Constraints on early Earth differentiation from hafnium and neodymium isotopes

INFERENCES about the early evolution of the Earth's crust and mantle have come largely from the study of isotope systematics-in particular, those of neodymium(1-5), Neodymium isotope data from the oldest preserved rocks have been interpreted(4,6-8) as reflecting early large-scale chemical depletion of the mantle (presumably resulting from the extraction of continental crust), but these data have remained controversial, in view of the potential for disturbances to the samarium-neodymium system during these rocks' long history(9-11). Here we provide an independent evaluation of the Nd isotope compositions of ten early Archaean (3.6-3.8 Gyr old) gneisses, by investigating the hafnium isotope systematics of zircons from these rocks, The Hf data are consistent with the Nd record in indicating early depletion of the mantle, but fail to verify the scale and variability of this depletion, We conclude that Nd isotopes of early Archaean gneisses do not faithfully record isotopic variations in the early Earth, and therefore that these data need to be examined more critically before they can be used to constrain the early history of crust-mantle differentiation.