Exogenous carbonaceous microstructures in Early Archaean cherts and BIFs from the Isua Greenstone Belt: implications for the search for life in ancient rocks
Exogenous carbonaceous microstructures in Early Archaean cherts and BIFs from the Isua Greenstone Belt: implications for the search for life in ancient rocks
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DOI：
10.1016/s0301-9268(03)00102-5
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发表时间：
2003-10
影响因子：
3.8
通讯作者：
F. Westall;R. Folk
中科院分区：
文献类型：
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作者：
F. Westall;R. Folk
使用配备电子衍射系统、能够分析轻元素的高分辨率扫描电子显微镜研究了来自超过 3.7 年历史的伊苏亚绿岩带（格陵兰岛西南部）的早期太古代带状铁地层 (BIF) 和燧石的高频蚀刻样品的微观结构。岩石含有内生（内部来源）和外生（外部来源）碳质微观结构。前者由石墨夹杂物和可能的小无定形碳质颗粒组成，两者均嵌入metacherts中（但是，需要进一步的原位TEM研究来验证无定形颗粒的内生性）。此外，这些岩石还含有内生微生物（即栖息在岩石裂缝中），以及存在于颗粒之间的裂缝和裂缝中的未分化碳质物质。这些微生物包括蓝细菌、丝状微生物如真菌菌丝和可能的细菌，以及大型的、未鉴定的细胞或孢子。大多数微生物似乎已经化石化。内石明显比主岩年轻，但一定是在不同时期渗透的，最有可能是在内陆冰消退之后（约 8000 年前）。这些岩石的裂缝和微裂隙中存在的内石碳质物质将影响对大块样品的任何分析，例如碳同位素和化学生物标记物，以及对酸浸渍残留物的分析。因此，之前对 Isua 的 BIF 和燧石进行的同位素测量可能反映的是较年轻的污染，而不是内源（原始）信号。同样，一些先前描述的 Isuan 微生物可能代表了最近的内石污染。
The microstructure of HF-etched samples of Early Archaean banded iron formations (BIFs) and cherts from the >3.7 b.y.-old Isua Greenstone Belt (southwestern Greenland) was investigated using high resolution scanning electron microscopy equipped with an electron diffraction system, capable of analysing light elements. The rocks contain both endogenous (of internal origin) and exogenous (of external origin) carbonaceous microstructures. The former consist of inclusions of graphite and, possibly, small, amorphous carbonaceous particles, both embedded in metacherts (however, further in situ TEM studies are needed to verify the endogeneity of the amorphous particles). Moreover, these rocks also contain endolithic microorganisms (i.e. inhabiting cracks in rocks), as well as undifferentiated carbonaceous matter, that occur in fractures and cracks between grains. The microorganisms include cyanobacteria, filamentous microorganisms such as fungal hyphae and possibly bacteria, as well as large, unidentified cells or spores. Most of the microorganisms appear to have been fossilised. The endoliths are evidently younger than the host rock, but must have infiltrated at different periods, most likely after the Inland Ice retreated (∼8000 years ago). The presence of endolithic carbonaceous matter in cracks and microfissures in these rocks will affect any analyses of bulk samples, such as carbon isotopes and chemical biomarkers, as well as analyses of acid-macerated residues. Thus, previous isotope measurements made on BIFs and cherts from Isua may reflect younger contamination rather than an endogenous (original) signal. Likewise, some of the previously described Isuan microorganisms probably represent recent, endolithic contamination.