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

3.2 ga。澳大利亚皮尔巴拉海岸克利弗维尔群中的迪克森岛组是保存了中太古宙海洋地层学的最佳序列之一，可能包含微生物物质。野外观测和地球化学证据表明，迪克森岛组在太古宙海洋中存在低温热液喷口系统和生物微生物群落。Dixon岛组厚度约350米，由流纹凝灰岩、黑燧石和杂色燧石组成。流纹岩凝灰岩段中有大量黑燧石脉群，表明黑燧石段在沉积过程中有强烈的低温热液活动，厚度为15 ~ 20 m。在层状黑燧石层中，保存了文石、重晶石和石膏之后丰富的二氧化硅假形态，以及明显连续的叠层石样生物岩层（厚10 ~ 20 cm）。叠层石状生物床在细粒二氧化硅中形成细铁或石英。陆源碎屑沉积物的缺失和大量脉状注入表明，该沉积相代表古深度约500 ~ 2000 m的远洋热液环境，不是很深的海洋平面，如未成熟岛弧背景的斜坡。黑燧石段微生物物质保存较好，由块状黑燧石、层状黑燧石、深绿色硅质页岩和凝灰质层状黑燧石组成。这些岩石主要由极细的石英组成。块状黑燧石具有与黑燧石脉相似的碳质球状体（直径0.3 mm ~ 2mm）。大块的黑燧石含有螺旋状、棒状和树突状的细菌物质。总有机碳（TOC）在0.05 ~ 0.16%（平均0.1%）之间变化，碳同位素（δ 13C）值为-32 ~ -27 / ml（平均30 / ml）。这些证据表明，黑燧石中的碳质颗粒和细菌状物质是生物成因的，形成于低温热液喷口系统附近。微生物群落可能是通过与热液活动有关的硅化作用而迅速变成化石的。少

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