VOLCANIC DEGASSING, HYDROTHERMAL CIRCULATION AND THE FLOURISHING OF EARLY LIFE ON EARTH: NEW EVIDENCE FROM THE C. 3460 MA WARRAWOONA GROUP, PILBARA CRATON, WESTERN AUSTRALIA

Detailed map data from Earth’s oldest fossiliferous chert horizons of the c. 3460 Ma Warrawoona Group contrasts with previous depositional models of simple evaporite precipitation from seawater. Results show that the cherts were fed by swarms of kerogenous chert (± barite ± carbonate) veins which contain phreatic breccia textures and caused extensive argillic/phyllic and/or carbonate alteration of underlying volcanics. These features are indicative of a hydrothermal origin.

Stromatolites grew under very shallow water conditions close to, or downflow from, hydrothermal vents in both the 3490 Ma Dresser Formation [DF] and 3430 Ma Strelley Pool Chert [SPC], and were probably constructed by hyperthermophilic phototrophs. Distinct stromatolite morphologies in sulphate [DF] and carbonate [SPC] host rocks suggest a diversity of microbes. Furthermore, reported microfossils from the 3460 Ma Apex Basalt occur in a phreatic breccia vein, suggesting some microbes were possibly subsurface chemoautotrophs.

Hydrothermal veins were emplaced into, and above, listric normal growth faults that formed local basins with restricted water circulation during periods of surface uplift. This was driven by intrusion of subvolcanic laccoliths during periods of waning volcanism; thus the basins are interpreted as calderas. Based on East African Rift analogues, stromatolitic laminites and weakly radiating aragonite crystal splays of the SPC are interpreted as deposited in an alkaline caldera lake fed by hotsprings. Component carbonate-quartz couplets, which locally display cyclical bundling, most likely reflect seasonal variations in lakewater composition. Coarse clastic successions disconformably overlying the cherts resulted from caldera collapse during magmatic deflation and cooling.

Petrographic evidence suggests that hydrothermal fluids introduced CO2 and SO2 from degassing subvolcanic magma chambers. Volcanic gasses combined with Si, Fe, Ba leached from country rocks to precipitate oxides and provide energy, C, and S used by growth of microbes, supporting the contention that the Archaean biosphere was oxidized from the bottom up.