Abstract
Pure Superior-type Banded Iron Formation (BIF) samples from the Krivoy Rog Supergroup (Ukraine) are excellent archives of ambient Early Precambrian seawater. They show low concentrations of incompatible elements such as Zr, Hf, and Th, and shale-normalized Rare Earths and Yttrium (REYSN) patterns similar to those of modern seawater, i.e. heavy REYSN enriched patterns with positive LaSN, GdSN and YSN anomalies. Lack of CeSN and presence of positive EuSN anomalies indicate REY contributions to anoxic ferruginous seawater from high-temperature hydrothermal fluids.The depositional age of the Krivoy Rog BIF is ill-defined, but a Late Archean to Paleoproterozoic age has been suggested based on U–Pb zircon ages for units stratigraphically above and below the BIF. We determined Sm–Nd isotopic compositions of pure and impure samples from the Krivoy Rog BIF, which yield an errorchron with an apparent age of 2406±350Ma (MSWD 15), that falls within this broad age range. All pure BIF samples show chondrite-normalized (subscript CN) REY patterns with strong positive EuCN anomalies that are typical for Archean but rather rare and much less pronounced in Proterozoic BIFs. Associated schists also show Archean – rather than post-Archean-style REY distributions. The REY geochemistry of both, chemical and epiclastic sediments, therefore, is more consistent with a Late Archean rather than a post-Archean depositional age of the Krivoy Rog Supergroup.Initial ɛNd values of impure BIFs and of associated schist reveal variable contributions from TTGs less radiogenic in Nd and a more radiogenic component possibly comprised of basement amphibolites or mafic volcanics of the stratigraphically underlying New Krivoy Rog Group. The purest Krivoy Rog BIF, representing local Krivoy Rog seawater, displays an ɛNd2.60Ga value of −2.3. This value is less radiogenic than impure Krivoy Rog BIFs or other near-contemporaneous Neoarchean pure chemical sediments. To preserve this specific local isotopic fingerprint in anoxic Archean seawater, the Krivoy Rog BIF must have been deposited in an isolated sea basin with limited exchange with ferruginous deep-waters of the open ocean.A compilation of REY data for high-purity Precambrian BIFs reveals that EuCN/Eu*CN ratios of Precambrian seawater follow a general global evolution curve, that shows specific peaks which reflect times of increased high-temperature hydrothermal REY input into seawater. Following declining EuCN/EuCN ratios from the Eoarchean to the Mesoarchean, the ratios suddenly rise at 2.7Ga and reach a maximum at 2.6Ga, indicating an increased flux of high-temperature hydrothermal REY to Neoarchean seawater, which supports the hypothesis that times of widespread BIF deposition coincided with periods of intense submarine hydrothermal activity, probably triggered by major mantle plume events. This association is supported by a strong increase of the ɛNd(t) values of pure seawater archives at 2.7–2.6Ga, which reflects an increased flux of mantle Nd into seawater. These results suggest that Eu-REY systematics (and potentially ɛNd systematics) are robust tools to indentify episodes of enhanced mantle plume activity.
Published Version
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