Ultradeep xenoliths from African kimberlites: Sr and Nd isotopic compositions suggest complex history

Rare kimberlite xenoliths have chemical characteristics that indicate an origin deep in the upper mantle, in the vicinity of the transition zone. The Nd isotopic compositions of minerals separated from several of these xenoliths suggest that these materials were emplaced into the African lithosphere at times ranging from approximately the time of kimberlite volcanism to many hundreds of millions of years earlier, possibly by periodic plume transport. They also indicate that these ultradeep samples have a complex chemical history involving both enrichment and depletion, and — assuming that there has not been re-equilibration within the lithosphere — that they sample an isotopically heterogeneous deep upper mantle. This would be expected if this part of the mantle contains a heterogeneous assemblage of subducted crustal and lithospheric components.