High-resolution Ge-Si-Fe, Cr isotope and Th-U data for the Neoarchean Temagami BIF, Canada, suggest primary origin of BIF bands and oxidative terrestrial weathering 2.7 Ga ago

Unique deposits that formed in the Precambrian oceans are the Banded Iron-Formations (BIFs) which are chemical sediments comprised of alternating layers of iron- and silica-dominated precipitates. The origin of this enigmatic banding is still debated, with most favoring a primary sedimentary and a few others arguing for a secondary diagenetic origin. We here use a high-resolution integrated multi-proxy approach and report on the Ge-Si-Fe and Th-U systematics and on the isotopic composition of Cr in exceptionally pure adjacent magnetite and metachert bands from the similar to 2.7 billion years old Temagami BIF, Ontario, Canada.While each of the geochemical proxies alone may be controversial, our multi-proxy approach sheds new light on their applicability, as the two types of adjacent BIF bands are each characterised by a distinct geochemical signature. The magnetite bands show super-crustal (i.e. fractionated) Ge/Si ratios approaching those of modern marine high-temperature hydrothermal fluids, unfractionated Th/U ratios similar to those of crustal rocks, and unfractionated Cr isotope ratios similar to Bulk Silicate Earth. Adjacent metachert bands, however, show lower (i.e. unfractionated) Ge/Si ratios close to those of modern seawater, sub-crustal (i.e. fractionated) Th/U ratios, and fractionated Cr isotope ratios above those of Bulk Silicate Earth. The composition of the magnetite bands suggests that the original iron-oxyhydroxide precipitated during periods when the dissolved Ge, Th, U, and Cr inventory of ambient seawater was dominated by input from mafic crust via anoxic high-temperature hydrothermal fluids. In contrast, the composition of the metachert bands indicates that these formed during times when ambient seawater was dominated by low-temperature riverine input from an evolved landmass that despite its Neoarchean age had provided environments for oxidative Cr and U mobilization during terrestrial weathering. This suggests that habitats with oxygenic photosynthesis existed on land at least 200 million years before the Great Oxidation Event. The systematic and internally consistent difference between the geochemical signatures shown by the two types of adjacent BIF bands also demonstrates that the prominent banding of BIFs is a primary depositional feature reflecting precipitation from different watermasses and is not the result of post-depositional separation of an initially homogeneous chemical sediment. (c) 2022 Published by Elsevier B.V.

Automated summary

This study investigated the origin of Banded Iron-Formations (BIFs) using a high-resolution integrated multi-proxy approach, revealing distinct geochemical signatures of adjacent BIF bands, indicating different formation processes and water sources.