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

Résumé: This talk summarizes research undertaken over the past 15 years upon the microbial alteration of originally glassy basaltic rocks from submarine environments. Textural, chemical and isotopic results from the youngest to the oldest in-situ oceanic crust are shown and compared with data obtained from ancient oceanic crust, i.e. ophiolite and greenstone belts dating back to c. 3.5 Ga. Petrographic characterizations are provided with granular and tubular microbial alteration textures found in (meta)-volcanic glasses from pillow lavas and volcanic breccias and contrasted with textures produced by abiotic alteration. 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. 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 suggest that microbial alteration of volcanic glass extends back to some of the earliest preserved seafloor crustal fragments. Observations collected from well preserved, in-situ oceanic crust provide a guide to interpreting comparable mineralized micro-textures from the ancient seafloor. Textural evidence is best preserved in undeformed to little-deformed, low grade, meta-volcanic rocks, whereas chemical tracers, in particular the 13Ccarb signatures, are more robust and can survive relatively strong deformation and metamorphic conditions. Drawing together all of this data a tentative model for microbial life in the Archean sub-seafloor is proposed. 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. -------------------------------------------------------------------------------- Séminaires généraux de l'Institut de Physique du Globe de Paris --------------------------------------------------------------------------------