Physico-Chemical Transport And Differentiation Processes In Subduction Zones: Mixing At The Slab-Mantle Interface And Melting Of Mélange Rocks In Mantle-Wedge Plumes?

Sediment-derived melts and hydrous fluids loaded with solutes generated in the subducting slab at elevated P-T conditions are thought to transport differing amounts of trace elements and variable isotopic signatures from the subducting slab into the overlying mantle, where they ultimately contribute to the source region of magmas produced at convergent plate margins. Conventional geochemical models of subduction zones consider trace elements to be derived from two distinct sources: hydrous fluids derived from subducted altered oceanic crust and partial melts derived from the thin sedimentary veneer. However, the physical processes of mixing of these contributors within the subduction factory are largely enigmatic. Studies on exhumed subduction mélanges suggest that transference of a significant portion of the trace elements, used to quantify crustal recycling, may not come directly from the subducted crust as conventionally modelled. New rock compositions form from metasomatic reactions, diffusive and mechanical mixing. The newly grown minerals concentrate, sequester and redistribute water and a variety of elements. The interface between the subducting slab and the mantle is characterized by strong petrologic and chemical contrasts: the aluminium-, silica- and alkali-rich slab that carries crustal isotopic signatures and trace-element abundances is juxtaposed with the Mg-rich ultramafic rocks of the harzburgitic mantle. Chemical disparities at the interface between subducting oceanic crustal rocks and the harzburgitic mantle lead to the formation of reaction zones in the mantle above the subducting slab composed of hybrid rocks that carry exotic trace-element patterns and isotopic signatures. Mechanical mixing of crustal and mantle rocks will propagate the formation of hybrid rocks, and fluxing by hydrous fluids derived from the dehydrating slab will enhance reactivity and lead to fluid saturation of the newly formed rocks. The identification of cold plumes in high-resolution numerical experiments (mainly by the Gerya group) provides a mechanism to emplace buoyant subducted crustal and metasomatic rocks from the slab-mantle interface into the mantle wedge. Heating followed by dehydration or melting of metasomatic rocks entrained in cold diapirs could efficiently recycle material from the subduction zone directly into the mantle where it would be subjected to P-T conditions dramatically different from those within the slab. These mantle wedge plumes may transport the hybrid rocks that are formed at the slab-mantle interface into the magma source region; effectively, their fractionated geochemical signatures and water are transported within solid rock. Partial melting of hybrid rocks may produce the large range of major- and trace-element compositions found in modern island arc volcanic rocks.