Subduction Zone Mantle Wedge Research Articles

Oligocene melting of subducted mélange and its mantle dynamics in northeast Asia

Melting of subducted mélange can potentially transport mass from the slab-mantle interface to the mantle wedge in subduction zones. The mélange diapir model was primarily proposed from the results of laboratory experiments and thermodynamic modeling. However, the melting mechanisms of mélange diapirs in subduction zones remain unclear. To further constrain the mantle dynamics of a mélange diapir, we studied Oligocene alkaline intermediate rocks on the northeast Asian continental margin. We report whole-rock geochemical and Sr-Nd-Pb-Mg-Zn isotope data and show that these rocks formed by partial melting of mélange. We conclude that a diapir was the mechanism for Oligocene melting of the mélange. We also identified younger rocks formed by melting of mélange in the eastern part of northeast Asia, implying an eastward shift in such magmatism since the Oligocene. Our results and the tectonic setting indicate that melting of mélange diapirs occurred preferentially during tectonic transitions, such as the formation of a back-arc basin triggered by trench-perpendicular mantle flow. The low-viscosity mantle with an incompressible stress field triggered melting of the mélange diapirs. Interactions occurred between the mélange diapirs and carbonated peridotites, constraining the depth of mélange-mantle interactions to the asthenosphere, which is deeper than the depth inferred in previous studies.

Segmentation and Radial Anisotropy of the Deep Crustal Magmatic System Beneath the Cascades Arc

AbstractVolcanic arcs consist of many distinct vents that are ultimately fueled by the common melting processes in the subduction zone mantle wedge. Seismic imaging of crustal‐scale magmatic systems can provide insight into how melt is organized in the deep crust and eventually focused beneath distinct vents as it ascends and evolves. Here, we investigate the crustal‐scale structure beneath a section of the Cascades arc spanning four major stratovolcanoes: Mt. Hood, Mt. St. Helens (MSH), Mt. Adams (MA), and Mt. Rainier, based on ambient noise data from 234 seismographs. Simultaneous inversion of Rayleigh and Love wave dispersion constrains the isotropic shear velocity (Vs) and identifies radially anisotropic structures. Isotropic Vs shows two sub‐parallel low‐Vs zones (∼3.45–3.55 km/s) at ∼15–30 km depth with one connecting Mt. Rainier to MA, and another connecting MSH to Mt. Hood, which are interpreted as deep crustal magma reservoirs containing up to ∼2.5%–6% melt, assuming near‐equilibrium melt geometry. Negative radial anisotropy, from vertical fractures like dikes, is prevalent in this part of the Cascadia, but is interrupted by positive radial anisotropy, from subhorizontal features like sills, extending vertically beneath MA and Mt. Rainier at ∼10–30 km depth and weaker and west‐dipping positive anisotropy beneath MSH. The positive anisotropy regions are adjacent to rather than co‐located with the isotropic low‐Vs anomalies. Ascending melt that stalled and mostly crystallized in sills with possible compositional differences from the country rock may explain the near‐average Vs and positive radial anisotropy adjacent to the active deep crustal magma reservoirs.

Reduction of iron with no systematic isotope fractionation during continental subduction observed in metamorphic rocks from the Dabie-Sulu orogen, China

• No resolvable Fe isotope fractionation in metabasites during prograde dehydration. • Eclogites have Fe 3+ /∑Fe systematically lower than the other metabasites. • Fe reduction likely occurred during eclogitization with simultaneous S oxidization. In order to better understand the mobility and charge balance of Fe in subducted slabs, here we report precise Fe isotopic data, supplemented by Fe 3+ /∑Fe and C-N-S elemental contents, in a well-characterized suite of metamorphic rocks from the Dabie-Sulu orogen, China, including mafic to felsic greenschists, amphibolites and eclogites. There is no systematic Fe isotopic difference among the mafic schists (δ 56 Fe = 0.08 ± 0.08‰, 2SD, N = 13), amphibolites (δ 56 Fe = 0.09 ± 0.04‰, 2SD, N = 19) and eclogites (δ 56 Fe = 0.07 ± 0.06‰, 2SD, N = 27), implying limited Fe mobility during prograde metamorphic dehydration. The overall δ 56 Fe variation of the metabasites and felsic schists ranging from 0.02 ± 0.03‰ to 0.20 ± 0.03‰ most likely mirrors heterogeneity in their Neoproterozoic magmatic protoliths, and can be interpreted by derivation from diverse sources and isotope fractionation during magma differentiation. In a contrary to limited Fe mobility, a decrease in Fe 3+ /∑Fe ratios has been identified for metabasites during eclogitization (on average 0.43 for greenschists, 0.42 for amphibolites and 0.24 for eclogites respectively). Reduction of Fe could have been compensated by oxidation of S where a concurrent expulsion of sulfate seems to be necessary. This study thus supports the hypothesis that Fe-S redox coupling in subducted slabs promotes transfer of oxidizing agents into the mantle wedge in subduction zones.

Physics of Melt Extraction from the Mantle: Speed and Style

Melt extraction from the partially molten mantle is among the fundamental processes shaping the solid Earth today and over geological time. A diversity of properties and mechanisms contribute to the physics of melt extraction. We review progress of the past ∼25 years of research in this area, with a focus on understanding the speed and style of buoyancy-driven melt extraction. Observations of U-series disequilibria in young lavas and the surge of deglacial volcanism in Iceland suggest this speed is rapid compared to that predicted by the null hypothesis of diffuse porous flow. The discrepancy indicates that the style of extraction is channelized. We discuss how channelization is sensitive to mechanical and thermochemical properties and feedbacks, and to asthenospheric heterogeneity. We review the grain-scale physics that underpins these properties and hence determines the physical behavior at much larger scales. We then discuss how the speed of melt extraction is crucial to predicting the magmatic response to glacial and sea-level variations. Finally, we assess the frontier of current research and identify areas where significant advances are expected over the next 25 years. In particular, we highlight the coupling of melt extraction with more realistic models of mantle thermochemistry and rheological properties. This coupling will be crucial in understanding complex settings such as subduction zones. ▪ Mantle melt extraction shapes Earth today and over geological time. ▪ Observations, lab experiments, and theory indicate that melt ascends through the mantle at speeds ∼30 m/year by reactively channelized porous flow. ▪ Variations in sea level and glacial ice loading can cause significant changes in melt supply to submarine and subaerial volcanoes. ▪ Fluid-driven fracture is important in the lithosphere and, perhaps, in the mantle wedge of subduction zones, but remains a challenge to model.

Evidence of intraplate magmatism and subduction magmatism during the formation of Nagaland–Manipur Ophiolites, Indo–Myanmar Orogenic Belt, north‐east India

Mafic extrusive rocks (basalts) and intrusive rocks (gabbros) from the Nagaland–Manipur Ophiolite (NMO) of the Indo–Myanmar Orogenic Belt (IMOB), north‐east India, are investigated to understand their magmatic evolution in diverse tectonic environments. Basalts are distinguished into two types: basalt‐I and basalt‐II. Basalt‐I type shows the sub‐alkaline character with Nb/Y < 0.50, low Nb/Th (2.36–7.94), and low to moderate La/Sm (1.00–4.12) indicating derivation from a slightly enriched mantle source and also supported by their enriched LREE pattern with flat HREE. They are depleted in HFSEs (Nb and Ti) but enriched in U and Pb, which is indicative of a typical subduction origin derived from an MORB‐type mantle source. Investigated samples of basalt‐II and gabbros have an equal composition with alkaline characteristics. They have Nb/Y > 0.50, high Nb/Th (8.38–13.37), and highly enriched LREE (La/Sm = 4.41–6.35) pattern. They show typical Ocean Island Basalt (OIB) characters of a plume source. The two sets of basalts and gabbros found in this study have no sign of genetic relationship, and therefore, it strongly suggests that they were derived from two different mantle sources of a plume and a subduction zone mantle wedge. Our study supports the theory that the NMO has records of different magmatic episodes produced ranging from plume‐related magmatism, to divergent and convergent plate magmatism that were generated at diverse tectonic settings.

>1.8 Ga cold subduction of lithospheric mantle: Evidences from the Fengzhen olivine-bearing garnet pyroxenite xenoliths in Trans-North China Orogen

Whether modern-style subduction exists in Paleoproterozoic has been hotly debated because of the scarcity of robust petrological evidence. Here, we present a comprehensive study of olivine-bearing garnet pyroxenite xenoliths hosted in the Paleoproterozoic Fengzhen carbonatite. The carbonatite is located in the conjugate area between the Paleoproterozoic Trans-North China Orogen (TNCO) and the Khondalite Belt in North China Craton (NCC) with a dated age of 1810 ± 3 Ma. Petrographically, four-stages of evolution have been identified in the studied olivine-bearing garnet pyroxenite: 1) the formation of the protolith spinel websterite (S1), 2) the prograde metamorphism of spinel-lherzolite facies to garnet-lherzolite facies (M1), 3) retrograde metamorphism to Ariegite subfacies (M2) with formation of symplectite-I, and 4) Seiland subfacies (M3) forming symplectite-II. The clinopyroxene in the xenoliths display high Mg# (Mg2+/(Mg2++Fe2+)*100 = 89–94), strongly depleted HREE (heavy REE) and HFSE (high field strength elements; e.g., Nb, Zr, Ti) and enriched LREE (light REE) and LILE (large ion lithophile elements; e.g., Th and U). Similarly, the whole-rock chemistry exhibits high Mg# (85–88) and high Al2O3 + CaO (20.2–21.9 wt%) with enrichment in LREE and LILE (Th, U) and depletion in HREE and HFSE (e.g., Nb, Ta, Zr, Hf, Ti). The geochemical signatures imply that they might originate from the refractory mantle wedge in subduction zone and have been metasomatized by crust-derived melts. Major mineral thermobarometry yields the peak PT conditions of 26–33 kbar and 890–962 °C corresponding to a cold subduction geothermal gradient (307 ± 35 °C/GPa) with a depth up to 110 km for olivine-bearing garnet pyroxenites, which is also consistent with the results by zirconium-in-rutile and REE thermobarometers. This studies suggest that that the modern-style subduction of the lithospheric mantle was initiated at least since 1.8 Ga.

Redox evolution of western Tianshan subduction zone and its effect on deep carbon cycle

Knowing the phase relations of carbon-bearing phases at high-pressure (HP) and high-temperature (HT) condition is essential for understanding the deep carbon cycle in the subduction zones. In particular, the phase relation of carbon-bearing phases is also strongly influenced by redox condition of subduction zones, which is poorly explored. Here we summarized the phase relations of carbon-bearing phases (calcite, aragonite, dolomite, magnesite, graphite, hydrocarbon) in HP metamorphic rocks (marble, metapelite, eclogite) from the Western Tianshan subduction zone and high-pressure experiments. During prograde progress of subduction, carbonates in altered oceanic crust change from Ca-carbonate (calcite) to Ca,Mg-carbonate (dolomite), then finally to Mg-carbonate (magnesite) via Mg–Ca cation exchange reaction between silicate and carbonate, while calcite in sedimentary calcareous ooze on oceanic crust directly transfers to high-pressure aragonite in marble or amorphous CaCO3 in subduction zones. Redox evolution also plays a significant effect on the carbon speciation in the Western Tianshan subduction zone. The prograde oxygen fugacity of the Western Tianshan subduction zone was constrained by mineral assemblage of garnet-omphacite from FMQ − 1.9 to FMQ − 2.5 ​at its metamorphic peak (maximum P-T) conditions. In comparison with redox conditions of other subduction zones, Western Tianshan has the lowest oxygen fugacity. Graphite and light hydrocarbon inclusions were ubiqutously identified in Western Tianshan HP metamorphic rocks and speculated to be formed from reduction of Fe-carbonate at low redox condition, which is also confirmed by high-pressure experimental simulation. Based on petrological observation and high-pressure simulation, a polarized redox model of reducing slab but oxidizing mantle wedge in subduction zone is proposed, and its effect on deep carbon cycle in subduction zones is further discussed.

H2O-rich mantle melting near the slab–wedge interface

To investigate the first melts of the mantle wedge in subduction zones and their relationship to primitive magmas erupted at arcs, the compositions of low degree melts of hydrous garnet lherzolite have been experimentally determined at 3.2 GPa over the temperature range of 925–1150 °C. Two starting compositions with variable H2O contents were studied; a subduction-enriched peridotite containing 0.61% Na2O, 0.16 K2O% (wt%) with 4.2 wt% H2O added (Mitchell and Grove in Contrib Mineral Petrol 170:13, 2015) and an undepleted mantle peridotite (Hart and Zindler in Chem Geol 57:247–267, 1986) with 14.5% H2O added (Till et al. in Contrib Mineral Petrol 163:669–688, 2012). Saturating phases include olivine, orthopyroxene, clinopyroxene, garnet and rutile. Melting extent is tracked from near solidus (~ 5 wt%) to 25 wt%, which is close to or beyond the point where clinopyroxene and garnet are exhausted. The beginning of melting is a peritectic reaction where 0.54 orthopyroxene + 0.17 clinopyroxene + 0.13 garnet react to produce 1.0 liquid + 0.88 olivine. The melt production rate near the solidus is 0.1 wt% °C−1 and increases to 0.3 wt% °C−1 over the experimentally studied interval. These values are significantly lower than that observed for anhydrous lherzolite (~ 1 wt% °C−1). When melting through this reaction is calculated for a metasomatized lherzolite source, the rare earth element characteristics of the melt are similar to melts of an eclogite, as well as those observed in many subduction zone magmas. Moreover, since rutile is stable up to ~ 8 wt% melting, the first melts of a hydrous lherzolite source could also show strong high field strength element depletions as is observed in many subduction zone lavas. The silicate melts measured at the lowest temperatures and melting extents (< 10 wt%) are high silica andesites (56–60 wt% SiO2) and contain very low Ca/Al and high alkalis. These deep low degree andesitic melts, if added to experimentally produced hydrous liquids from melting (20–25 wt%) of harzburgite residues at shallow pressures (1.0–1.2 GPa, Mitchell and Grove in Contrib Mineral Petrol 170:13, 2015), can match the compositional characteristics of primitive natural basaltic andesite and magnesian andesite lavas found globally in arcs. In addition to a silicate melt phase, there is a small amount of silicate dissolved in the H2O supercritical fluid that coexists with the silicate liquid and solids in our experiments. The composition of this fluid is in equilibrium with the Mg-rich minerals and it is granitic. The results presented here are used to develop a model for producing hydrous arc magmas. We hypothesize that mantle wedge melting produced by the flux of hydrous fluid from the slab occurs over a range of depths that begins at the base of the mantle wedge and ends at shallow mantle depths. These melts ascend and remain isolated until they mix in the shallow, hottest part of the mantle wedge. In this melting scenario, the metasomatic “slab melt” contributions to arc magmas is small (~ 5 wt%), but its effect on the alkali, REE and incompatible trace element budget of the derivative magmas is large and able to reproduce the trace elemental characteristics of the primitive andesites. Higher proportions of slab or sediment melt do not resemble primitive high magnesian arc andesites and basaltic andesites.

East‐West Differential Underthrusting of the Indian Lithospheric Plate Beneath Central Tibet Revealed by Imaging VP/VS

AbstractAlthough numerous research studies have already provided basic information on deeper lithospheric structure under Tibet, the northernmost edge and the geometry of the northward underthrusting of the Indian mantle lithosphere are yet to be understood. The P wave to S wave velocity ratio (VP/VS) can provide more insight into the mantle physical state than either VP or VS alone, owing to their different sensitivities to partial melting, temperature, and composition. High VP/VS usually indicates that partial melting exists in the upper mantle, while anomalously low VP/VS in the upper mantle reflects the enrichment of orthopyroxene in the subduction zone mantle wedge. Using teleseismic records of 164 broadband stations deployed in central Tibet, we obtained a high‐resolution velocity ratio image of the upper mantle. A striking difference is seen between western and eastern Tibet. A distinct high VP/VS zone exists at relatively a shallow depth (&lt;200 km) to the west of the Yadong‐Gulu Rift, while the eastern part exhibits a shallow region (south of the Bangong‐Nujiang suture in the depth range of 120–160 km) with abnormally low VP/VS. Moreover, there are high VP/VS areas existing below 270‐km depth, and it appears further east at depth of 340 km than that at depth of 270 km. We propose that this difference reflects different geometries of the underthrusting Indian mantle lithosphere from west to east. Slab tearing under eastern Tibet and lithospheric delamination under western Tibet are likely good candidates to explain the observation.

Seismic Anisotropy Evidence for Ductile Deformation of the Forearc Lithospheric Mantle in Subduction Zones

AbstractSeismic anisotropy provides important information on the structure and geodynamics of the Earth. The forearc mantle wedge in subduction zones mainly exhibits trench‐parallel azimuthal anisotropy globally, which is inconsistent with the model of olivine a axis aligning with the slab‐driven corner flow. Its formation mechanism is currently unclear. Here we present high‐resolution 3‐D P wave anisotropic tomography of the Tohoku subduction zone. We suggest that ductile deformation of the forearc lithospheric mantle of the overriding plate induces the trench‐parallel azimuthal anisotropy and positive radial anisotropy (i.e., horizontal velocity &gt; vertical velocity) in Tohoku. Our results provide the first seismic anisotropic evidence for the slab‐mantle decoupling at a common depth of ~70 km. On the basis of the high‐resolution seismic images, we propose a geodynamic model suggesting that the forearc mantle wedge anisotropy is produced via ductile deformation of dry olivine or hydrous antigorite lithospheric mantle, which accords well with the trench‐parallel shear wave splitting measurements dominant in subduction zones globally.

An experimental investigation of C–O–H fluid-driven carbonation of serpentinites under forearc conditions

The carbonation of serpentinites in the forearc region of the mantle wedge in subduction zones and of serpentinites within the subducting slab by fluids derived from prograde dehydration and decarbonation has important implications for the deep Earth carbon cycle. This study shows that the carbonation of serpentinites under the forearc can establish, over time, a significant reservoir for carbon within a partially hydrated mantle wedge and that carbonation of (ultra-) mafic rocks within the subducting slab contributes to C-transfer to greater depths and might supply carbon for arc volcanism or the deep mantle.We report a new high pressure experimental investigation of the interactions between oxidised C–O–H fluids and serpentinite and model the reaction progress with time series experiments. The CO2, H2O and alkane (CnH2n+2, n=1–6; e.g. methane, ethane) contents in the fluid phase from quenched experimental run products have been analysed by gas chromatography and the results are compared with thermodynamic calculations. With progressive carbonation, the formation of magnesite + chlorite together with quartz, quartz + talc or talc at 1–2 GPa and 500–650 °C was observed. At temperatures above antigorite stability (T≳700 °C and 2 GPa) magnesite + chlorite is stable together with talc, talc + enstatite, enstatite or enstatite + forsterite for decreasing CO2-content in the fluid. Carbonation of serpentinite is a rapid process where magnesite forms within the first hour of the experiments, filtering CO2 from the fluid effectively and equilibrium is approached within 48 h. The CO2-sequestration and magnesite production are less pronounced at higher temperatures. Therefore, C–O–H fluids released under subarc conditions might migrate through the mantle with only minor changes in their carbon budget whereas significant carbonation likely occurs under the forearc. Furthermore, partition coefficients for Ca, Ba, Sr and Pb between magnesite and a C-poor aqueous fluid have been established to demonstrate the potential of newly formed magnesite to sequester fluid mobile elements.

Seismic imaging of mantle wedge corner flow and arc magmatism.

I reviewed studies on the inhomogeneous seismic structure of the mantle wedge in subduction zones, in relation to corner flow and its implications for arc magmatism. Seismic studies in Tohoku clearly imaged the descending flow portion of the corner flow as a thin seismic low-velocity layer right above the slab. Slab-derived H2O is fixed to the layer as hydrous minerals, which are brought down by the slab and eventually decompose. The released H2O rises and encounters the ascending flow, formed to fill the gap caused by the descending flow. The combination of H2O addition and adiabatic decompression causes partial melting within the ascending flow. For many subduction zones, seismic tomography has distinctly imaged the ascending flow of the corner flow as a seismic low-velocity and/or high-attenuation layer in the mantle wedge inclined nearly parallel to the slab. These observations indicate that the volcanic front in subduction zones is formed both by the ascending flow and the addition of slab-derived H2O.

Olivine-antigorite orientation relationships: Microstructures, phase boundary misorientations and the effect of cracks in the seismic properties of serpentinites

Antigorite-bearing rocks are thought to contribute significantly to the seismic properties in the mantle wedge of subduction zones. Here we present a detailed study of the microstructures and seismic properties in a sample of antigorite-olivine schist previously studied by Kern et al. (1997, 2015). We have measured crystallographic orientations and calculated the seismic properties in three orthogonal thin sections. Microstructures indicate that deformation is localized in the bands with high antigorite fractions, resulting in strong crystallographic preferred orientations (CPOs) with point maxima of poles to (100) parallel to lineation and poles to (001) to the foliation normal. Olivine CPO suggests deformation under high temperature and low stress, with a [100] fiber texture. The CPO strength varies with grain size, but is strong even in fine-grained antigorite, and larger grains tend to display higher internal misorientation. Orientation relationships between olivine and antigorite are evident in phase boundary misorientation analysis, (100)ol||(001)atg being more frequent than [001]ol||[010]atg. Two new orientation relationships between olivine and antigorite have been documented. Seismic velocities decrease while anisotropy increases with increasing antigorite modal content. Antigorite grain shape has a weak effect on seismic velocities, but it is important on anisotropy. Comparison between CPO-derived seismic velocities using Voigt, Reuss, Hill averages and geometric mean only showed good agreement in 1/3 of experimental velocities. If the crack porosity of 1.63% measured experimentally at 600 MPa was used in the self-consistent model with two crack orientations with planes normal to Z and Y, good match with all experimental velocities was achieved. The self-consistent model implies important crack porosity in the foliation plane at 600 MPa that reduces Vp normal to the foliation by 0.3 km/s.

Fluid migration in the mantle wedge: Influence of mineral grain size and mantle compaction

AbstractMineral grain size in the mantle affects fluid migration by controlling mantle permeability; the smaller the grain size, the less permeable the mantle is. Mantle shear viscosity also affects fluid migration by controlling compaction pressure; high mantle shear viscosity can act as a barrier to fluid flow. Here we investigate for the first time their combined effects on fluid migration in the mantle wedge of subduction zones over ranges of subduction parameters and patterns of fluid influx using a 2‐D numerical fluid migration model. Our results show that fluids introduced into the mantle wedge beneath the forearc are first dragged downdip by the mantle flow due to small grain size (&lt;1 mm) and high mantle shear viscosity that develop along the base of the mantle wedge. Increasing grain size with depth allows upward fluid migration out of the high shear viscosity layer at subarc depths. Fluids introduced into the mantle wedge at postarc depths migrate upward due to relatively large grain size in the deep mantle wedge, forming secondary fluid pathways behind the arc. Fluids that reach the shallow part of the mantle wedge spread trench‐ward due to the combined effect of high mantle shear viscosity and advection by the inflowing mantle and eventually pond at 55–65 km depths. These results show that grain size and mantle shear viscosity together play an important role in focusing fluids beneath the arc.

Antigorite crystallographic preferred orientations in serpentinites from Japan

Abstract Foliated antigorite serpentinites are contributing to seismic anisotropy in subduction zones. However, uncertainty remains regarding the development of antigorite crystal preferred orientations (CPO). This study analyzes the CPOs of antigorite and olivine in variably serpentinized samples of antigorite serpentinite from three localities in Southwest Japan, using U-stage, EBSD, and synchrotron X-rays. In all samples (010) poles show a maximum parallel to the lineation, and (001) poles concentrate normal to the foliation with a partial girdle about the lineation. (100) poles are less distinct and scatter widely. The three measurement methods give similar results, though orientation distributions from X-ray analysis which averages over larger sample volumes are more regular and weaker than U-stage and EBSD which focuses on local well-crystallized grains. In olivine-bearing serpentinite, the olivine CPO does not appear to control the antigorite CPO, though there are some topotactic relationships. The antigorite CPO formed contemporaneously with serpentinization and shear deformation. Based on our analyses, in a subduction zone mantle wedge, the (010) pole of antigorite is parallel to the direction of the subduction, and (001) lattice planes are parallel to the interface of the subducting oceanic plate. By averaging single crystal elastic properties over orientation distributions wave velocities have been calculated. Fast P velocities are parallel to the subduction direction and slow velocities are perpendicular to the subducting plate with an anisotropy of 10–15%.

Small-scale convection in the subduction zone mantle wedge

Small-scale convection in the upper mantle can appreciably influence large-scale mantle dynamics and has important implications for geophysical observables such as seismic anisotropy and surface heat flow. We develop numerical models to evaluate the likelihood of small-scale convection in the mantle wedge above subducting slabs. The characteristics of small-scale convection are analyzed using the 3-D single-mode approximation, in which one characteristic wavenumber represents small-scale convective motions with rotational axes parallel to the convergence direction. Numerical simulations are run for a range of characteristic wavenumbers, Rayleigh numbers, and subduction parameters (e.g., slab dip angle, convergence velocity, and downgoing plate age). For each simulation, we quantify the excitation due to small-scale convection and determine under what conditions and characteristic wavenumbers this excitation is maximized. We find that of the parameters examined, mantle wedge viscosity plays the most significant role in dictating the occurrence and strength of small-scale convective motions in the mantle wedge. Numerical models run with subduction parameters similar to that of northeast Japan, where it has been proposed that small-scale convection may be occurring in the mantle wedge, require a mantle wedge viscosity of ∼1018Pas for significant small-scale convection to occur.

Grain-size distribution in the mantle wedge of subduction zones

[1] Mineral grain size plays an important role in controlling many processes in the mantle wedge of subduction zones, including mantle flow and fluid migration. To investigate the grain-size distribution in the mantle wedge, we coupled a two-dimensional (2-D) steady state finite element thermal and mantle-flow model with a laboratory-derived grain-size evolution model. In our coupled model, the mantle wedge has a composite olivine rheology that incorporates grain-size-dependent diffusion creep and grain-size-independent dislocation creep. Our results show that all subduction settings lead to a characteristic grain-size distribution, in which grain size increases from 10 to 100 μm at the most trenchward part of the creeping region to a few centimeters in the subarc mantle. Despite the large variation in grain size, its effect on the mantle rheology and flow is very small, as >90% of the deformation in the flowing part of the creeping region is accommodated by grain-size-independent dislocation creep. The predicted grain-size distribution leads to a downdip increase in permeability by ∼5 orders of magnitude. This increase is likely to promote greater upward migration of aqueous fluids and melts where the slab reaches ∼100 km depth compared with shallower depths, potentially providing an explanation for the relatively uniform subarc slab depth. Seismic attenuation derived from the predicted grain-size distribution and thermal field is consistent with the observed seismic structure in the mantle wedge at many subduction zones, without requiring a significant contribution by the presence of melt.

Electrical conductivity of the serpentinised mantle and fluid flow in subduction zones

In the mantle wedge of subduction zones, electromagnetic profiles reveal high electrical-conductivity bodies. In hot areas (> 700 °C), water released by dehydration of the slab induces melting of the mantle under volcanic arcs that can explain the observed high conductivities. In the cold (< 700 °C) melt-free fore-arc mantle wedge, fluid water migrates and causes serpentinisation detected as low seismic wave velocities in the mantle wedge. High conductivities in the serpentinized wedge may picture serpentinisation or instantaneous fluid flow depending on serpentine electrical conductivity. We measured the electrical conductivities of three natural serpentine samples from subduction zone context using complex impedance measurements, and find they have low electrical conductivities (< 10 − 4 S.m − 1 ) similar to those of dry mantle minerals below 700 °C. Because of the negligible conductivity of serpentine, electrical conductivity in the hydrated mantle wedge is only sensitive to the fluid content and salinity, not to serpentinisation. A small fraction ( ca. 1% in volume) of connective high-salinity fluids accounts for the highest observed conductivities. The low-salinity fluids (≤ 0.1 m) released by slab dehydration evolve towards high-salinity (≥ 1 m) fluids during progressive serpentinisation of the mantle wedge. These fluids can mix with arc magmas at depths and account for high chlorine/water ratios in arc lavas.

Elasticity of antigorite, seismic detection of serpentinites, and anisotropy in subduction zones

Serpentinization of the mantle wedge is an important process that influences the seismic and mechanical properties in subduction zones. Seismic detection of serpentines relies on the knowledge of elastic properties of serpentinites, which thus far has not been possible in the absence of single-crystal elastic properties of antigorite. The elastic constants of antigorite, the dominant serpentine at high-pressure in subduction zones, were measured using Brillouin spectroscopy under ambient conditions. In addition, antigorite lattice preferred orientations (LPO) were determined using an electron back-scattering diffraction (EBSD) technique. Isotropic aggregate velocities are significantly lower than those of peridotites to allow seismic detection of serpentinites from tomography. The isotropic V P/ V S ratio is 1.76 in the Voigt–Reuss–Hill average, not very different from that of 1.73 in peridotite, but may vary between 1.70 and 1.86 between the Voigt and Reuss bonds. Antigorite and deformed serpentinites have a very high seismic anisotropy and remarkably low velocities along particular directions. V P varies between 8.9 km s − 1 and 5.6 km s − 1 (46% anisotropy), and 8.3 km s − 1 and 5.8 km s − 1 (37%), and V S between 5.1 km s − 1 and 2.5 km s − 1 (66%), and 4.7 km s − 1 and 2.9 km s − 1 (50%) for the single-crystal and aggregate, respectively. The V P/ V S ratio and shear wave splitting also vary with orientation between 1.2 and 3.4, and 1.3 and 2.8 for the single-crystal and aggregate, respectively. Thus deformed serpentinites can present seismic velocities similar to peridotites for wave propagation parallel to the foliation or lower than crustal rocks for wave propagation perpendicular to the foliation. These properties can be used to detect serpentinite, quantify the amount of serpentinization, and to discuss relationships between seismic anisotropy and deformation in the mantle wedge. Regions of high V P/ V S ratios and extremely low velocities in the mantle wedge of subduction zones (down to about 6 and 3 km.s −1 for V P and V S, respectively) are difficult to explain without strong preferred orientation of serpentine. Local variations of anisotropy may result from kilometer-scale folding of serpentinites. Shear wave splittings up to 1–1.5 s can be explained with moderately thick (10–20 km) serpentinite bodies.

Geochemistry of the Davis and Aurora Banks: Possible implications on evolution of the North Scotia Ridge

Geochemistry of volcanic rocks of the Davis and Aurora Banks, North Scotia Ridge, is presented with the aim to infer their petrogenesis and implications in regional tectonics. The analyses show consistent major and trace element patterns which are conformable with calc-alkaline series volcanics with typical signatures of subduction zone magma. The source melt of the studied volcanics is deciphered to be subduction zone mantle wedge metasomatised by sediments. Geochemical analyses of the volcanics from the Davis and Aurora Banks are compared with geochemical data of volcanics of the South Sandwich island arc and Jurassic, Cretaceous and Paleogene arc volcanics of Patagonia, South America. This comparative study shows that the Davis and Aurora Banks' volcanics are akin to Cretaceous arc volcanics of the Hardy Formation, South America. Rocks of Hardy Formation are also reported from South Georgia. Further, based on established trace element ratio diagrams, it is deciphered that source magma characteristics of arc volcanics from South Georgia, Hardy Formation and Davis and Aurora Banks are the same. Integration of our findings with previous studies, we propose a geodynamic model for the Davis and Aurora Banks in conformity with accepted model for the evolution of the West Scotia Sea.