Oceanic pillow lavas and hyaloclastites as habitats for microbial life through time – a review

This chapter summarizes research undertaken over the past 15 years upon the microbial alteration of originally glassy basaltic rocks from submarine environments. We report textural, chemical and isotopic results from the youngest to the oldest in-situ oceanic crust and compare these to data obtained from ophiolite and greenstone belts dating back to c. 3.8. Ga. Petrographic descriptions of the granular and tubular microbial alteration textures found in (meta)-volcanic glasses from pillow lavas and volcanic breccias are provided and contrasted with textures produced by abiotic alteration (palagonitization). The geological setting in particular the degree of deformation and metamorphism experienced by each study site is documented in outcrop photographs, geological maps and stratigraphic columns (where possible). In addition, X-ray mapping evidence and carbon isotopic data that are consistent with a biogenic origin for these alteration textures is explained and a model for their formation is presented. Lastly, the petrographic observations and direct radiometric dating techniques that have been used to establish the antiquity and syngenicity of these microbial alteration textures are reviewed. The combined dataset presented herein suggests that the microbial alteration of volcanic glass extends back to some of the earliest preserved seafloor crustal fragments. We use observations collected from well preserved, in-situ oceanic crust as a guide to interpreting comparable mineralized micro-textures from the ancient seafloor. It emerges that textural evidence is best preserved in undeformed to littledeformed, low grade, meta-volcanic rocks, and that chemical tracers, in particular the Ā¹³C carb signatures, are more robust and can survive relatively strong deformation and metamorphic conditions. Drawing together all of this data we propose a tentative model for microbial life in the Archean sub-seafloor. Overall, it is argued that bioalteration textures in (meta)-volcanic glasses provide a valuable tracer of the deep oceanic biosphere, which constitutes one of the largest and least explored portions of the modern, and especially the ancient, biosphere.