Pyrite is consistently found in modern seafloor vesicular basalts and has a sulphur isotopic composition consistent with microbial activity. We present S isotope data from pyrite in Ordovician pillow lavas at localities across the British Isles, which yield a range of δ34S compositions consistent with two components of sulphur: groundmass pyrite (~0±4‰) and 32S-enriched pyrite in the vesicles (-8‰ to -27‰). The latter are recorded from vesicular lavas in four different terranes, representing different settings at the margins of the Iapetus Ocean. Whereas a component of magmatic sulphate is recognized in the groundmass values, the isotopically light data in the vesicles are consistent with microbial reduction of seawater sulphate in sub-seafloor lavas. This is consistent with modern examples and indicates considerable longevity for this sub-seafloor microbial habitat.
We are grateful to A. Light, R. Parr, A. Williamson and S. Foster for help with sample collection and processing and the Garda of Co. Leitrim for their understanding. A. McDonald, A. Sandison and C. Taylor provided skilled technical support. J. Ponicka drafted Figure 6. AJB is funded by NERC support of the Isotope Community Support Facility at SUERC. NERC supported the project through facility grant IP-1235-0511.
- Deep biosphere
- Pillow lava