Distribution of high field strength elements (Y, Zr, REE, Hf, Ta, Th, U) in adjacent magnetite and chert bands and in reference standards FeR-3 and FeR-4 from the Temagami iron-formation, Canada, and the redox level of the Neoarchean ocean

Adjacent magnetite and chert bands and reference standards FeR-3 and FeR-4 from the Neoarchean Temagami iron-formation (IF) show shale-normalized rare earth and yttrium (REY) patterns with low Nd/Yb ratios and positive Eu, Gd and Y anomalies, indicating that they formed as marine chemical sediments. In contrast to previous claims. none of the samples shows any Cc anomaly, indicating the absence of oxidative Ce-REY decoupling and arguing against oxic conditions in the wider vicinity of the Neoarchean Temagami seabasin The distribution of Zr. Hf and Ta yields Zr/Hf and Hf/Ta ratios that differ from those of chondrites, average upper continental crust and local shales, Suggesting that the Temagami IF is the only case observed so far in which a significant fraction of these elements is non-detrital but sourced from seawater If Neciarchean seawater was characterized by Zr/Hf and Hf/Ta ratios similar to those of modern seawater these ratios point towards preferential scavenging of Hf over Zr and Ta, as is typical of the modern ocean Similar to the 2 9 Ga old Mozaan IF in the Pongola Supergroup, South Africa, the Temagami IF shows low Th/U ratios that differ from those of the respective local shales and from I hat of aver age upper continental crust. Decoupling of U and Th results from U4+ oxidation in the Earth's surface system and fractionated Th/U ratios in these marine chemical sediments are. therefore. at odds with the lack of Cc anomalies. This suggests a different redox-sensinvity of the two paleo-redox-proxies Th-U and Ce-REY and demonstrates that the Temagami IF and the Mozaan IF warrant further Study of other paleo-redox-proxies. (C) 2009 Elsevier B.V. All rights reserved