Mantle Wedge (mantle + wedge)

Distribution by Scientific Domains


Selected Abstracts


Aqueous fluids at elevated pressure and temperature

GEOFLUIDS (ELECTRONIC), Issue 1-2 2010
A. LIEBSCHER
Abstract The general major component composition of aqueous fluids at elevated pressure and temperature conditions can be represented by H2O, different non-polar gases like CO2 and different dissolved metal halides like NaCl or CaCl2. At high pressure, the mutual solubility of H2O and silicate melts increases and also silicates may form essential components of aqueous fluids. Given the huge range of P,T,x regimes in crust and mantle, aqueous fluids at elevated pressure and temperature are highly variable in composition and exhibit specific physicochemical properties. This paper reviews principal phase relations in one- and two-component fluid systems, phase relations and properties of binary and ternary fluid systems, properties of pure H2O at elevated P,T conditions, and aqueous fluids in H2O,silicate systems at high pressure and temperature. At metamorphic conditions, even the physicochemical properties of pure water substantially differ from those at ambient conditions. Under typical mid- to lower-crustal metamorphic conditions, the density of pure H2O is , the ion product Kw = 10,7.5 to approximately 10,12.5, the dielectric constant , = 8,25, and the viscosity , = 0.0001,0.0002 Pa sec compared to , Kw = 10,14, , = 78 and , = 0.001 Pa sec at ambient conditions. Adding dissolved metal halides and non-polar gases to H2O significantly enlarges the pressure,temperature range, where different aqueous fluids may co-exist and leads to potential two-phase fluid conditions under must mid- to lower-crustal P,T conditions. As a result of the increased mutual solubility between aqueous fluids and silicate melts at high pressure, the differences between fluid and melt vanishes and the distinction between fluid and melt becomes obsolete. Both are completely miscible at pressures above the respective critical curve giving rise to so-called supercritical fluids. These supercritical fluids combine comparably low viscosity with high solute contents and are very effective metasomatising agents within the mantle wedge above subduction zones. [source]


P - and S -velocity images of the lithosphere,asthenosphere system in the Central Andes from local-source tomographic inversion

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2006
Ivan Koulakov
SUMMARY About 50 000 P and S arrival times and 25 000 values of t* recorded at seismic arrays operated in the Central Andes between 20S and 25S in the time period from 1994 to 1997 have been used for locating more than 1500 deep and crustal earthquakes and creating 3-D P, S velocity and Qp models. The study volume in the reference model is subdivided into three domains: slab, continental crust and mantle wedge. A starting velocity distribution in each domain is set from a priori information: in the crust it is based on the controlled sources seismic studies; in slab and mantle wedge it is defined using relations between P and S velocities, temperature and composition given by mineral physics. Each iteration of tomographic inversion consists of the following steps: (1) absolute location of sources in 3-D velocity model using P and S arrival times; (2) double-difference relocation of the sources and (3) simultaneous determination of P and S velocity anomalies, P and S station corrections and source parameters by inverting one matrix. Velocity parameters are computed in a mesh with the density of nodes proportional to the ray density with double-sided nodes at the domain boundaries. The next iteration is repeated with the updated velocity model and source parameters obtained at the previous step. Different tests aimed at checking the reliability of the obtained velocity models are presented. In addition, we present the results of inversion for Vp and Vp/Vs parameters, which appear to be practically equivalent to Vp and Vs inversion. A separate inversion for Qp has been performed using the ray paths and source locations in the final velocity model. The resulting Vp, Vs and Qp distributions show complicated, essentially 3-D structure in the lithosphere and asthenosphere. P and S velocities appear to be well correlated, suggesting the important role of variations of composition, temperature, water content and degree of partial melting. [source]


Dissecting large earthquakes in Japan: Role of arc magma and fluids

ISLAND ARC, Issue 1 2010
Dapeng Zhao
Abstract We synthesized information from recent high-resolution tomographic studies of large crustal earthquakes which occurred in the Japanese Islands during 1995,2008. Prominent anomalies of low-velocity and high Poisson's ratio are revealed in the crust and uppermost mantle beneath the mainshock hypocenters, which may reflect arc magma and fluids that are produced by a combination of subducting slab dehydration and corner flow in the mantle wedge. Distribution of 164 crustal earthquakes (M 5.7,8.0) that occurred in Japan during 1885,2008 also shows a correlation with the distribution of low-velocity zones in the crust and uppermost mantle. A qualitative model is proposed to explain the geophysical observations recorded so far in Japan. We consider that the nucleation of a large earthquake is not entirely a mechanical process, but is closely related to the subduction dynamics and physical and chemical properties of materials in the crust and upper mantle; in particular, the arc magma and fluids. [source]


Long-term changes in distribution and chemistry of middle Miocene to Quaternary volcanism in the Chokai-Kurikoma area across the Northeast Japan Arc

ISLAND ARC, Issue 1 2004
Hirofumi Kondo
Abstract To understand the characteristics of long-term spatial and temporal variation in volcanism within a volcanic arc undergoing constant subduction since the cessation of back-arc opening, a detailed investigation of middle Miocene to Quaternary volcanism was carried out within the Chokai-Kurikoma area of the Northeast Japan Arc. This study involved a survey of available literature, with new K,Ar and fission track dating, and chemical analyses. Since 14 Ma, volcanism has occurred within the Chokai-Kurikoma area in specific areas with a ,branch-like' pattern, showing an east,west trend. This is in marked contrast to the widespread distribution of volcanism with a north,south trend in the 20,14 Ma period. The east,west- trending ,branches' are characterized by regular intervals (50,100 km) of magmatism along the arc. These branches since 14 Ma are remarkably discrepant to the general northwest,southeast or north-northeast,south-southwest direction of the crustal structures that have controlled Neogene to Quaternary tectonic movements in northeast Japan. In addition, evidence indicating clustering and focusing of volcanism into smaller regions since 14 Ma was verified. Comparison of the distribution and chemistry of volcanic rocks for three principal volcanic stages (11,8, 6,3 and 2,0 Ma) revealed that widely but sparsely distributed volcanic rocks had almost the same level of alkali and incompatible element concentrations throughout the area (with the exception of Zr) in the 11,8 Ma stage. However, through the 6,3 Ma stage to the 2,0 Ma stage, the concentration level in the back-arc cluster increased, while that in the volcanic front cluster remained almost constant. Therefore, the degree of partial melting has decreased, most likely with a simultaneous increase in the depth of magma segregation within the back-arc zone, whereas within the volcanic front zone, the conditions of magma generation have changed little over the three stages. In conclusion, the evolution of the thermal structure within the mantle wedge across the arc since 14 Ma has reduced the extent of ascending mantle diapirs into smaller fields. This has resulted in the tendency for the distribution of volcanism to become localized and concentrated into more specific areas in the form of clusters from the late Miocene to Quaternary. [source]


,Forbidden zone' subduction of sediments to 150 km depth, the reaction of dolomite to magnesite + aragonite in the UHPM metapelites from western Tianshan, China

JOURNAL OF METAMORPHIC GEOLOGY, Issue 6 2003
L. Zhang
Abstract The solid-state reaction magnesite (MgCO3) + calcite (aragonite) (CaCO3) = dolomite (CaMg(CO3)2) has been identified in metapelites from western Tianshan, China. Petrological studies show that two metamorphic stages are recorded in the metapelites: (1) the peak mineral assemblage of magnesite and calcite pseudomorphs after aragonite which is only preserved as inclusions within dolomite; and (2) the retrograde glaucophane-chloritoid facies mineral assemblage of glaucophane, chloritoid, dolomite, garnet, paragonite, chlorite and quartz. The peak metamorphic temperatures and pressures are calculated to be 560,600 C, 4.95,5.07 GPa based on the calcite,dolomite geothermometer and the equilibrium calculation of the reaction dolomite = magnesite + aragonite, respectively. These give direct evidence in UHP metamorphic rocks from Tianshan, China, that carbonate sediments were subducted to greater than 150 km depth. This UHP metamorphism represents a geotherm lower than any previously estimated for subduction metamorphism (< 3.7 C km,1) and is within what was previously considered a ,forbidden' condition within Earth. In terms of the carbon cycle, this demonstrates that carbonate sediments can be subducted to at least 150 km depth without releasing significant CO2 to the overlying mantle wedge. [source]


A general model for the intrusion and evolution of ,mantle' garnet peridotites in high-pressure and ultra-high-pressure metamorphic terranes

JOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2000
Brueckner
Garnet-bearing peridotite lenses are minor but significant components of most metamorphic terranes characterized by high-temperature eclogite facies assemblages. Most peridotite intrudes when slabs of continental crust are subducted deeply (60,120 km) into the mantle, usually by following oceanic lithosphere down an established subduction zone. Peridotite is transferred from the resulting mantle wedge into the crustal footwall through brittle and/or ductile mechanisms. These ,mantle' peridotites vary petrographically, chemically, isotopically, chronologically and thermobarometrically from orogen to orogen, within orogens and even within individual terranes. The variations reflect: (1) derivation from different mantle sources (oceanic or continental lithosphere, asthenosphere); (2) perturbations while the mantle wedges were above subducting oceanic lithosphere; and (3) changes within the host crustal slabs during intrusion, subduction and exhumation. Peridotite caught within mantle wedges above oceanic subduction zones will tend to recrystallize and be contaminated by fluids derived from the subducting oceanic crust. These ,subduction zone peridotites' intrude during the subsequent subduction of continental crust. Low-pressure protoliths introduced at shallow (serpentinite, plagioclase peridotite) and intermediate (spinel peridotite) mantle depths (20,50 km) may be carried to deeper levels within the host slab and undergo high-pressure metamorphism along with the enclosing rocks. If subducted deeply enough, the peridotites will develop garnet-bearing assemblages that are isofacial with, and give the same recrystallization ages as, the eclogite facies country rocks. Peridotites introduced at deeper levels (50,120 km) may already contain garnet when they intrude and will not necessarily be isofacial or isochronous with the enclosing crustal rocks. Some garnet peridotites recrystallize from spinel peridotite precursors at very high temperatures (c. 1200 C) and may derive ultimately from the asthenosphere. Other peridotites are from old (>1 Ga), cold (c. 850 C), subcontinental mantle (,relict peridotites') and seem to require the development of major intra-cratonic faults to effect their intrusion. [source]


Petrochemical constraints for dual origin of garnet peridotites from the Dabie-Sulu UHP terrane, eastern-central China

JOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2000
Zhang
Garnet peridotites occur as lenses, blocks or layers within granulite,amphibolite facies gneiss in the Dabie-Sulu ultra-high-pressure (UHP) terrane and contain coesite-bearing eclogite. Two distinct types of garnet peridotite were identified based on mode of occurrence and petrochemical characteristics. Type A mantle-derived peridotites originated from either: (1) the mantle wedge above a subduction zone, (2) the footwall mantle of the subducted slab, or (3) were ancient mantle fragments emplaced at crustal depths prior to UHP metamorphism, whereas type B crustal peridotite and pyroxenite are a portion of mafic,ultramafic complexes that were intruded into the continental crust as magmas prior to subduction. Most type A peridotites were derived from a depleted mantle and exhibit petrochemical characteristics of mantle rocks; however, Sr and Nd isotope compositions of some peridotites have been modified by crustal contamination during subduction and/or exhumation. Type B peridotite and pyroxenite show cumulate structure, and some have experienced crustal metasomatism and contamination documented by high 87Sr/86Sr ratios (0.707,0.708), low ,Nd(t) values (,6 to ,9) and low ,18O values of minerals (+2.92 to +4.52). Garnet peridotites of both types experienced multi-stage recrystallization; some of them record prograde histories. High- P,T estimates (760,970 C and 4.0,6.50.2 GPa) of peak metamorphism indicate that both mantle-derived and crustal ultramafic rocks were subducted to profound depths >100 km (the deepest may be ,180,200 km) and experienced UHP metamorphism in a subduction zone with an extremely low geothermal gradient of <5 C km,1. [source]


What Happened in the Trans-North China Orogen in the Period 2560-1850 Ma?

ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 6 2006
Guochun ZHAO
Abstract: The Trans-North China Orogen (TNCO) was a Paleoproterozic continent-continent collisional belt along which the Eastern and Western Blocks amalgamated to form a coherent North China Craton (NCC). Recent geological, structural, geochemical and isotopic data show that the orogen was a continental margin or Japan-type arc along the western margin of the Eastern Block, which was separated from the Western Block by an old ocean, with eastward-directed subduction of the oceanic lithosphere beneath the western margin of the Eastern Block. At 2550-2520 Ma, the deep subduction caused partial melting of the medium-lower crust, producing copious granitoid magma that was intruded into the upper levels of the crust to form granitoid plutons in the low- to medium-grade granite-greenstone terranes. At 2530-2520 Ma, subduction of the oceanic lithosphere caused partial melting of the mantle wedge, which led to underplating of mafic magma in the lower crust and widespread mafic and minor felsic volcanism in the arc, forming part of the greenstone assemblages. Extension driven by widespread mafic to felsic volcanism led to the development of back-arc and/or intra-arc basins in the orogen. At 2520-2475 Ma, the subduction caused further partial melting of the lower crust to form large amounts of tonalitic-trondhjemitic-granodioritic (TTG) magmatism. At this time following further extension of back-arc basins, episodic granitoid magmatism occurred, resulting in the emplacement of 2360 Ma, ,2250 Ma 2110,21760 Ma and ,2050 Ma granites in the orogen. Contemporary volcano-sedimentary rocks developed in the back-arc or intra-arc basins. At 2150-1920 Ma, the orogen underwent several extensional events, possibly due to subduction of an oceanic ridge, leading to emplacement of mafic dykes that were subsequently metamorphosed to amphibolites and medium- to high-pressure mafic granulites. At 1880-1820 Ma, the ocean between the Eastern and Western Blocks was completely consumed by subduction, and the closing of the ocean led to the continent-arc-continent collision, which caused large-scale thrusting and isoclinal folds and transported some of the rocks into the lower crustal levels or upper mantle to form granulites or eclogites. Peak metamorphism was followed by exhumation/uplift, resulting in widespread development of asymmetric folds and symplectic textures in the rocks. [source]


Petrogenesis of Cenozoic Potassic Volcanic Rocks in the Nangqn Basin

ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 1 2001
SUN Hongjuan
Abstract The Nangqn basin is one of the Tertiary pull-apart basins situated in the east of the Qiangtang block. Similar to the adjacent Dengqn basin and Baxoi basin, there occurred a series of potassic volcanic and sub-volcanic rocks, ranging from basic, intermediate to intermediate-acid in lithology. Based on the study of petrology, mineralogy and geochemistry, including REEs, trace elements, isotopic elements and chronology, the authors concluded that the Cenozoic potassic volcanic rocks in the Nangqn basin were formed in the post-collisional intraplate tectonic settings. The relations between the basic, intermediate and intermediate-acid rocks are neither differentiation nor evolution, but instead the geochemical variability is mainly attributable to the different partial melting degrees of the mantle sources formed at depths of 50,80 km. The sources of the potassic rocks are enriched metasomatic mantle that has experienced multiple mixing of components mainly derived from the crust. The recycling model can be described as follows: after they had subducted to the mantle wedge, the crust-derived rocks were metasomatized with the mantle materials. In view of the fact that the ratio of crust-derived rocks increases by the age of volcanism, it can be concluded that the sources of the potassic rocks moved upwards progressively with time. The underplating of small scattered magmas upwelling from the asthenosphere may have induced partial melting of the sources of the volcanic rocks in some pull-apart basins in the Hengduanshan area and the intense tectonic movements of large-scale strike-slip belts provided conduits for the ascending melts. [source]


A general model for the intrusion and evolution of ,mantle' garnet peridotites in high-pressure and ultra-high-pressure metamorphic terranes

JOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2000
Brueckner
Garnet-bearing peridotite lenses are minor but significant components of most metamorphic terranes characterized by high-temperature eclogite facies assemblages. Most peridotite intrudes when slabs of continental crust are subducted deeply (60,120 km) into the mantle, usually by following oceanic lithosphere down an established subduction zone. Peridotite is transferred from the resulting mantle wedge into the crustal footwall through brittle and/or ductile mechanisms. These ,mantle' peridotites vary petrographically, chemically, isotopically, chronologically and thermobarometrically from orogen to orogen, within orogens and even within individual terranes. The variations reflect: (1) derivation from different mantle sources (oceanic or continental lithosphere, asthenosphere); (2) perturbations while the mantle wedges were above subducting oceanic lithosphere; and (3) changes within the host crustal slabs during intrusion, subduction and exhumation. Peridotite caught within mantle wedges above oceanic subduction zones will tend to recrystallize and be contaminated by fluids derived from the subducting oceanic crust. These ,subduction zone peridotites' intrude during the subsequent subduction of continental crust. Low-pressure protoliths introduced at shallow (serpentinite, plagioclase peridotite) and intermediate (spinel peridotite) mantle depths (20,50 km) may be carried to deeper levels within the host slab and undergo high-pressure metamorphism along with the enclosing rocks. If subducted deeply enough, the peridotites will develop garnet-bearing assemblages that are isofacial with, and give the same recrystallization ages as, the eclogite facies country rocks. Peridotites introduced at deeper levels (50,120 km) may already contain garnet when they intrude and will not necessarily be isofacial or isochronous with the enclosing crustal rocks. Some garnet peridotites recrystallize from spinel peridotite precursors at very high temperatures (c. 1200 C) and may derive ultimately from the asthenosphere. Other peridotites are from old (>1 Ga), cold (c. 850 C), subcontinental mantle (,relict peridotites') and seem to require the development of major intra-cratonic faults to effect their intrusion. [source]