Crustal recycling and the Aleutian arc
Two types of crustal recycling transfer continental crust back into its mantle source. The first of these, upper crustal recycling, involves elements that have been fractionated by the hydrosphere-sediment system, and are subducted as a part of the oceanic crust. The subduction process (S-process) then fractionates these elements, and those not removed at shallow tectonic levels and as “excess” components of arc magmas are returned to the mantle. Newly determined trace element compositions of Pacific oceanic sediments are variable and mixing is necessary during the S-process, if sediment is to provide “excess” elements in the ratios observed in Aleutian arc magmas. Only a small fraction of the total sediment subducted at the Aleutian trench is required to furnish the “excess” elements in Aleutian arc magmas. Ba and 10Be data indicate that this small fraction includes a contribution from the youngest subducted sediment. The second type of recycling, lower crustal recycling, involves crystal cumulates of both arc and oceanic crustal origin, and residues from crustal melting within arc crust. Unlike the silicic sediments, recycled lower crust is mafic to ultramafic in composition. Trace element analyses of xenoliths representing Aleutian arc lower crust are presented. Recycling by delamination of lower crust and attached mantle lithosphere may occur following basalt eclogite phase transformations that are facilitated by terrane suturing events that weld oceanic island arcs to the continents. The relative importance of upper and lower crustal recycling exerts a primary control on continental crustal composition.