Abstract
The isotopic composition of Nd is reported for the Hamersley (Western Australia) and the Michipicoten (Ontario) banded iron formations (BIFs). Their initialε Nd values are variable ranging from ≈ 0 to + 4; this range most likely reflects Nd isotopic variability in Archean seawater. Since samples from both Australia and Ontario yield dominantly positiveε Nd values, it is likely that positveε Nd values are a global feature of the Archean oceans. Furthermore, theε Nd-Al P Nd systematics of the samples suggest that the ≈ 0 to + 4 range reflects regional variability in the initialε Nd values of authigenic components in these BIFs, and not varying proportions of clastic contamination. The positive, mantle-likeε Nd values of these BIFs contrast strongly with the negative, continental-like values of Phanerozoic seawater. Therefore, the REE budget of the oceans during the deposition of these BIFs 2.5–2.7 Ga ago was probably dominated by the hydrothermal circulation of seawater through mid-ocean ridge basalts and not by river waters, as is the case today. An order of magnitude higher hydrothermal input of Nd to the oceans is suggested by the data for the Archean. This is probably a result of both a higher hydrothermal water to river water mass flux ratio and higher hydrothermal water Nd concentration caused by hotter hydrothermal waters (> 375°C) during the Archean. The Fe/Nd ratio in modern hydrothermal waters and BIFs are both ≈ 10 5 suggesting that the source of much of the Fe in BIFs (and Archean seawater) was hydrothermal water circulating through Archean mid-ocean ridge systems.
Published Version
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