Mantle WedgeEdit
Mantle wedge is the sector of the upper mantle that lies above a subducting tectonic plate at convergent margins, forming a wedge-shaped region between the descending slab and the overlying lithosphere. This region plays a central role in the generation of arc magmas, the transport of fluids released from the slab, and the overall dynamics of subduction zones. Through a combination of petrological processes, fluid flux, and mantle flow, the mantle wedge links deep geophysical processes with surface volcanism and crustal evolution. Mantle Subduction zone Arc volcanism
The mantle wedge operates within a complex tectonic system in which a cold, dense slab sinks into the mantle while the overriding plate remains at the surface or near-surface. The interaction between the slab and the surrounding mantle drives a distinctive flow pattern, often described as corner flow, in which mantle material moves toward the trench at shallow depths and then returns to depth beneath the wedge. This circulation is thought to couple the slab with the wedge, facilitating the transfer of fluids and influencing melting processes that produce arc magmas. Subduction zone Geodynamics
Discussions of the mantle wedge encompass a range of observations and models, from petrological experiments on hydrous melting to seismic imaging that reveals hydration signatures and low-velocity zones within the wedge. These lines of evidence are synthesized to understand how water, fluids, and trace elements are transported from the subducting slab into the mantle above it, and how these inputs modify the mantle’s mineralogy and melting behavior. Seismic tomography Serpentinization Hydration of mantle Geochemistry
Geological setting
At convergent margins, the subducting plate begins to descend beneath the overriding plate, descending into the mantle where high pressures and relatively low temperatures permit unique metamorphic and melting processes. The mantle wedge occupies the mantle domain just above the slab, extending from the trench toward deeper levels where the slab interacts with the surrounding mantle. The wedge is bounded laterally by the overlying crust and the mantle beneath, and its geometry is influenced by slab dip, convergence rate, and the dynamics of slab rollback or bite. Debates persist about the exact depth extent and lateral variation of the wedge across different subduction zones, but a common feature is the reliance on slab-derived fluids to modify mantle rheology and melting behavior. Subduction zone Mantle Seismology Geodynamics
Structure and composition
The mantle wedge is composed mainly of peridotitic mantle rock that has undergone hydration and metasomatic alteration as fluids released from the slab percolate upward. Serpentinization and related hydrous mineral phases can form in the cooler, shallower portions of the wedge, altering its rheology and melting point. At greater depths, mantle rocks may still be hydrous but experience high-temperature reactions that lead to flux melting, generating silicate melts that rise and contribute to arc volcanism. The chemical signature of wedge-derived magmas often records a mix of depleted mantle, slab-derived components, and crustal assimilation, which is detected in isotopic and trace-element data. Serpentinization Mantle Isotopic geochemistry Arc volcanism
Dynamics and processes
A central process in the mantle wedge is flux melting driven by fluids released from the subducting slab. Water released from hydrous minerals lowers the solidus of adjacent mantle rocks, promoting partial melting and the generation of intermediate to felsic arc magmas. The fluids also metasomatize the mantle wedge, introducing trace elements and altering mineral assemblages. Mantle flow within the wedge, including corner flow and potential shear-driven deformation, helps transport these melts toward the volcanic arc and distributes heat and material within the wedge. Regional variations in slab geometry and convergence rates lead to differing melting regimes and arc magnitudes. Flux melting Hydration of mantle Arc volcanism Geodynamics
Role in arc volcanism
Most subduction-zone volcanoes are fed by magmas with components derived from the mantle wedge, modified by slab-derived fluids and crustal processes. The mantle wedge supplies melts that become the primary magma source for many volcano chains, with geochemical signatures that reflect mixing between mantle wedge material and slab-derived fluids. The resulting arc lavas are typically calc-alkaline and display distinctive isotopic systematics that reveal the wedge’s role as a melting and transport reservoir. Studies of arc volcanism frequently invoke the mantle wedge as a key source of water, incompatible elements, and heat necessary to sustain volcanic activity in these regions. Arc volcanism Isotopic geochemistry Mantle wedge Seismic tomography
Geophysical and geochemical signatures
Imaging techniques such as Seismic tomography reveal hydrated low-velocity zones and high-water-content features within the wedge, consistent with fluid-rich mantle that has interacted with slab-derived fluids. Geochemically, arc lavas exhibit enrichment in volatile components and trace elements that trace back to the mantle wedge source, with isotopic signatures indicating contributions from both depleted mantle and slab materials. Experimental petrology supports the idea that hydrous melting in the wedge can occur at temperatures and pressures characteristic of subduction zones, producing magmas that feed arc volcanism. Seismic tomography Isotopic geochemistry Flux melting Serpentinization
Variations and regional examples
Mantle-wedge characteristics vary across subduction zones, reflecting differences in slab age, angle, and convergence rate, as well as in the thermal structure of the overriding plate. Some regions show more extensive serpentinization and fluid networks, while others exhibit more limited hydration and a greater emphasis on decompression melting or crustal assimilation. Comparative studies across the Andes, the Cascades, the Japan-led arcs, and other systems illustrate how regional tectonics sculpt wedge rheology, melting, and magma evolution. Cascadia subduction zone Andes Japan subduction zone Mantle wedge
Controversies and debates
Relative importance of slab dehydration versus corner-flow–driven melting: While many models emphasize fluids released directly from the subducting slab as the primary driver of wedge melting, others argue that the geometry and dynamics of wedge flow can amplify or even limit melting in different settings. Both viewpoints recognize fluids as essential but differ on how hot, how much, and where melts originate within the wedge. Hydration of mantle Flux melting
Geometry and depth extent of the wedge: The precise shape and depth extent of the mantle wedge remain subjects of debate, with some models favoring relatively shallow, well-defined hydrated zones and others proposing deeper, more diffuse zones of interaction. Seismic anisotropy, hot-spots of melt generation, and variations in isotopic signatures contribute to these discussions. Seismic tomography Mantle
Role of slab rollback and mantle flow in arc development: The interplay between slab rollback, trench retreat, and mantle flow in the wedge influences melting regimes and arc evolution. Competing models interpret these dynamics differently, leading to ongoing discussions about how much of arc volcanism is controlled by steady-state wedge processes versus transient tectonic events. Geodynamics Arc volcanism
Interpretation of geochemical signals: The mixing lines between mantle wedge material and slab-derived inputs can be complex, and there is debate about the relative contributions of different reservoirs to arc magmas. Isotopic data are powerful but require careful modeling of crustal processing and mantle heterogeneity. Isotopic geochemistry Geochemistry