Résumé
Application of the 147Sm-143Nd and 146Sm-142Nd chronometers has suggested that the initial differentiation of Earth's mantle into enriched and depleted reservoirs may have begun within the first 100-200million years of Earth's history. However, little is known about the differentiation of the early crust; although evidence has suggested the presence of enriched crustal material, data regarding the nature and composition of this crust are limited. Here we present 147Sm-143Nd data from the weakly metamorphosed basalt and layered chert-barite successions from the Dresser Formation of the Pilbara Craton, Western Australia. The Sm-Nd isochron indicates an age of 3.49+/-0.10 billion years, in agreement with previous estimates from Pb-Pb (ref. 6) and U-Pb (ref. 7) dating, which indicates that the Sm-Nd system has not been reset. Our measured ɛNd value of -3.3+/-1.0 for the rocks at this site is consistent with formation from an older protolith. On the basis of our modelling of trace element and isotopic compositions from these rocks, we suggest that the older component was crustal in nature, and differentiated from the convective mantle more than 4.3 billion years ago.