Restore the Archean oceanic hydrothemai cross section: Geologicai and geochemical approach for frnding of early biosphere

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The 3.2-Ga. Dixon Island Formation in the Cleaverville Group of the coastal Pilbara terrane, Australia, is one of the best sequences that preserves an example of middle Archean oceanic stratigraphy and contains possible microbial material. Field observations and geochemical evidence suggest that the Dixon Island Formation contains low temperature hydrothermal vents system with a biogenic microbial colony in the Archean ocean. The Dixon Island Formation is approximately 350 m thick and consists of the Rhyolite Tuff, Black Chert and Varicolored Chert members. Many black-chert vein swarms in the Rhyolite Tuff Member imply intensive low-temperature hydrothermal activity during deposition of the Black Chert Member, which is 15〜20 m thick. Abundant pseudomorph of silica after aragonite, barite, and gypsum, and a distinctly continuous, stromatolite-like biomat layer (10〜20 cm in thick), are preserved within the laminated black-chert bed. The stromatolite-like biomat bed formed fine iron or ir … More on-coated quartz pisoids within fine-grained silica. The absence of detrital sediments of continental origin and many vein injection implies that this sedimentary facies represents a pelagic hydrothermal environment at about 500〜2000 m in paleodepth, which is not very deep oceanic plane, such as a slope of immature island arc setting.Microbial material has been preserved well in the Black Chert Member, which is composed of massive black chert, laminated black chert, dark greenish siliceous shale and tuffaceous laminated chert. These rocks are mainly composed very fine quartz. The massive black chert has carbonaceous peloids (0.3 mm〜2 mm in diameter) similar to those in the black chert veins. The massive black chert contains spiral-, rod- and dendrite-shaped bacterial material. Total organic carbon (TOC in the black chert and black chert veins varies within 0.05〜0.16% (average 0.1%) and the carbon isotope (delta 13C values of these rocks are -32〜-27 per mil (average 30 per mill). This evidence suggests that the carbonaceous grains and bacteria-shaped material in the black cherts are biogenic origin and formed close to a low temperature hydrothermal vent system. The microbial colony might have been quickly fossilized by silicification related to hydrothermal activity. Less