In summary, recycling of material in subduction zones occurs on different scales: (i) return of expelled fluids to the surface or to near-surface levels in the forearc system, (ii) enrichment of the source region of arc magmas by metasomatic agents originating in the subducting slab and eruption of those magmas in the volcanic arc, and (iii) storage of deeply subducted crust in the mantle and later sampling of these heterogeneities by plume-related volcanoes.Subduction zones are therefore the most important sites of chemical interaction between the Earth's crust and mantle and are critical in understanding the long-term evolution of the two geochemical reservoirs.