Subduction zone geochemistry

Crustal recycling at convergent plate boundaries is essential to mantle heterogeneity. However, crustal signatures in the mantle source of basaltic rocks above subduction zones were primarily incorporated in the form of liquid rather than solid phases. The physicochemical property of liquid phases is determined by the dehydration behavior of crustal rocks at the slab-mantle interface in subduction channels. Because of the significant fractionation in incompatible trace elements but the full inheritance in radiogenic isotopes relative to their crustal sources, the production of liquid phases is crucial to the geochemical transfer from the subducting crust into the mantle. In this process, the stability of specific minerals in subducting crustal rocks exerts a primary control on the enrichment of given trace elements in the liquid phases. For this reason, geochemically enriched oceanic basalts can be categorized into two types in terms of their trace element distribution patterns in the primitive mantle-normalized diagram. One is island arc basalts (IAB), showing enrichment in LILE, Pb and LREE but depletion in HFSE such as Nb and Ta relative to HREE. The other is ocean island basalts (OIB), exhibiting enrichment in LILE and LREE, enrichment or non-depletion in HFSE but depletion in Pb relative to HREE. In either types, these basalts show the enhanced enrichment of LILE and LREE with increasing their incompatibility relative to normal mid-ocean ridge basalts (MORB).