Geochemistry and Secular Geochemical Evolution of the Earth's Mantle and Lower Crust

Abstract

The incompatible elements U and Th are related to Pb via radioactive decay. Extraction, modifi cation and storage of continental crust have, over time, left an isotopic record in the continental crust itself and in the depleted portion of the mantle. Ancient lower crustal xenoliths require that crust has matured by upward transport of radioactive heat - producing elements; hundreds of millions of years after formation. Recycling of continental material has contributed in at least three ways to the generation of enriched mantle - melt sources. First, this has occurred by delamination of lower crustal segments back into the mantle. Second, sediment has been recycled back into the mantle in subduction zones, and third, since the oxygenation of the atmosphere, seawater U, weathered from the continents, has been incorporated into hydrated oceanic crust with which it has ultimately been recycled back into the mantle. The joint treatment of the lower continental crust and the mantle in terms of their geochemgeochemistry and their isotopic evolution may seem, at fi rst, a less than obvious choice. They are, however, related in the sense that the evidence for their evolution is largely of indirect nature, either inferred from rare xenoliths or via products of partial melting. Any joint treatment of these two geochemical reservoirs also inherently carries with it the assumption that they have, at least in part, mutually infl uenced each other's temporal evolution. Before attempting to condense into an opening book chapter the relevant aspects of the exhaustive body of knowledge about the geochemistry of the mantle and the much sparser information regarding the lower crust, it is necessary to remind ourselves of the evidence for their mutually related evolutions. © 2011 Blackwell Publishing Ltd. All rights reserved.