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Black carbon-rich chert dikes are common features of early Archean terranes and are especially prominent on the Pilbara Craton of Western Australia. The dikes cut across the local stratigraphy before truncating abruptly at specific stratigraphic horizons where they can be seen to interfinger with the stratigraphic succession. These uniquely Archean dike complexes are enigmatic and raise significant questions about the source of the carbon they contain - is it biogenic, and therefore indicative of the early biosphere, or abiotic, and perhaps indicative of prebiotic conditions that led to the appearance of the first life on Earth? The Strelley Pool Chert, which is exposed on the Pilbara Craton of Western Australia, forms part of the Warrawoona Group and rests unconformably on older ultramafic volcanics. The formation, which is generally less than 30 m thick, consists of a relatively consistent lithologic succession found over a wide area. Locally, a basal transgressive sandstone is overlain by a sequence of carbonate, barite?, black and white laminated chert and grey-green chert which is locally stained blue green by weathered fuchsite. The formation is intersected by normal faults about which the depositional successions thickens and thins. The mineralogy and geochemistry (including stable isotope data) of the underlying dike complex closely parallels that of the lithologies preserved in the Strelley Pool Chert. Similarly, the depositional sequence of minerals in voids within the dike complex parallels that of the depositional sequence above. This new data from the Strelley Pool Chert and its associated dike complex show that the black chert dikes are an integral part of a hydrothermal mineralogical complex. The complex accumulated around vents fed by growth-fault-controlled dikes that extended to a depth of more than 2 km below the Archean paleo-seafloor. The mineralogy of the sedimentary succession reflects deposition during the declining temperature regime of an evolving hydrothermal system driven by an intruding granitoid complex below. The fault system and associated hydrothermal complex evolved in the tensional environment developed during doming above the intermittently intruding granitoid complex that forms the framework of the Pilbara Protocontinent. Carbonaceous compounds contained within the black chert dikes could have been generated abiotically by a process similar to Fischer-Tropsch synthesis at elevated temperatures (200-300 °C) and pressures (500 bar) at a paleo-depth of 2 km within the hydrothermal system. The "primordial medium" in which life first evolved may thus have been a silica-rich slurry with a low pH and a high CO 2 content which contained basic abiotic organic building blocks fundamental to the origins of life. The abiotic organic output from these hydrothermal systems may overwhelm any early biospheric geochemical signal and may therefore be the most difficult environment in which to recognise the early biospheric record. © 2005 Elsevier B.V. All rights reserved. |
