In Situ Rb/Sr Geochronology and Stable Isotope Geochemistry Evidence for Neoproterozoic and Paleozoic Fracture‐Hosted Fluid Flow and Microbial Activity in Paleoproterozoic Basement, SW Sweden

Abstract

Recent studies have shown that biosignatures of ancient microbial life exist in mineral coatings in deep bedrock fractures of Precambrian cratons, but such surveys have been few and far between. Here, we report results from southwestern Sweden in an area of 1.6–1.5 Ga Paleoproterozoic rocks heavily reworked by the 1.14–0.96 Ga Sveconorwegian orogeny, a terrane previously scarcely explored for ancient microbial biosignatures. Calcite‐pyrite‐adularia‐illite‐coated fractures were analyzed for stable isotopes via Secondary Ion Mass Spectrometry (δ13C, δ18O, δ34S) and in situ Rb/Sr geochronology via Laser‐ablation inductively coupled plasma mass spectrometry. The Rb/Sr ages for calcite‐adularia and calcite‐illite show that several fluid flow events can be discerned (797 ± 18–769 ± 7, 391 ± 5–387 ± 6, 356 ± 5–347 ± 4, and 301 ± 7 Ma). The δ13C, δ18O and 87Sr/86Sr values of different calcite growth zones further confirmed episodic fluid flow. Pyrite δ34S values down to −49.9‰V‐CDT, together with systematically increased δ34S from crystal core to rim, suggest formation following microbial sulfate reduction under semi‐closed conditions. Assemblages involving MSR‐related pyrite generally have Devonian to Permian Rb/Sr ages, indicating an association to extension‐related fracturing and fluid mixing during foreland‐basin formation linked to Caledonian orogeny in the northwest. An assemblage with an age of 301 ± 7 Ma is potentially related to Oslo Rift extension, whereas the Neo‐Proterozoic ages relate to post‐Sveconorwegian extensional tectonics. Remnants of short‐chained fatty acids in the youngest calcite coatings further indicate a biogenic origin, while the absence of organic molecules in older calcite is in line with thermal degradation, potentially related to heating during Caledonian foreland basin burial.