Mantle Wedge

Subduction zone kinematics predict that, assuming a fixed lower plate, the velocity of the subduction equals the velocity of the subduction hinge (Vs=−Vh). In all subduction zones the subduction hinge migrates toward the lower plate. However, two main types of subduction zones can be distinguished: 1) those where the upper plate converges toward the lower plate slower than the subduction hinge (mostly W-directed), and 2) those in which the upper plate converges faster than the subduction hinge (generally E- or NE-directed). Along the first type, there generally is an upward flow of the asthenosphere in the hanging wall of the slab, whereas along the opposite second type, the mantle is pushed down due to the thickening of the lithosphere. The kinematics of W-directed subduction zones predict a much thicker asthenospheric mantle wedge, larger volumes and faster rates of subduction with respect to the opposite slabs. Moreover, the larger volumes of lithospheric recycling, the thicker column of fluids-rich, hotter mantle wedge, all should favor greater volumes of magmatism per unit time. The opposite, E–NE-directed subduction zones show a thinner, if any, asthenospheric mantle wedge due to a thicker upper plate and shallower slab. Along these settings, the mantle wedge, where the percolation of slab-delivered fluids generates melting, mostly involves the cooler lithospheric mantle. The subduction rate is smaller, andesites are generally dominant, and the lithosphere thickens, there appears to be a greater contribution to the growth of the continental lithosphere.
Another relevant asymmetry that can be inferred is the slab-induced corner flow in the mantle along W-directed subduction zones, and an upward suction of the mantle along the opposite E- or NNE-directed slabs. The upward suction of the mantle inferred at depth along E–NE-directed subduction zones provides a mechanism for syn-subduction alkaline magmatism in the upper plate, with or without contemporaneous rifting in the backarc. Positive δ11B and high 143Nd/144Nd characterize W-directed subduction zones where a thicker and hotter mantle wedge is present in the hanging wall of the slab. However, this observation disappears where large amounts of crustal rocks are subducted as along the W-directed Apennines subduction zone.

The relationship between mantle perturbation, penetrative magmatism, crustal uplift and the processes of rifting and rift propagation was studied. Quasi-scale model experiments and theoretical analyses by Boussinesq's three function method were used. Three dimensional stress analyses around a rising mantle intrusion and semiquantitative model experiments on mantle upwelling and magma penetration into brittle and brittle-ductile lithospheric materials indicate the origin of ridges, rifts, and rift-basin structures. These structures are commonly initiated by broad domal or plateau uplift. The magnitude and nature of the uplift, fissure or rift development, branching and pattern(s) are controlled primarily by flexural rigidity, magnitude of stress development due to mantle intrusion and magma penetration, and the degree of preferred orientation of weakness (anisotropy) in the lithosphere. Continued magma penetration into the crust produces en echelon fissures, vertical and lateral dike propagation, graben and graben and horst structures with step or block faulting. This is followed by axial subsidence, fault opening and periodic volcanism with continued crustal extension, stretching and slow spreading. Mantle and magma penetration into the crust are shown to be important in developing in situ lateral compressive stress adjacent to and around the active rift zones. Transverse fissures, dike swarms, volcanoes and anticlines between two rifts (as in Iceland) result from such lateral compressive stress. This stress is also important in the development of marginal fissures and faults (as in fast-spreading centers). Time progressive model experiments on magma formation and magma penetration from the asthenosphere-lithosphere boundary show the importance of magma chambers. These chambers cause thinning and stretching of the composite lithosphere, vertical and lateral propagation of dikes and dike swarms, and initiate rifts with or without significant uplift. This study also suggests that multiple dike wedging with rapid extension fracturing can generate excess magma pressure and push force, and may contribute seismicity to spreading centers.

The subsidence and uplift history of the forearc system of southwestern Colombia and northern Ecuador margin is complex and reveals several stages of deformation. The sequential stratigraphy of the forearc area shows the development of three megasequences (M1 to M3). The basal megasequence corresponds to the basement of the forearc, which was formed at the end of the Mesozoic and at the beginning of the Cenozoic and accreted against the Northwestern part of South America related to the accretion of the Late Cretaceous – Paleoceneoceanic plateau. This accretion occurred in a transpressional regime. The second megasequence is composed by deep water sediments, recording the transition between transpressional to compressional stages of the margin from the Late Eocene to the Middle Miocene. The third megasequence is characterized by shallow water sediments strongly constrained by the compressional stage of the margin and the uplift activity of the structural highs since the Late Miocene ...

Abstract  Triassic granitoids related to Palaeo- and Neo-Tethyan events occur widely in the metamorphic terranes largely affected by the Alpine orogeny. A first recorded unmetamorphosed plutonic body intruded into the Palaeotethyan mélange in western Turkey, called the Karaburun granodiorite, is composed of two small intrusive stocks that were emplaced between 240 and 220 Ma. It is compositionally diverse, ranging from granodiorite and tonalite to diorite. These rocks show heterogeneous compositions with 54 to 65 wt % SiO2 and are calc-alkaline in character. They are also subalkaline with molar ratios of Al2O3/(Na2O + K2O) from 0.74 to 1.00 and are metaluminous. Most samples are diopside-normative (0.36–8.64), with Na2O > K2O. Chondrite normalized rare earth element (REE) patterns show various degrees of light REE (LREE) enrichment, with LaN = 57.79 to 99.59 and (La/Yb)N = 5.98–7.85 and Eu negative anomalies (Eu/Eu* = 0.62–0.86). These rocks have coherent patterns in ocean ridge granite (ORG) normalized trace-element plots, marked by variable enrichment in K, Rb, Ba, Th, Ce and depletion in Ta and Nb, similar to I-type granites from subduction zones. In primitive mantle-normalized multi element variation diagrams, the granodiorites show pronounced depletions in the high-field-strength elements (HFSE; Nb, Ta, Zr), Sr, P, and Ti. Trace-element modeling of the Karaburun granodiorite suggests an origin through partial melting of the subduction-modified mantle wedge with minor contribution of crustal components through a process of strong fractional crystallization (FC) combined with slight assimilation-fractional crystallization (AFC). Exposures of typical continental-arc granodiorites in the Karaburun Mélange support the validity of the subduction-accretion model that implies the presence of an active continental margin following closure of the Palaeotethyan Ocean during the Triassic.

One characteristic of many subduction-zone garnet peridotites is that they contain titanium-bearing phases not otherwise found in mantle rocks. In particular, titanoclinohumite and/or its breakdown assemblage consisting of symplectic intergrowths of olivine and ilmenite is common in many of these bodies. The Alpe Arami garnet lherzolite of the Swiss Alps, while lacking titanoclinohumite, displays instead large numbers of FeTiO3 rod-shaped precipitates in the oldest generation of olivine, amounting to approximately 1% by volume, indicating that at some time in its past, the peridotite experienced conditions under which the solubility of TiO2 in olivine was >0.6 wt.%. In order to test the hypothesis that the environment of very high solubility of TiO2 in olivine is to be found at very high pressures, we have conducted experiments on lherzolite compositions with added ilmenite at pressures between 5 and 12 GPa and temperatures of 1350–1700 K. Our results on anhydrous compositions show that whereas solubility of TiO2 was not detected in olivine at 5 GPa, 1400 K where it coexists with rutile, when rutile disappeared from the paragenesis, the solubility climbed to 0.4 wt.% at 8 GPa, 0.5 wt.% at 10 GPa and to >1.0 wt.% at 12 GPa, 1700 K. These results support our previous interpretations from titanate morphology and abundance that the Alpe Arami massif has surfaced from P=10 GPa but remove the need to suggest a deeper origin and possible precursor phase such as wadsleyite. They also support the hypothesis that garnet peridotites with unusual Ti-bearing phases reflect a unique mantle environment occurring in the mantle wedge overlying subduction zones.

The Mersin ophiolite, Turkey, is of typical arc type based on geochemistry of crustal rocks without any signs of mid-ocean ridge (MOR) affinity. We examined its ultramafic rocks to reveal sub-arc mantle processes. Mantle peridotites, poor in clinopyroxene (<1.0 vol.%), show high Fo content of olivine (90–92) and Cr# [=Cr/(Cr + Al) atomic ratio] (=0.62–0.77) of chromian spinel. NiO content of olivine is occasionally high (up to 0.5 wt.%) in the harzburgite. Moho-transition zone (MTZ) dunite is also highly depleted, i.e., spinel is high Cr# (0.78–0.89), clinopyroxene is poor in HREE, and olivine is high Fo (up to 92), but relatively low in NiO (0.1–0.4 wt.%). The harzburgite is residue after high-degree mantle melting, possibly assisted by slab-derived fluid. The high-Ni character of olivine suggests secondary metasomatic formation of olivine-replacing orthopyroxene although replacement textures are unclear. The MTZ dunite is of replacive origin, resulted from interaction between M...

The subsidence and uplift history of the forearc system of southwestern Colombia and northern Ecuador margin is complex and reveals several stages of deformation. The sequential stratigraphy of the forearc area shows the development of three megasequences (M1 to M3). The basal megasequence corresponds to the basement of the forearc, which was formed at the end of the Mesozoic and at the beginning of the Cenozoic and accreted against the Northwestern part of South America related to the accretion of the Late Cretaceous – Paleoceneoceanic plateau. This accretion occurred in a transpressional regime. The second megasequence is composed by deep water sediments, recording the transition between transpressional to compressional stages of the margin from the Late Eocene to the Middle Miocene. The third megasequence is characterized by shallow water sediments strongly constrained by the compressional stage of the margin and the uplift activity of the structural highs since the Late Miocene ...

Subduction-zone magmatism is triggered by the addition of H2O-rich slab-derived flux: aqueous fluids, hydrous partial melts or supercritical fluids from the subducting slab through reactions. Whether the slab-derived flux is an aqueous fluid, a partial melt, or a supercritical fluid remains an open question. In general, with increasing pressure, aqueous fluids dissolve more silicate components and silicate melts dissolve more H2O. Under low-pressure conditions, those aqueous fluids and hydrous silicate melts remain isolated phases due to the miscibility gap. As pressure increases, the miscibility gap disappears and the two liquid phases becomes one phase. This vanishing point is regarded as critical end point or second critical end point. X-ray radiography experiments locate the pressure of the second critical end point at 2.5 GPa（ 83 km depth）and 700 °C for sediment-H2O, and at 2.8 GPa （92 km depth） and 750 °C for high-Mg andesite（HMA）-H2O. These depths correspond to the depth range of a subducted oceanic plate beneath volcanic arcs. Sediment-derived supercritical fluids, which are fed to the mantle wedge from the subducting slab, may react with the mantle peridotite to form HMA supercritical fluids due to peritectic reaction between silica-rich fluids and olivine-rich mantle peridotite. Such HMA supercritical fluids may separate into aqueous fluids and HMA melts at 92 km depth during ascent. HMA magmas can be erupted as they are, if the HMA melts segregate without reacting to the overriding peridotite. Partitioning behaviors between aqueous fluids and melts are determined with and without（Na, K）Cl using synchrotron X-ray fluorescence. The data indicate that highly saline fluids effectively transfer large-ion lithophile elements. If the slab-derived supercritical fluids contain Cl and subsequently separate into aqueous fluids and melts in the mantle wedge, then such aqueous fluids inherit much more Cl and also more or less amounts of large ion lithophile elements than the coexisting melts. In contrast, Cl-free aqueous fluids cannot effectively transfer Pb and alkali earth elements to the magma source. Enrichment of some large-ion lithophile elements in arc basalts relative to mid-oceanic ridge basalts has been attributed to mantle source fertilization by such aqueous fluids from a dehydrating oceanic plate. Such aqueous fluids are likely to contain Cl, although the amount remains to be quantified. If such silica-rich magmas survive as andesitic melts under a limited reaction with mantle minerals, they may erupt as HMA magmas having slab-derived signatures.