts in the boreholes, associated heavy average values up to +75‰ and heavy minimum δ34Spyrite values, suggest isolation of significant amounts of isotopically light sulfide in other parts of the fracture system. Large fracture-specific δ34Spyrite variability and overall average δ34Spyrite values (+11 to +16‰) lower than the anticipated δ34Ssulfate,0 support this hypothesis. The superheavy pyrite found locally in the borehole intercepts thus represents a late stage in a much larger fracture system undergoing Rayleigh fractionation. Microscale Rb–Sr dating and U/Th–He dating of cogenetic minerals reveal that most pyrite formed in the early Paleozoic era, but crystal overgrowths may be significantly younger. The δ13C values in cogenetic calcite suggest that the superheavy δ34Spyrite values are related to organotrophic MSR, in contrast to findings from marine sediments where superheavy pyrithas been proposed to be linked to anaerobic oxidation of methane. The findings provide new insights into MSR-related S-isotope systematics, particularly regarding formation of large fractions of 34S-rich pyrite.

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F I G U R E 6 Details of the highly 34S-enriched pyrite sample KLX14A:80. (a) Photograph showing the pyrite together with cogenetic calcite in a cavity within an older calcite-dominated fracture filling. (b) Backscattered SEM image showing intergrown calcite (scalenohedral) and pyrite (cubic). (c–e) S-isotope ion images draped over SEM images of pyrite cross sections with values indicated by color scale (spot analysis values also shown) and (f-h) as graphs (more examples in Supporting Information Figure S3). (f) The transect from crystal core (left) to rim (right) reveals several growth phases, where δ34S (1) starts at −19‰ (spot analysis calibrated); (2) increases to >90‰ due to closed system MSR; (3) drops to 40‰ at fracture reactivation; (4) increases to 120‰ at closed system MSR; (5) fluctuates by 15‰; (6) drops to around 30‰ at fracture reactivation; and (7) increases to 80‰. (g, h) Transects from rim to rim of crystals lacking the first precipitation phase seen in f but showing similar fluctuation as in f around 120 ± 15‰ at two episodes (h) and three episodes (g). (i) Histogram of all δ34S spot measurements in KLX14A:80. Analytical error of ratios derived from ion images and the SIMS spot analyses are ±3‰ and ±0.14‰, respectively (see method description for data handling).

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Geobiology. 2018;1–19. wileyonlinelibrary.com/journal/gbi