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).
The thermal regime of subduction zones can be categorized into two stages in both time and space. The first stage is characterized by compressional tectonism at low thermal gradients. As a consequence, metamorphic dehydration of the subducting crust prevails at forearc to subarc depths due to the breakdown of hydrous minerals such as mica and amphibole in the stability field of garnet and rutile, resulting in the liberation of aqueous solutions with the trace element composition that is considerably enriched in LILE, Pb and LREE but depleted in HFSE and HREE relative to normal MORB. This provides the crustal signature for the mantle sources of IAB. The second stage is indicated by extensional tectonism at high thermal gradients, leading to the partial melting of metamorphically dehydrated crustal rocks at subarc to postarc depths. This involves not only the breakdown of hydrous minerals such as amphibole, phengite and allanite in the stability field of garnet but also the dissolution of rutile into hydrous melts. As such, the hydrous melts can acquire the trace element composition that is significantly enriched in LILE, HFSE and LREE but depleted in Pb and HREE relative to normal MORB, providing the crustal signature for the mantle sources of OIB. In either case, these liquid phases would metasomatize the overlying mantle wedge peridotite at different depths, generating ultramafic metasomatites such as serpentinized and chloritized peridotites, and olivine-poor pyroxenites and hornblendites. As a consequence, the crustal signatures are transferred by the liquid phases from the subducting slab into the mantle.