A mantle wedge is a triangular shaped piece of mantle that lies above a

subducting Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

tectonic plate Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...

and below the overriding plate. This piece of mantle can be identified using seismic velocity imaging as well as earthquake maps. Subducting oceanic slabs carry large amounts of

water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

; this water lowers the melting temperature of the above mantle wedge. Melting of the mantle wedge can also be contributed to depressurization due to the flow in the wedge. This melt gives rise to associated

volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...

on the earth's surface. This volcanism can be seen around the world in places such as Japan and

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...

Cross-section of a subduction zone and back-arc basin

Water in mantle wedge

Magmas produced in subduction zone regions have high volatile contents. This water is derived from the breakdown of hydrous minerals in the subducting slab, as well as water in the oceanic plate from percolation of seawater. This water rises from the subducting slab to the overriding mantle wedge. The water lowers the melting temperature of the wedge and leaves behind melt inclusions that can be measured in the associated arc volcanic rocks.

Structure of the mantle wedge

The

forearc Forearc is a plate tectonic term referring to a region between an oceanic trench, also known as a subduction zone, and the associated volcanic arc. Forearc regions are present along a convergent margins and eponymously form 'in front of' the vo ...

mantle extends from where the subducting slab meets the cold nose of the mantle wedge, this occurs at depths from 10–40 km. Low seismic attenuation, and high seismic velocities characterize this region. There is a boundary between this low attenuation region and a high attenuation region on the forearc side of the arc volcanoes. To image the mantle wedge region below volcanic arcs P-wave, S-wave and seismic attenuation images should be used in coordination. These tomographic images show a low velocity, high attenuation region above the subducting slab. The slowest velocities in these volcanic arc regions are Vp= 7.4 km·s⁻¹ and Vs= 4 km·s⁻¹. Mantle wedge regions that do not have associated arc volcanism do not show such low velocities. This can be attributed to the melt production in the mantle wedge.

Mantle wedge flow

Flow in mantle wedges has important effects on the thermal structure, overall mantle circulation and melt within the wedge.

Mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

s are anisotropic and have the ability to align themselves within the mantle when exposed to strain. These mineral alignments can be seen using

seismic imaging Geophysical imaging (also known as geophysical tomography) is a minimally destructive geophysical technique that investigates the subsurface of a terrestrial planet. Geophysical imaging is a noninvasive imaging technique with a high parametrical a ...

, as waves will travel through different orientations of a mineral at different speeds.

Shear strain In physics, deformation is the continuum mechanics transformation of a body from a ''reference'' configuration to a ''current'' configuration. A configuration is a set containing the positions of all particles of the body. A deformation can ...

associated with mantle flow will align the fast direction of pyroxene and

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

grains in the direction of flow. This is the most common theory on flow within the mantle, although opposing theories do exist (6). Flow within the mantle wedge is parallel to the crust until it reaches the relatively cooler nose of the wedge, then is overturned and is parallel to the subducting slab. The nose of the wedge is generally isolated from the overall mantle flow.

Oxidation in the mantle wedge

Studies have shown that

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

s that produce island arcs are more

oxidized Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

than the magmas that are produced at

mid-ocean ridges A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverg ...

. This relative degree of oxidation has been determined by the iron oxidation state of

fluid inclusions image:Inclumed.gif, 250px, Trapped in a time capsule the same size as the diameter of a human hair, the ore-forming liquid in this inclusion was so hot and contained so much dissolved solids that when it cooled, crystals of halite, sylvite, gypsum, ...

in glassy volcanic rocks. It has been determined that this state of oxidation is correlated with the water content of the mantle wedge. Water itself is a poor oxidant and therefore the oxidizing agent must be transported as a dissolved ion in subducting slab.

References

{{Reflist Tectonics Subduction