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

Abstract

In the deep biosphere, microbial sulfate reduction (MSR) is exploited for energy. Here, we show that, in fractured continental crystalline bedrock in three areas in Sweden, this process produced sulfide that reacted with iron to form pyrite extremely enriched in S relative to S. As documented by secondary ion mass spectrometry (SIMS) microanalyses, the δ S values are up to +132‰V-CDT and with a total range of 186‰. The lightest δ S values (-54‰) suggest very large fractionation during MSR from an initial sulfate with δ S values (δ S ) 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 δ S 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 δ S values, suggest isolation of significant amounts of isotopically light sulfide in other parts of the fracture system. Large fracture-specific δ S variability and overall average δ S values (+11 to +16‰) lower than the anticipated δ S 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 δ C values in cogenetic calcite suggest that the superheavy δ S values are related to organotrophic MSR, in contrast to findings from marine sediments where superheavy pyrite has 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 S-rich pyrite.

Sulfur isotope analysis of microcrystalline iron sulfides using secondary ion mass spectrometry imaging: Extracting local paleo‐environmental information from modern and ancient sediments

“…Analysis of fragments of hydrothermal pyrite and marcasite crystals, used as S‐isotopic standards for our other SIMS experiments, resulted in little inter‐ (Figures S1 and S2, supporting information) or intra‐fragment (Figures S3 and S4, supporting information) variability in the δ 34 S value (i.e., the standard deviation of measurements between or within fragments was always smaller than the average standard error associated with those inter‐ or intra‐grain measurements). Therefore, the method is suitable for the detection of the potentially large variations in δ 34 S values that may exist within or between sedimentary iron sulfide grains …”

“…The method described here, scanning ion imaging by SIMS, is designed to address several of the limitations of currently available methods for micro‐scale measurement of δ 34 S values in sedimentary pyrite. First, by rastering over grains, scanning ion imaging can generate a continuous record of isotope variations, one that can be interrogated at variable spatial resolution after data collection. Previously, most SIMS studies of pyrites relied on analyses of fairly large (≥10 μm‐diameter) spots within grains .…”

“…Progress has been made using spot analyses by secondary ion mass spectrometry (SIMS) 1,23,[28][29][30][31][32][33][34][35][36][37][38] or laser ablation (LA)-MC-ICP-MS, 20,39,40 to make spatially resolved δ 34 S measurements. Some have already attributed detectable intra-sample pyrite δ 34 S variability to temporal changes in the S-isotopic composition of the fluids from which the pyrites precipitated.…”

“…Previously, most SIMS studies of pyrites relied on analyses of fairly large (≥10 μm-diameter) spots within grains. 1,23,[28][29][30][31][32][33][34][35][36][37][38] This precludes analyses of many sedimentary pyrites (i.e., those with diameters <10 μm). Moreover, while spot analyses can be sufficient to determine the presence of inter-grain isotopic variability on larger grains, they are not able to discern intra-grain variability, [31][32][33][34][35][36][37] except for the case of unusually large (diameters >~100 μm) pyrites.…”

“…Relatively high primary beam currents (e.g., ≥1 nA) inherently limit the three-dimensional resolution of isotopic measurements. 28,38 As scanning ion imaging requires the use of very low primary beam currents (e.g., ≤20 pA) to prevent the saturation of the electron multiplier detector, 29 the associated low sputter rates and small diameter of the focused primary beam (≤1 μm) result in excellent three-dimensional resolution. 29 Finally, we introduce a physical extraction procedure to enable pyrite to be concentrated for optimized SIMS analyses for samples where pyrite is a trace phase.…”