Nd isotopic variations in Precambrian banded iron formations

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Abstract
The isotopic composition of Nd is reported for 8 banded iron formations (BIFs) ranging in age from 0.65 to 3.4 Ga. The data suggest a trend of increasingly positive ϵNd values with age and is interpreted to reflect isotopic variations in Precambrian seawater. The Urucum (0.65 Ga) and the Gunflint (1.9 Ga) BIFs yield negative ϵNd values between −6 and 0. The remaining BIFs, with ages of 1.84 to 3.4 Ga, have predominantly positive values between −1 and +4. The Nd isotopic signature of BIFs changes from a principally continental source to a dominantly depleted mantle source from the present into the Archean. The positive, mantle‐like values of Archean BIFs contrast strongly with the negative, continental‐like values of Phanerozoic seawater. Therefore, the REE budget of the oceans during most of the Archean was probably dominated by hydrothermal circulation of seawater through mid‐ocean ridge basalts and not by river waters, as today.