The isotopic composition of Nd is reported for 8 banded iron formations (BIFs) ranging in age from 0.65 to 3.4 Ga. Th data suggest a trend of increasingly positive &egr;Nd 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 &egr;Nd 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. ¿ American Geophysical Union 1988