The early terrestrial crust

Recent geochemical evidence based on the 146Sm–142Nd system and Hadean zircons shows that the Earthʹs mantle experienced depletion approximately 100 Ma after the formation of the solar system, and possibly even before (earlier than 30 Ma), due to the extraction of a crust enriched in incompatible elements. Depending on the model 142Nd abundance assumed for the Bulk Earth, the early crust may have been stored in the deep mantle, or may have been remixed in the mantle with a timescale of ∼1 Ga. If the Earth is considered to have a 142Nd composition identical to that of ordinary chondrites, then it implies that the early crust (or the enriched reservoir) is now present at the core–mantle boundary and has remained isolated from the rest of the Earth for the past 4.5 Ga. If the primordial crust had a basaltic composition, then it is unlikely that this enriched reservoir remained isolated for more than 4.5 Ga due to entrainment; yet, there is no signature of this reservoir in hotspot lavas that should sample this enriched reservoir. In contrast, if the Bulk Earth has an Nd isotopic composition slightly distinct from that of chondrites, then there is no need to invoke a hidden reservoir and the early crust must have been remixed by mantle convection prior to the formation of the modern continents.