The problem of deep carbon - An Archean paradox
The problem of deep carbon - An Archean paradox
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DOI：
10.1016/j.precamres.2005.09.003
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发表时间：
2005-12-15
影响因子：
3.8
通讯作者：
Mertzman, SA
中科院分区：
文献类型：
--
作者：
Lindsay, JF;Brasier, MD;Mertzman, SA
富含碳的黑色硅质岩是太古宙早期地体的共同特征，在西澳大利亚的皮尔巴拉克拉通尤为突出。这些岩脉穿过当地地层，然后在特定的地层层位处突然截断，在那里可以看到它们与地层序列交错。这些独特的太古宙岩墙令人费解，并引发了关于它们所含碳的来源的重大问题--它是生物源的，因此表明早期生物圈，还是非生物的，或许表明导致地球上第一个生命出现的生前条件？暴露在西澳大利亚皮尔巴拉克拉通上的斯特雷利池冰川，是瓦拉乌纳群的一部分，不整合地坐落在更古老的超镁铁质火山岩上。该地层厚度一般不到30米，由大范围内发现的相对一致的岩性序列组成。在局部，底部海侵砂岩被一系列碳酸盐、重晶石、黑白相间的层状硅质岩和灰绿色硅质岩所覆盖，灰绿色硅质岩被风化的玄武岩局部染成蓝绿色。该组与正断层相交，其周围的沉积序列增厚变薄。下伏岩脉杂岩的矿物学和地球化学(包括稳定同位素数据)与保存在Strelley Pool Chert中的岩性密切相关。同样，岩墙杂岩内部空隙中矿物的沉积序列与上面的沉积序列相似。来自Strelley Pool Chert及其伴生的岩墙杂岩的新数据表明，黑色硅质岩岩脉是热液矿物学杂岩的组成部分。该杂岩聚集在由生长断层控制的岩脉提供的喷口周围，这些岩脉延伸到太古宙古海底以下2公里深的地方。沉积序列的矿物学反映了在下面侵入的花岗岩杂岩驱动的演化热液系统的降温制度下的沉积。断裂系统及其伴生的热液杂岩是在间歇性侵入的花岗岩杂岩上方的穹隆形成的张性环境中演化出来的，形成了皮尔巴拉原生大陆的框架。黑色硅质岩脉中的碳质化合物可能是在高温(200-300摄氏度)和压力(500巴)的古热液系统内2公里的高温(200-300摄氏度)和压力(500巴)下通过类似费托合成的过程非生物地生成的。因此，生命最初进化的“原始介质”可能是一种富含二氧化硅的泥浆，具有低pH值和高二氧化碳含量，其中包含对生命起源至关重要的基本非生物有机构件。来自这些热液系统的非生物有机产出可能会压倒任何早期生物圈地球化学信号，因此可能是识别早期生物圈记录的最困难的环境。(C)2005 Elsevier B.V.保留所有权利。
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-control led 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 degrees C) and pressures (500 bar) at a paleo-depth of 2km 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 CO2 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. (c) 2005 Elsevier B.V. All rights reserved.