Hydrothermal alteration and microfossil artefacts of the 3,465-million-year-old Apex chert
Hydrothermal alteration and microfossil artefacts of the 3,465-million-year-old Apex chert
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34.65 亿年前的 Apex 燧石的热液蚀变和微化石文物
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
V. Clément
中科院分区：
文献类型：
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作者：
D. Pinti;R. Mineau;V. Clément
最早的蓝藻化石来自西澳大利亚 34.65 亿年前的 Apex 燧石，这一报道颇具争议。对化石地点附近岩石的矿物学分析表明，该地点在低温和中温下经历了反复的变化，因此不太可能保存任何早期的生命形式。据报道，西澳大利亚皮尔巴拉克拉通 34.65 亿年前的顶端燧石含有一些最古老的生命化石证据：被解释为蓝细菌的微小碳质丝1。然而，基于重晶石和天然金属的存在，有人认为燧石是由温度高于 250 ∘C 的热液流体形成的（参考文献 2、3），这对于蓝藻来说不太可能是环境。在这里，我们使用扫描电子显微镜来评估含有微化石的节点周围燧石脉的矿物学和沉积环境。除了稀有的天然金属之外，我们还发现了氧化铁和粘土矿物的组合，我们认为它们是在中低温度下重复的热液蚀变脉冲过程中形成的。我们还发现了类似于微生物外聚合物的微米级二氧化硅结构，以及由管状矿物部分溶解形成的纹理，看起来类似于微生物垫化石5。其他分支的微观结构表明微生物在沉积后对微裂纹和裂缝进行定殖5。这些观察结果并不直接适用于据称是最早的生命化石证据的碳质丝，因为我们没有在样本中发现任何碳质丝。然而，由于观察到的热液和地下水变化，我们得出结论，顶端燧石不太可能保存任何早期生命形式。
The earliest fossils of cyanobacteria have been controversially reported from the 3,465-million-year-old Apex chert in Western Australia. Mineralogical analyses of the rock near the fossil site indicate that the site experienced repeated alterations at low and medium temperatures, and is therefore unlikely to have preserved any early forms of life. The 3,465-million year-old Apex chert of the Pilbara Craton in Western Australia has been controversially reported to contain some of the oldest fossil evidence for life: tiny carbonaceous filaments interpreted as cyanobacteria1. Yet, on the basis of the presence of barite and native metals, it has been suggested that the chert formed from hydrothermal fluids at temperatures greater than 250 ∘C (refs 2, 3), an unlikely environment for cyanobacteria. Here we use scanning electron microscopy to assess the mineralogy and depositional setting of the chert veins surrounding the nodes that contain the microfossils. In addition to rare native metals, we find an assemblage of iron oxides and clay minerals, which we interpret to have formed during repeated pulses of hydrothermal alteration at low to medium temperatures. We also find micrometre-sized silica structures that resemble microbial exopolymers4, and textures formed by the partial dissolution of tubular minerals that look similar to fossilized microbial mats5. Other branched microstructures suggest post-depositional colonization of microcracks and fissures by microbes5. These observations are not directly applicable to the carbonaceous filaments purported to be the earliest fossil evidence of life, as we did not recover any in our sample. However, because of the observed hydrothermal and groundwater alteration, we conclude that the Apex chert is unlikely to have preserved any early forms of life.