Micro- and nanobands in late Archean and Palaeoproterozoic banded-iron formations as possible mineral records of annual and diurnal depositions

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• 作者：Yi-Liang Li ; ^{yiliang@hku.hk" class="auth_mail}
• 关键词：Precambrian ; banded-iron formation ; microbands ; diurnal precipitation ; annual depositions
• 刊名：Earth and Planetary Science Letters
• 出版年：1 April, 2014
• 年：2014
• 卷：391
• 期：Complete
• 页码：160-170
• 全文大小：4925 K

文摘

The microbands in Precambrian banded-iron formations (BIFs) have been conjectured to record annual or even diurnal depositions. However, these bands have rarely been observed in high resolution at their true (micro) scale. Here, I suggest that nanobands of fine-grained hematite represent possible diurnal depositions and that microbands of chert/jasper represent possible annual depositions in three sets of BIFs: 2460-Myr BIFs from the Kuruman Iron Formation, Transvaal Supergroup of South Africa; 2480-Myr BIFs from the Dales Gorge Member of the Brockman Iron Formation, Western Australia; and 2728-Myr BIFs from the Hunter Mine Group, Abitibi Greenstone Belt, Canada. Observations made using scanning electron microscopy indicate that hematite and chert were syngenetic, and that there was a hiatus between their precipitation and the genesis of the remainder of the minerals containing structural Fe(II). Spindle-like grains of hematite, monocrystals of magnetite, and ferro-dolomite formed microbands of 鈭?0-70 渭m in thickness, which appear cyclically in the matrix of the chert. Neither the band-bound magnetite and dolomite nor the linear formations of the hematite spindles represent annual depositions due to their diagenetic features. The thinnest microbands () were observed in the chert and jasper, and indicate depositional rates of 6.6-22.2 m/Myr in the BIFs. These rates are consistent with the integrated deposition rates calculated by geochronologic methods for the BIFs, if annual deposition is assumed. The 鈭?6-nm nanobands observed only in hematite grains reflect an annual deposition of 鈭?8.6 渭m, or 鈭?8.6 m/Myr, which is also consistent with the depositional rate calculated by geochronologic methods. It is tentatively suggested that these 鈭?6-nm nanobands were formed from the diurnal precipitation of Fe(III) resulting from the circadian metabolism of Fe(II)-oxidizing or oxygen-evolving photosynthetic microorganisms, which slowed down the rise of atmospheric oxygen. The diurnal precipitation of Fe(III) as hematite and the annual deposition of silica as chert/jasper in the BIFs provide internal clocks that may facilitate the examination of short-term processes, such as ecological, oceanographic and climatic cycles, that are recorded by the mineral or chemical compositions of BIFs.