کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
4723917 | 1639685 | 2009 | 10 صفحه PDF | دانلود رایگان |
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 Ce 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 Neoarchean 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 that of average 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 Ce anomalies. This suggests a different redox-sensitivity 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.
Journal: Precambrian Research - Volume 174, Issues 3–4, November 2009, Pages 337–346