Significant improvements to the analytics and calibration of in situ Rb–Sr dating; extending the concept to in situ K–Ca dating.

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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.

Conventional dating of molybdenite (187 Re-187 Os) provides one of few options for direct dating of sulfide mineralization. Unfortunately, in situ dating of molybdenite is considered unreliable due to intra-granular decoupling of 187 Re-187 Os. In this study, we developed a new analytical protocol for studies of micron-to grain scale 187 Re-187 Os systematics in molybdenite. Online chemical separation using ICP-MS/MS technology enables in situ dating by β-decay systems (e.g., Rb-Sr and K-Ca in micas) using laser ablation. Here, the methodology is extended to the 187 Re-187 Os system, another β-decay system that cannot be resolved by mass spectrometry. Several reaction gases were evaluated, and production of OsCH 2 by reaction with CH 4 was found to produce strong separation of Os from Re. However, in contrast to the e.g., 87 Rb-87 Sr system, 1-2% of the parent isotope Re also reacted to ReCH 2 , leaving a significant interference. A mathematical correction of this remaining interference is possible, and 187 Re-187 Os (mass-shifted) can be measured accurately even for fairly extreme ratios. For laser ablation, standards were developed by pressing particulate pellets of conventionally dated molybdenite (Moly Hill and Merlin), because there are no appropriate reference materials available. Six natural molybdenite samples from a range of geological settings, containing > 10 ppm Re, were analyzed by 70 μm laser ablation spots, and ages were calibrated by analysis of molybdenite pellets. Contrary to our expectation, weighted average ages obtained were in agreement (within 1%) with conventional age determinations, with fairly good precision (from~1 to 5% 2σ depending on Re concentration), suggesting limited or essentially nonexistent decoupling within crystals. Two important implications of this result are that decoupling Re-Os is not universal, and that our new analytical protocol is useful both for dating and for studies of decoupling. The benefit of in situ dating compared to conventional dating is, apart from lower cost and time consumption, the possibility of targeting smaller molybdenite crystals (≥ 100 μm) in thin sections and epoxy mounts. The youngest sample in the study is 920 Ma, but we see potential of dating significantly younger Re-rich molybdenite.

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