CONSTRAINTS ON EARLY EARTH DIFFERENTIATION FROM HAFNIUM AND NEODYMIUMISOTOPES

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