Topography of the 660-km discontinuity within the Izu-Bonin subduction zone and evidence of slab penetration near the Bonin Super Deep Earthquake(~680 km)

The Izu-Bonin subduction zone in the Northwest Pacific is an ideal location for understanding mantle dynamics such as cold lithosphere subduction. The slab produces a lateral thermal anomaly, inducing local topographic changes at the boundary of a post-spinel phase transformation, considered to be the origin of the ‘660-km discontinuity.’ In this study, the short-period(1–2 Hz) S-to-P conversion phase S660P was used to obtain the fine-scale structure of the discontinuity. More than 100 earthquakes that occurred from the 1980s to the 2020s and were recorded by high-quality seismic arrays in the United States and Europe were analyzed. A discontinuity in the ambient mantle with an average depth of ~670 km was found beneath the 300–400-km event zone in the northern Bonin region near 33°N. Meanwhile, the ‘660-km discontinuity’ has been pushed upward, away from the slab, possibly because of a hot upwelling mantle plume. In the central part of the subduction zone, the 660-km discontinuity is depressed to an average depth of(690 ± 5) km within the slab at approximately 150 km below the coldest slab core, indicating a(300 ± 100) ℃ cold anomaly estimated using a post-spinel transformation Clapeyron slope of(-2.0 ± 1.0) MPa/K. In southern Bonin near 28°N, the discontinuity was found to be further depressed at an average depth of(695 ± 5) km below the deepest event and with a focal depth of ~550 km. The discontinuity is located where the slab bends abruptly to become sub-horizontal toward the west-southwest. Near the zone of the isolated Bonin Super Deep Earthquake, which occurred at ~680 km on May 30,2015, the discontinuity is depressed to ~700 km, suggesting a near-vertical penetrating slab and an S-to-P conversion in the coldest slab core, where a large low-temperature anomaly should exist.

Thermal structure of continental subduction zone: high temperature caused by the removal of the preceding oceanic slab

The thermal structure of the continental subduction zone can be deduced from high-pressure and ultra-high-pressure rock samples or numerical simulation.However,petrological data indicate that the temperature of subducted continental plates is generally higher than that derived from numerical simulation.In this paper,a two-dimensional kinematic model is used to study the thermal structure of continental subduction zones,with or without a preceding oceanic slab.The results show that the removal of the preceding oceanic slab can effectively increase the slab surface temperature of the continental subduction zone in the early stage of subduction.This can sufficiently explain the difference between the cold thermal structure obtained from previous modeling results and the hot thermal structure obtained from rock sample data.

Finite element modelling of the geodynamic processes of the Central Andes subduction zone:A Reference Model

This paper presents preliminary results of three-dimensional thermomechanical finite-element models of a parameter study to compute the current temperature and stress distribution in the subduction zone of the central Andes （16°S-26°S） up to a depth of 400 km, the bottom of the asthenosphere. For this purpose a simulation running over c. 50,000 years will be realized based on the geometry of a generic subduction zone and an elasto-viscoplastic Drucker-Prager rheology. The kinematic and thermal boundary conditions as well as the rheological parameters represent the current state of the study area. In future works the model will be refined using a systematic study of physical parameters in order to estimate the influence of the main parameters （e.g. viscosity, fault friction, velocity, shear heating） on the results of the reference model presented here. The reference model is kept as simple as possible to be able to estimate the influence of the parameters in future studies in the best possible way, whilst minimizing comnutational time.

Upper Limit for Rheological Strength of Crust in Continental Subduction Zone:Constraints Imposed by Laboratory Experiments

The transitional pressure of quartz coesite under the differential stress and highly strained conditions is far from the pressure of the stable field under the static pressure. Therefore, the effect of the differential stress should be considered when the depth of petrogenesis is estimated about ultrahigh pressure metamorphic (UHPM) rocks. The rheological strength of typical ultrahigh pressure rocks in continental subduction zone was derived from the results of the laboratory experiments. The results indicate the following three points. (1) The rheological strength of gabbro, similar to that of eclogite, is smaller than that of clinopyroxenite on the same condition. (2) The calculated strength of rocks (gabbro, eclogite and clinopyroxenite) related to UHPM decreases by nearly one order of magnitude with the temperature rising by 100 ℃ in the range between 600 and 900 ℃. The calculated strength is far greater than the faulting strength of rocks at 600 ℃, and is in several hundred to more than one thousand mega pascals at 700-800 ℃, which suggests that those rocks are located in the brittle deformation region at 600 ℃, but are in the semi brittle to plastic deformation region at 700-800 ℃. Obviously, the 700 ℃ is a brittle plastic transition boundary. (3) The calculated rheological strength in the localized deformation zone on a higher strain rate condition (1.6×10 -12 s -l ) is 2-5 times more than that in the distributed deformation zone on a lower strain rate condition (1.6×10 -14 s -1 ). The average rheological stress (1 600 MPa) at the strain rate of 10 -12 s -1 stands for the ultimate differential stress of UHPM rocks in the semi brittle flow field, and the average rheological stress (550-950 MPa) at the strain rate of l0 -14 - 10 -13 s -l stands for the ultimate differential stress of UHPM rocks in the plastic flow field, suggesting that the depth for the formation of UHPM rocks is more than 20-60 km below the depth estimated under static pressure condition due to the effect of the differential stress.

The Formation and Evolution of Uvarovite in UHP Serpentinite and Rodingite and its Constraints on Chromium Mobility in the Oceanic Subduction Zone

The uvarovite-andradite and uvarovite-andradite-grossular solid-solution series are rare in nature.The discovery of uvarovite-andradite in serpentinite and rodingite from the ultra-high pressure(UHP)metamorphic belt in southwestern Tianshan provided an opportunity to investigate its behavior in the subduction zone.Uvarovite(defined as chromiumgarnet)from serpentinite is homogeneous in a single grain,covering compositions in the uvarovite-andradite solid solution series of Adr_(58-66)Uv_(33-41),with few grossular components.Uvarovite from rodingites contain various Cr_(2)O_(3) contents(1.7-17.9 wt%)and mineral compositions being in the range of Adr_(21-31)Uv_(41-50)Grs_(22-37),Adr_(52-90)Uv_(5-25)Grs_(0-21) and Adr_(19-67)Uv_(3-63)Grs_(13-42).Discontinuous chemical variation of uvarovite from core to rim indicates that uvarovite formed by consuming andradite and chromite,which could provide Ca,Cr,Al and Fe.Raman signals of water were identified for uvarovite from both serpentinite and rodingite,with high water content in uvarovite from serpentinite.The high pressure mineral assemblage,as well as the association with perovskite,indicated that the studied uvarovite from serpentinite and rodingite was formed through high pressure metamorphism,during the subduction zone serpentinization and rodingitization.High alkaline and highly reduced fluids released from serpentinization or rodingitization in the oceanic subduction zone promote the mobility of chromium and enable its long-distance migration.

Lithium Isotopic Geochemistry in Subduction Zones:Retrospects and Prospects

Subduction zones involve many complex geological processes, including the release of slabderived fluids, fluid/rock interactions, partial melting, isotopic fractionations, elemental transporting, and crust/mantle interactions. Lithium（Li） isotopes（~6Li and ~7Li） have relative mass difference up to 16%, being the largest among metal elements. Thus, Li isotopes have advantage to interprete trace various geological processes. Most importantly, during crust/mantle interactions in deep subduction zones, surface materials and mantle rocks usually have distinct Li isotopic compositions. Li isotopes can be potential tracer for subduction processes, from the onset of subduction to the release of Li from subducted slabs and interaction with mantle wedge, as well as the fate of Li in slab-derived fluids and residual slabs. Moreover, the Li isotopic composition of subducting output materials can provide useful information for understanding global Li circulation. With developments in measurement and expansion of Li isotopic database, Li isotopic geochemistry will provide more inference and be a powerful tracer for understanding subduction-related processes. This work retrospected the application of Li isotopes in tracing successive subduction processes, and made some prospects for further studies of Li isotopes.

Micro-textures in plagioclase from 1994-1995 eruption, Barren Island Volcano: Evidence of dynamic magma plumbing system in the Andaman subduction zone

A systematic account of micro-textures and a few compositional profiles of plagioclase from high-alumina basaltic aa lava erupted during the year 1994-1995, from Barren Island Volcano, NE India ocean, are presented for the first time. The identified micro-textures can be grouped into two categories： （i） Growth related textures in the form of coarse/fine-sieve morphology, fine-scale oscillatory zoning and resorption surfaces resulted when the equilibrium at the crystal-melt interface was fluctuated due to change in temperature or H20 or pressure or composition of the crystallizing melt; and （ii） morphological texture, like glomerocryst, synneusis, swallow-tailed crystal, microlite and broken crystals, formed by the influence of dynamic behavior of the crystallizing magma （convection, turbulence, degassing, etc.）. Each micro-texture has developed in a specific magmatic environment, accordingly, a first order magma plumbing model and crystallization dynamics are envisaged for the studied lava unit. Magma generated has undergone extensive fractional crystallization of An-rich plagioclase in stable magmatic environment at a deeper depth. Subsequently they ascend to a shallow chamber where the newly brought crystals and pre-existing crystals have undergone dynamic crystallization via dissolution-regrowth processes in a convective self- mixing environment. Such repeated recharge-recycling processes have produced various populations of plagioclase with different micro-textural stratigraphy in the studied lava unit. Intermittent degassing and eruption related decompression have also played a major role in the final stage of crystallization dynamics.

Spatial variations of the 660-km discontinuity in the western Pacific subduction zones observed from CEArray triplication data

We examined the spatial variation of velocity structures around the 660-kin discontinuity at the western Pacific subduction zones by waveform modeling of triplication data. Data from two deep earthquakes beneath Izu-Bonin and Northeast China are used. Both events were well recorded by a dense broadband seismic network in China （CEArray）. The two events are located at approximately the same distance to the CEArray, yet significant differences are observed in their records： （1） the direct arrivals traveling above the 660-km discontinuity （AB branch） are seen in a different distance extent： -29° for the NE China event, -23° for Izu-Bonin event; （2） the direct （AB） and the refracted waves at the 660-km （CD branch） cross over at 19.5° and 17° for the NE China and the Izu-Bonin event, respectively. The best fitting model for the NE China event has a broad 660-km discontinuity and a constant high velocity layer upon it; while the Izu-Bonin model differs from the standard IASP91 model only with a high velocity layer above the 660-km discontinuity. Variations in velocity models can be roughly explained by subduction geometry.

On the generation of deep focus earthquakes in subduction zones

Following a quasi-dynamic scheme proposed by Minear and Toksoz (1970), thermal structures of subduction zonesfor different models by finite element method (FEM) were calculated. Density distribution and p-wave anomaly ofsubduction zones were calculated at the same time. Comparing with seismological evidences and results of laboratories. it is proposed that earthquakes occurred below 400 km depth are probably controlled by anti-crackmechanism.

Discrete Element Modeling of a Subduction Zone with a Seafloor Irregularity and its Impact on the Seismic Cycle

Seafloor irregularities influence rupture behavior along the subducting slab and in the overriding plate,thus affecting earthquake cycles.Whether seafloor irregularities increase the likelihood of large earthquakes in a subduction zone remains contested,partially due to focus put either on fault development or on rupture pattern.Here,we simulate a subducting slab with a seafloor irregularity and the resulting deformation pattern of the overriding plate using the discrete element method.Our simulations illustrate the rupture along three major fault systems:megathrust,splay and backthrust faults.Our results show different rupture dimensions of earthquake events varying from tens to ca.140 km.Our results suggest that the recurrence interval of megathrust events with rupture length of ca.100 km is ca.140 years,which is overall comparable to the paleoseismic records at the Mentawai area of the Sumatran zone.We further propose the coseismic slip amounts decrease and interseismic slip amounts increase from the surface downwards gradually.We conclude that the presence of seafloor irregularities significantly affects rupture events along the slab as well as fault patterns in the overriding plate.

Geological characteristics of the Nankai Trough subduction zone and their tectonic significances

The Nankai Trough subduction zone is a typical subduction system characterized by subduction of multiple geological units of the Philippine Sea Plate(the Kyushu-Palau Ridge,the Shikoku Basin,the Kinan Seamount Chain,and the Izu-Bonin Arc)beneath the Eurasian Plate in the southwest of Japan.This study presents a geophysical and geochemical analysis of the Nankai Trough subduction zone in order to determine the features and subduction effects of each geological unit.The results show that the Nankai Trough is characterized by lowgravity anomalies(–20 mGal to–40 mGal)and high heat flow(60–200 mW/m2)in the middle part and low heat flow(20–80 mW/m2)in the western and eastern parts.The crust of the subducting plate is 5–20 km thick.The mantle composition of the subducting plate is progressively depleted from west to east.Subduction of aseismic ridges(e.g.,the Kyushu-Palau Ridge,the Kinan Seamount Chain,and the Zenisu Ridge)is a common process that leads to a series of subduction effects at the Nankai Trough.Firstly,aseismic ridge or seamount chain subduction may deform the overriding plate,resulting in irregular concave topography along the front edge of the accretionary wedge.Secondly,it may have served as a seismic barrier inhibiting rupture propagation in the 1944 Mw 8.1 and 1946 Mw 8.3 earthquakes.In addition,subduction of the Kyushu-Palau Ridge and hot and young Shikoku Basin lithosphere may induce slab melting,resulting in adakitic magmatism and the provision of ore-forming metals for the formation of porphyry copper and gold deposits in the overriding Japan Arc.Based on comparisons of their geophysical and geochemical characteristics,we suggest that,although the Izu-Bonin Arc has already collided with the Japan Arc,the Kyushu-Palau Ridge,which represents a remnant arc of the Izu-Bonin Arc,is still at the subduction stage characterized by a single-vergence system and a topographic boundary with the Japan Arc.

Effective Elastic Thickness of the Lithosphere in the Mariana Subduction Zone and Surrounding Regions and Its Implications for Their Tectonics

To understand the rheology,structure,and tectonics of the lithosphere in the Mariana subduction zone and surrounding regions,we calculated the effective elastic thickness of the lithosphere(Te)in these areas using the improved moving window admittance technique(MWAT)method.We find that smaller data grid spacing can better reflect Te variations in the subduction zone.The Te of the study region ranges from 0 to 47 km.The Te is reduced from 40 km on the seaward side of the outer-rise region to 1-2 km along the trench axis.The lithospheric breaking distance from the trench axis ranges from 0 to 250 km.We suggest that the intermediate Te values in seamounts and high Te values on the seaward side of the outer-rise region respectively reflect the‘fossil’rheological state and current lithospheric strength of the Pacific plate.The faulting induced by the downward bending of subducting plate not only ruptures the lithosphere but also contributes to the mantle serpentinization,significantly reducing the lithospheric strength.The largest breaking distance of the Ogasawara Plateau may be due to the increase in the mass load of the subducting plate in the Ogasawara Plateau and the significant horizontal bending force in the plate caused by the resistance of seamounts to subduction.Furthermore,a good positive correlation exists between the breaking distance and subduction dip angle along the trench axis.We suggest that the subducting plate with a larger breaking distance is likely to form a larger subduction angle.

Chemical composition of sediments from the subducting Cocos Ridge segment at the Southern Central American subduction zone

Subducted sediments play an important role in the magmatism at subduction zones and the formation of mantle heterogeneity,making them an important tracer for shallow crustal processes and deep mantle processes.Therefore,ascertaining the chemical compositions of different subduction end-members is a prerequisite for using subducted sediments to trace key geological processes.We reports here the comprehensive major and trace element analyses of 52 samples from two holes(U1414 A and U1381 C)drilled on the subducting Cocos Ridge segment at the Southern Central American(SCA)subduction zone during Integrated Ocean Drilling Program(IODP)Expedition 344.The results show that the SCA subducting sediments contain 51%(wt%)Ca CO_(3),27%(wt%)terrigenous material,16%(wt%)opal,and 6%(wt%)mineral-bound H2 O+.Compared to the global trenches subducting sediment,the SCA subducting sediments are enriched in biogenic elements(Ba,Sr,and Ca),and depleted in high field strength elements(Nb,Ta,Zr,Hf,and Ti)and alkali elements(K,Rb,and Cs).Meanwhile,the sediments in this area were affected by the carbonate crash event,which could have been caused by a~800 m rise in the carbonate compensation depth at 11 Ma in the Guatemala Basin.The reason for the sedimentary hiatus at Hole U1381 C may be the closure of the Panama Isthmus and the collision between the Cocos Ridge and the Middle America Trench.In addition,the sediments from the subducting Cocos Ridge segment have influenced the petrogenesis of volcanic lavas erupted in the SCA.

Fluidizate-Explosive Occurrences in Ophiolites as Indicator of the Subduction Zone Activity: The Urals Example

It is known that the formation of oceanic crust occurs in different geodynamic settings,accompanying by the emergence of mantle-magmatic ophiolite complexes having a distinctive properties.In the process of mantle-crustal evolution of the ophiolites are undergoing significant changes with the formation of peculiar(on structure and composition)rocks,sometimes with unusual mineral paragenesis.The presence of such rocks in mélange tectonic zones greatly complicates to determine their origin.In the Ural folded belt(length more than 2,000 km)separating the East European Platform and the West Siberian sedimentary basin,ophiolites are widespread forming a chain of mafic-ultramafic massifs(Fig.1)located in the allochthonous position with mélange at the bottom(Puchkov,2013).With the Urals ophiolites are associated occurrences of eclogites,jadeites,ruby and other rocks of unclear nature,sometimes regarded as potentially diamondiferous.Such formations of unclear genesis include the associating with ophiolites metabasites of higher alkalinity composing the body in the mantle peridotites of the mélange Main Uralian Fault(MUF)zone(Shmelev,2005).By this time they are determined in different parts of the fault zone,but most completely are known in the Sub Polar Urals,where are distinguished under the name of Sertynya alkaline-ultramafic complex,which is located just 25 km east of Hartes kimberlitic complex(Fig.1).Formally,its affiliation to diamond-bearing associations is confirmed by finding of grains and fragments of natural diamond in the weathering crust.A detailed study of the rock complexes shows that in reality they have a polygenic nature,combine theelements of proper magmatic and fluidizate-explosive formations,the appearance of which was interfaced with the processes at the slab-mantle wedge boundary in subduction zones.Polygenic nature of the rocks is reflected in the existence of three interrelated structural-geological units:(1)bodies and dikes of uniformmetadiabasesanddensefine-grainedmetadolerites(lamprophyres),(2)fluidal-brecciated dolerites('tuff breccias')and(3)structural weathering crust with angular or rounded fragments(blocks)of metadolerites and serpentinites.The rocks have experienced rodingitization and permeated with net of veins a vesuvianite composition.The host peridotite matrix(harzburgites and dunites)has undergone serpentinizationandchloritization.Structural relationships give grounds for distinguishing in the history of the complex formation the magmatic proper(dolerite dyke and lamprophyre intrusion)and infiltration fluidizate-explosive(metasomatic transformation of dolerite)stages.Peculiarities of petrography and mineralogy of rock complexes does not allow to compare them with lamproites and kimberlites.Metadiabases demonstrate relics of ophitic structure,as primary paragenesis is completely replaced by aggregate of chlorite,zoisite and leucoxene.Dolerites(lamprophyres)have a uniform fine-grained or porphyry structure with phenocrysts of clinopyroxene,brown amphibole and leucoxene(sphene),which are immersed in a fine-scaly aggregate of light green mica.In the rocks amphibole,garnet and vesuvian are present.Clinopyroxene corresponds to augite with moderate content of titanium and alumina(up to 3.5wt.%),showing a normal magmatic zonation in composition.Mica previously wrongly called as phlogopite,actually has an extremely ferrous composition and corresponds to biotite(annite).Amphibole is presented by magmatic titaniferous tschermakite hornblende and metamorphic(bluish)variety of sodium-calcium composition(taramite).Garnet is presented by exceptionally grossular of rodingite type.Mineralogy of weathering crust reveals similar features,but in the samples it is marked the presence of muscovite,orthoclase and weakly ferrous diopside.An important feature of the weathering crust is the presence of shear surfaces on minerals,resulting in fracturing due to internal stress,confirming the explosive nature of protolith.The bulk chemical composition of rocks is characterized by significant variations in the content of silica(30-46 wt.%)and alkalis(0-6.5 wt.%).These metabasites have consistently a low magnesia number and high titanium oxide content(1.5-3.0 wt.%).Side by side with these are been established the uniform slope REE distribution trends similar to the trend of oceanic basalts N-MORB type(Fig.2).The level of trace element compositions does not depend on variations in the alkalinity of the rocks,but clearly correlates with the titanium content.Unlike them the Hartes kimberlites demonstrate the distribution with deficit of HREE,andthe level of the elements content is correlated with the alkalinity of rocks(Mahotkin et al.,1998).Another important geochemical feature of the Sertynya complex rocks is a regular behavior of the mobile LILE elements(Cs,Rb,Ba,K).In the varieties of rocks with mica enriched by alkalis,it is recorded extremely high level of LILE,exceeding the level of contents in N-MORB basalts at 10-10000 times!In the metabasites varieties with low level of alkalinity,LILE content is sharply(except Cs)reduced to minimum values(Fig.2).The observed pattern of the element distribution is undoubtedly the result of postmagmatic fluid-metasomatic alteration of the original rocks.Tectonic position and the primary composition characteristics of the metadolerites give reason to consider them as fragments of the ophiolite sheeted dike complex(Shmelev,2005).The famous dike complexes in the ophiolite massifs of the MUF zone(east of mélange)belong to suprasubduction formations of Paleozoic age.However the obtained mainly ancient U-Pb zircon dating(up to Archean inclusive)for metadolerites of the Sertynya complex,make it possible to assume its Vendian-Early Cambrian(530-617 Ma)age.It permits to compare the Sertynya metabasites with the Vendian metaophiolites of the MUF zone in the Middle Urals(Petrov et al.,2010).It is noteworthy that similar age datings(520-550 Ma)are also established for kimberlites of the Hartes complex located to the west of ophiolites.Therefore,thepresenceofthe Vendian-Cambrian ophiolite of MOR-type in the MUF mélange zone,'changing'to the east of Ordovician ophiolites SSZ-type,seems quite possible.The obtained data allow to suggest an original interpretation of nature of the Urals fluidizate-explosive formations considering the process specifics in the subduction zones(Bebout and Barton,2002).Accordingto this model,the pre-Ordovician(?)oceanic crust has undergone transformations and deformations on the slab-mantle wedge boundary during the subduction.As a result of slab dehydration it occurred a flow of aqueous fluids,which were enriched with the extracted from sedimentary rocks the LILE elements and percolated through the mantle substrate with dolerite dyke complex.Interaction with them led to the formation of chlorite-zoisite and/or mica(biotite-bearing)fluidizates and in the presence of a gas phase-fluidizate-explosive breccias with subsequent development of weathering crust.In the surrounding peridotites an explosive process is marked by the formation of pseudokimberlite breccias.Fluidized-explosive occurrences in mantle peridotites of mélange zones should be considered as indicators of the subduction slab-mantle interaction at relatively shallow levels involving enriched LILE fluids(without melts participation),rising as the front from the subduction zone.In this interpretation,there is no need toappealtothealkaline-ultramaficor lamproit-kimberlite hypothesis of the genesis of these formations,however,the question of their potential diamondiferous remains to be open.The proposed interpretation of the fluidizate-explosive occurrences makes it possible to comprehendthat in reality the mélange is a complex formation with signs of not onlycollisional(as usually is considered),but also of earlier subduction events.

Carbon Isotope Fraction during Subduction Zone Metamorphism

Carbon isotope derived from mantle rocks and diamonds occurring worldwide show a narrow interval of-8‰to-2‰,with a very broad distribution to lower values（;41‰）and higher values（;‰）(Cartigny et al.,2014).

Neogene subduction initiation models in the western Pacific and analysis of subduction zone parameters

The Neogene is an important period for studying the onset of subduction,with numerous subduction zones forming in the western Pacific,including the Ryukyu,Manila,Philippine,North Sulawesi,Halmahera,New Britain,Solomon,and New Hebrides subduction zones.However,studies on these subduction zones are relatively independent,so it is important to conduct systematic comparative studies.In this paper,we review the initiation models of Neogene subduction in the western Pacific,with the three typical types of subduction initiation models including polarity-reversal,induced subduction re-initiation,and noninherited subduction initiation to form new ruptures.In addition,the parameters of different subduction zones are collated to form five categories:basic features,subducting plate features,upper plate features,kinematic features,and subsequent activity.The regularity of the subduction processes,the specificity of the different subduction cases,and the possible constraints between the subduction initiation types and the characteristics of the subduction zone parameters are discussed and analyzed.The compiled dataset of the subduction zone parameters can provide data support for related studies.

Redox processes in subduction zones:Progress and prospect

Oxygen fugacity(fO_(2))is an intensive variable that describes the redox state of a system.By controlling the valence state of multivalent elements,fO_(2)affects the stability of iron-bearing minerals,dominants the species of volatile elements(e.g.,carbon and sulfur),and controls the partitioning behaviors of multivalent elements(e.g.,iron,vanadium,cerium,europium).Thus,fO_(2)plays a key role in understanding the generation and differentiation of arc magmas,the formation of magmatic-hydrothermal deposits,and the nature of magmatic volatiles.Subduction zones are an important site for arc magmatism and fluid action,and the study of redox processes is indispensable in subduction zone geochemistry.In this paper,we first introduce the concept,expression,and estimation methods of fO_(2).Then we retrospect the history and progress about the oxidation state of the metasomatized mantle wedge,summarize the redox property of slab-derived fluids,and review the latest progress on redox evolution of arc magmas during magma generation and differentiation.The main conclusions include:(1)despite its wide variation range,fO_(2)of the mantle wedge is generally higher than that of the oceanic mantle;(2)the redox property of the subducting slab-derived fluids is still controversial and the mechanism for the oxidization of the mantle wedge remains unclear;(3)how the fO_(2)varies during the generation and differentiation of the arc magmas is debated.We propose that the crux in deciphering the oxidization mechanism of the mantle wedge is to determine the mobility of iron,carbon and sulfur in subducting slab-derived fluids(especially solute-rich fluid or supercritical fluid);the key in understanding the redox evolution during arc magma generation and differentiation is to determine the partition coefficients of Fe^(3+)and Fe^(2+)between ferromagnesian minerals and silicate melts.

Metallogenesis and major challenges of porphyry copper systems above subduction zones

Porphyry copper±molybdenum±gold deposits(PCDs) are the most representative magmatic-hydrothermal metallogenic system above subduction zones with important economic value. Previous studies revealed that large PCDs are generally formed from initial arc magmas(from subduction-induced partial melting of the mantle wedge), which eventually ascend to the shallow crust(3–5 km) for mineralization after a series of complex evolution processes. These processes include(1) the dehydration or partial melting of subducting slab, which induces partial melting of the metasomatized mantle wedge;(2)the ascent of mantle-derived magma to the bottom of the lower crust, which subsequently undergoes crustal processes such as assimilation plus fractional crystallization(AFC) or melting, assimilation, storage and homogenization(MASH);(3) the magma chamber formation at the bottom of the lower, middle and upper crust;(4) the final emplacement and volatilization of porphyry stocks;and(5) the accumulation of ore-forming fluids and metal precipitation. Despite the many decades of research, many issues involving the PCD metallogenic mechanism still remain to resolve, such as(1) the tectonic control on the geochemical characteristics of ore-forming magma;(2) the reason for the different lifespans of the long-term magmatic arc evolution and geologically "instantaneous" mineralization processes;(3) the source of ore-forming materials;(4) the relative contributions of metal pre-enrichment to mineralization by the magma source and by magmatic evolution;and(5) the decoupling behaviors of Cu and Au during the pre-enrichment. These issues point out the direction for future PCD metallogenic research, and the resolution to them will deepen our understanding of the metallogenesis at convergent plate boundaries.

Metamorphism, fluid behavior and magmatism in oceanic subduction zones

Based on the updated results of experimental petrology and phase equilibria modelling and combined with the available thermal structure models of subduction zones, this paper presents an overview on the dehydration and melting of basic,sedimentary and ultrabasic rocks that occur in the different stages during oceanic subduction processes and their influences on magmatism above subduction zones. During the subduction at the forearc depth of <90–100 km, the basic and ultrabasic rocks from most oceanic slabs can release very small amounts of water, and significant dehydration may occur in the slab superficial sediments. Strong dehydration occurs in both basic and ultrabasic rocks during subduction at the subarc depth of 90–200 km. For example, more than 90% water in basic rocks is released by the successive dehydration of chlorite, glaucophane, talc and lawsonite in the subarc depths. This is diversely in contrast to the previous results from synthetic experiments. Ultrabasic rocks may undergo strong dehydration through antigorite, chlorite and phase 10 ? at the subarc depth of 120–220 km. However,sediments can contribute minor fluids at the subarc depth, one main hydrous mineral in which is phengite(muscovite). It can stabilize to ~300 km depth and transform into K-hollandite. After phengite breaks down, there will be no significant fluid release from oceanic slab until it is subducted to the mantle transition zone. In a few hot subduction zones, partial melting(especially flux melting) can occur in both sediments and basic rocks, generating hydrous granitic melts or supercritical fluids, and in carbonates-bearing sediments potassic carbonatite melts can be generated. In a few cold subduction zones, phase A occurs in ultrabasic rocks, which can bring water deep into the transition zone. The subducted rocks, especially the sediments, contain large quantities of incompatible minor and trace elements carried through fluids to greatly influence the geochemical compositions of the magma in subduction zones. As the geothermal gradients of subduction zones cannot cross the solidi of carbonated eclogite and peridotite during the subarc subduction stage, the carbonate minerals in them can be carried into the deep mantle.Carbonated eclogite can melt to generate alkali-rich carbonatite melts at >400 km depth, while carbonated peridotite will not melt in the mantle transition zone below a subduction zone.

Dehydration at subduction zones and the geochemistry of slab fluids

Subducting oceanic slabs undergo metamorphic dehydration with the increase of temperature and pressure during subduction.Dehydration is an essential step for element recycling,and slab fluids are critical agents for mediating slab-mantle interaction.Dehydration is mainly controlled by the thermal structure of subduction zones and the stability of hydrous minerals.At fore-arc depths,slab dehydration produces aqueous fluid with dissolved salts such as NaCl.As subduction proceeds deeper,the content of silicate components increases.At sub-arc and post-arc depths,a hydrous silicate melt is likely to form,or a supercritical fluid could arise from complete miscibility between silicates and H_(2)O.The partitioning of elements between slab fluid and the residual solid rock is controlled by the type of fluid,and generally it is the supercritical fluid that is the most capable of mobilizing trace elements,being an effective carrier even for high field strength elements.Understanding the chemistry of slab fluids relies on sophisticated integration of experiments,theoretical computation and investigation of natural rock samples.This contribution focuses on the content and speciation of key volatiles,including carbon,nitrogen and sulfur,in slab fluids as well as important fluid properties such as oxygen fugacity and acidity.The properties of slab fluids show complicated variation under the control of mineral assemblages and T-P conditions.Slab fluids at great depths of subductions have been inferred to be modestly alkaline and not necessarily very oxidizing as often assumed.Further progress in the research of slab dehydration and the chemistry and properties of slab fluids demands urgently the development of innovative experimental and computational technology including in situ analytical methods at high T-P.