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

Abstract

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 ∼30–70 μm 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 (∼3–∼12 μm) 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 ∼26-nm nanobands observed only in hematite grains reflect an annual deposition of ∼18.6 μm, or ∼18.6 m/Myr, which is also consistent with the depositional rate calculated by geochronologic methods. It is tentatively suggested that these ∼26-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.