Near-infrared focused heating method for the rotational diamond anvil cell

We developed a near-infrared focused heating system (IRrDAC) for deformation experiments using a rotational diamond anvil cell. This study reports the results of annealing tests on silver and antigorite conducted at SPring-8 (BL47XU) using the IRrDAC system. The experimental results demonstrated the melting of silver and the dehydration of antigorite, confirming the capability of this system. The reproducible relationships between temperature and input power were also confirmed. The IRrDAC system enables deformation experiments at pressures equivalent to those of the lower mantle under homogeneous and stable temperatures and is expected to contribute to the understanding of deep Earth rheology.

Steam caps in geothermal reservoirs can be monitored using seismic noise interferometry

Harvesting geothermal energy often leads to a pressure drop in reservoirs, decreasing their profitability and promoting the formation of steam caps. While steam caps are valuable energy resources, they also alter the reservoir thermodynamics. Accurately measuring the steam fraction in reservoirs is essential for both operational and economic perspectives. However, steam content estimations are very limited both in space and time since current methods rely on direct measurements within production wells. Besides, these estimations normally present large uncertainties. Here, we present a pioneering method for indirectly sampling the steam content in the subsurface using the ever-present seismic background noise. We observe a consistent annual velocity drop in the Hengill geothermal field (Iceland) and establish a correlation between the velocity drop and steam buildup using in-situ borehole data. This application opens new avenues to track the evolution of any gas reservoir in the crust with a surface-based and cost-effective method.

Subduction thermal regime, petrological metamorphism and seismicity under the Mariana arc

Because of the steep subduction of a highly concave slab, researchers have characterized megathrusts under the Marianas as among the coldest and curviest plate coupling interfaces in various circum-Pacific subduction zones. Seismic tomography indicates that the heterogeneous underlying plate varies markedly in its subduction angle, velocity, and flexure along the strike and dip, while their effects on the thermal structure and intraslab earthquake occurrence remain enigmatic. By incorporating the 3-D MORVEL velocity and state-of-the-art slab geometry into thermomechanical modeling, we estimated the 3-D subduction thermal state and hydrothermal regime below the Marianas. We find that (1) the concave slab geometry and the complexity of the intraslab velocity variation in the Marianas are associated with a heterogeneous along-strike thermal regime and a cold mantle wedge beneath the central Marianas; (2) amphibolitization and eclogitization of subducted oceanic crust cause variations in fluid pressure and fluid release from the subduction interface, which may influence the distribution of interface seismicity in the Mariana system; (3) the concentration of active hydrothermal vents in the trench > 8 km deep is accompanied by a large temperature gradient and subsequent remarkable slab dehydration in the southern Marianas; and (4) slab dehydration (> 0.02 wt%/km) from 30 to 80 km indicates notable fluid release and potential fluid migration in subduction channels, which may correspond to the large water flux at depth beneath the Marianas.

Venus' light slab hinders its development of planetary-scale subduction

Terrestrial planet Venus has a similar size, mass, and bulk composition to Earth. Previous studies proposed that local plume-induced subduction existed on both early Earth and Venus, and this prototype subduction might initiate plate tectonics on Earth but not on Venus. In this study, we simulate the buoyancy of submerged slabs in a hypothesized 2-D thermo-metamorphic model. We analyze the thermal state of the slab, which is then used for calculating density in response to thermal and phase changes. The buoyancy of slab mantle lithosphere is primarily controlled by the temperatures and the buoyancy of slab crust is dominated by metamorphic phase changes. Difference in the eclogitization process contributes most to the slab buoyancy difference between Earth and Venus, which makes the subducted Venus' slab consistently less dense than Earth's. The greater chemical buoyancy on Venus, acting as a resistance to subduction, may have impeded the transition into self-sustained subduction and led to a different tectonic regime on Venus. This hypothesis may be further tested as more petrological data of Venus become available, which will further help to assess the impact of petro-tectonics on the planet's habitability.

Subduction Evolution Controlled Himalayan Orogenesis: Implications from 3-D Subduction Modeling

Himalayan orogenesis remains enigmatic in terms of Tibetan Plateau geodynamics originating from the Cenozoic India–Eurasian continental collision. India underthrusts below Tibet to the Yarlung–Tsangpo suture, which has been identified as the northernmost boundary for underplating. However, the way in which the historical evolution of continental subduction induces plateau uplift and the way it controls the variation in uplift between outboard and inboard areas is still unclear. To interpret the evolutionary mechanisms involved in the Himalayan growth history, we constructed different 3-D dynamic models at important stages to address these questions related to the formation of the Himalayas on the basis of paleoenthalpy evidence encoded in fossil leaves from recently documented assemblages in southern Tibet. The results show that (1) the effect of crustal thickening was the predominant factor in the early evolution from the Paleocene to the early Eocene, which resulted in a moderate growth rate. (2) The consecutive slab break-off eastward from the western syntaxis and the associated slab rebound significantly accelerated orogenesis from the late Eocene to the Oligocene. The upwelling asthenospheric flow was a key control of increasing crustal buoyancy, which resulted in the fastest growth of the Himalayas during the early Miocene. (3) Thereafter, the gradually enhanced monsoon and surface erosion during accompanying the increasing mountain height resulted in a slowdown of the orogenic rate, which counterbalanced the buoyant force produced by asthenospheric flow driving continuous Himalayan growth.

3-D data of thermal regime, water content, and slab dehydration in Alaska

The data include the 3-D temperature field (degrees Celsius), water content (wt%), dehydration rate (wt%/km), and subduction velocity field (cm/yr) of the subducting plate, as well as the coastline and volcano distribution in Alaska. The data of the model region have dimensions of 800 × 1600 × 400 km (length × width × depth). The geometry of the subducted plate is well constrained by Slab2.0, and the plate ages are provided by EarthByte. The subduction velocities inside a prescribed 3-D constrained volume of the oceanic lithosphere are given based on the kinematic plate subduction modeling method and the MORVEL plate motion data. The observation of surface heat flow and Curie point depths are used to constrain the model thermal regime. The geophysical calculation is ensured after the subduction thermal regime reaches a steady state. Data are deposited in the TPDC repository, which has granted a persistent identifier https://data.tpdc.ac.cn/en/disallow/8b266d22-fea7-4259-9a5f-8ac0bd9e7869/. Data include (1) paraview_eq_USGS.vtk (earthquake catalog by IRIS, 2000-2010, Trabant et al., 2012), (2) paraview_slab.vtk (3-D thermal regime, slab water content and slab dehydration), (3) paraview_volcano.vtk (global volcanoes at NCEI, Siebert et al., 2010), and (4) paraview_map.vtk (coastline, GMT).

Differential Fluid Activity in a Single Exhumed Continental Subduction Unit from Local P-T-M(H2O) Records of Zoned Amphiboles (North Muya, Eastern Siberia)

The behavior of the continental lithosphere in the Alpine-type subduction zones, which primarily depends on its thickness, thermal regime of subduction and availability of fluids/melts, remains an important issue for both metamorphic petrology and geochemistry as well as for resolving the thermomechanical properties of subduction paleo-interfaces. Rehydrated (amphibole- and zoisite-bearing) eclogites from the Neoproterozoic North Muya high-pressure complex (northern Central Asian Orogenic belt, eastern Siberia) were studied in order to assess their peak burial depths, degree of prograde dehydration, and further retrograde hydration extent within a subducted and exhumed continental unit. Three medium-grained eclogites from different localities of HP complex show similarly dry peak assemblages of pyrope-almandine-grossular garnet (XGrs up to 0.30, XPrp up to 0.25) + Na-rich omphacite (up to 44 mol % of jadeite) + rutile + quartz, which are variably replaced by secondary plagioclase + clinopyroxene ± amphibole symplectitic aggregate. The eclogites were subjected to burial at similar peak depths (up to ~17–21 kbar) but different peak temperatures (within ~600–730 °C) with or without notable heating and re-equilibration due to crustal thickening. Variable degrees of exhumation-induced pervasive rehydration led to growth of individual zoned porphyroblastic barroisite-hornblende amphibole ([B]Na = 0.03–0.45) ± zoisite over the primary eclogitic assemblage or after notable thermally-driven development of symplectitic aggregate after omphacite. Amphibole compositions together with the zoisite presence/absence in different samples reflect continuous rehydration by addition of ~0.5–1.5 wt.% at different exhumation conditions, from nearly peak eclogitic P–T (~17–21 kbar) to granulite- and amphibolite-facies depths within the plagioclase stability field (<14 kbar). This diversity most likely required irregular distribution of internally sourced, low-volume, hydrous metamorphic fluid (i.e., from host felsic rocks or metasediments) acting at different depths of the subduction interface. From the performed PTX calculations, I suggest that nearly isochemical (i.e., without any significant modification of the bulk-rock composition other than incorporation of additional H2O), retrograde hydration by only at lower- to middle-crust conditions did not significantly influence the density and the rheology of the subducted continental slices due to both (1) a limited abundance of dense metabasic rocks, which are commonly more fluid-rich (e.g., due to chlorite or amphibole alteration), and (2) the initially dry nature of mafic and felsic continental rocks. The limited dehydration and rehydration scales exemplified by the North Muya eclogites and therefore low availability of hydrous metamorphic fluids may have accounted for the high buoyancy of the eclogitic crust and explained the absence of contemporaneous suprasubduction magmatism in the regional context at ca. ~630 Ma.

Structural and Thermal Evolution of an Infant Subduction Shear Zone: Insights From Sub-Ophiolite Metamorphic Rocks Recovered From Oman Drilling Project Site BT-1B

Subduction interface thermal structure changes drastically within the first few million years of underthrusting (i.e., subduction infancy). Metamorphic soles beneath ophiolites record snapshots of dynamic conditions and mechanical coupling during subduction infancy. Beneath the Samail Ophiolite (Oman), the sole comprises structurally higher high-temperature (HT) and lower low-temperature (LT) units. This inverted metamorphic gradient has been attributed to evolving metamorphic Pressure-Temperature (P-T) conditions during infancy; however, peak P-T and timing of LT sole subduction are poorly constrained. Oman Drilling Project core BT-1B sampled the base of the ophiolite in a location lacking the HT sole. Metasedimentary and meta-mafic samples collected from 104 m of core reveal that the LT sole subducted to similar peak P as HT rocks preserved elsewhere in Oman, but experienced ∼300°C lower peak T. Prograde fabrics record Si-in-phengite and amphibole chemistries consistent with peak P-T of ∼7-10 kbar and ∼450-550°C in the epidote-amphibolite facies. Retrograde fabrics record a transition from near-pervasive ductile to localized brittle strain under greenschist facies conditions. Titanite U-Pb ages (n = 2) constrain timing of peak LT sole subduction to ∼91 Ma (post-dating initial HT sole subduction by ∼12-13 Myr) and dynamic retrogression through ∼90 Ma. Combined with existing geo/thermo-chronology, our results support a model of protracted subduction and accretion while the infant subduction zone experienced multi-phase, slow-fast-slow cooling. Temporal overlap of HT sole cooling (rehydration?) and ophiolite formation suggests that cooling may lead to interface weakening, facilitating upper-plate extension and spreading. The LT sole formed in a rapidly-refrigerating forearc after ophiolite formation and may reflect the transition to self-sustaining subduction.

Weakening Mechanisms in a Basalt-Hosted Subduction Megathrust Fault Segment, Southern Alaska

Basaltic and gabbroic rocks that define the seafloor have been suggested to act as sources of rheological heterogeneity during subduction, with the capacity to enhance or dampen seismicity. Despite this, relatively little is known from the rock record regarding the progression and conditions of mafic oceanic crust deformation during subduction, particularly in the shallow megathrust region of the seismogenic zone. We describe subduction-related deformation structures and characterize deformation conditions from an exhumed, basalt-hosted megathrust in the Chugach accretionary complex of south-central Alaska. Rocks in the Chugach preserve a record of seafloor mineralogical changes from pre-subduction, hydrothermal circulation that produced sheet silicates with a lower frictional strength than intact basalt. Pre-subduction alteration also served to introduce hydrous phases that can expel water during deformation and raise the pore fluid pressure. Once strain localized within basalts onto a megathrust fault plane at lithostatic pore fluid pressures, the basalt weakened further through a combination of cataclasis, dilatational shear fracturing, and slip on chlorite-rich shear bands. This process occurred in a narrower fault zone, and at higher maximum differential stress and greater pore fluid pressure fluctuations than recorded in some sediment-hosted megathrusts at similar pressure and temperature conditions. Our data indicate that when the lower plate contains basalt bathymetric features, basalt dismembers during subduction into a chlorite-rich fault gouge that surrounds lenses or slices of intact, less-altered basalt.

Insights Into the Origin and Deformation Style of the Continental Moho: A Case-Study From the Western Alps (Italy)

Several hypotheses on the origin of the continental Moho are still debated and multiple mechanisms may contribute to its formation. Here, we present quantitative estimation of the seismic properties and anisotropy of the crust-mantle transition in the Western Alps where an example of newly formed (proto)-continental Moho is unusually shallow. We make use of teleseismic P-to-S converted-waves recorded by stations deployed on top of the Ivrea Body (IB), a volume of possibly serpentinized mantle peridotite below exhumed (ultra-)high pressure crustal rocks. The IB has been mapped by gravity, magnetic, active and passive seismic surveys suggesting an extremely shallow Moho. We demonstrate that the P-to-S converted waves propagating through this region display coupled features: (a) they record expected presence of strong seismic velocity contrast at shallow depth as due to the lower crustal and upper mantle transition; (b) they are decomposed due to anisotropic properties of rocks involved. The proto-continental Moho is recognized as an increase in S-wave velocity (∼0.4-1 km/s) at shallow depths of 5-10 km. The presence of anisotropy within the IB and overlying crustal rocks is evidenced by back-azimuthal dependence of the amplitude of P-to-S phases. The strength of anisotropy is ∼-14% on average pointing out the presence of metamorphosed/hydrated material (e.g., serpentinite) below the Moho. Anisotropic directions are consistent across Moho in both crust and upper mantle. The similarity of the anisotropy parameters between crust and upper mantle suggests they have been shaped by the same deformation event.

The stability of subducted glaucophane with the Earth's secular cooling

The blueschist to eclogite transition is one of the major geochemical–metamorphic processes typifying the subduction zone, which releases fluids triggering earthquakes and arc volcanism. Although glaucophane is an index hydrous mineral for the blueschist facies, its stability at mantle depths in diverse subduction regimes of contemporary and early Earth has not been experimentally determined. Here, we show that the maximum depth of glaucophane stability increases with decreasing thermal gradients of the subduction system. Along cold subduction geotherm, glaucophane remains stable down ca. 240 km depth, whereas it dehydrates and breaks down at as shallow as ca. 40 km depth under warm subduction geotherm or the Proterozoic tectonic setting. Our results imply that secular cooling of the Earth has extended the stability of glaucophane and consequently enabled the transportation of water into deeper interior of the Earth, suppressing arc magmatism, volcanism, and seismic activities along subduction zones.

Along the cold subduction geotherm, glaucophane remains stable down to pressure and temperature (P–T) conditions of ca. 240 km depth, whereas under the warm subduction geotherm, it dehydrates and breaks down into pyroxenes and silica between ca. 50 and 100 km depths.

Lattice Preferred Orientation and Deformation Microstructures of Glaucophane and Epidote in Experimentally Deformed Epidote Blueschist at High Pressure

To understand the lattice preferred orientation (LPO) and deformation microstructures at the top of a subducting slab in a warm subduction zone, deformation experiments of epidote blueschist were conducted in simple shear under high pressure (0.9–1.5 GPa) and temperature (400–500 °C). At low shear strain (γ ≤ 1), the [001] axes of glaucophane were in subparallel alignment with the shear direction, and the (010) poles were subnormally aligned with the shear plane. At high shear strain (γ > 2), the [001] axes of glaucophane were in subparallel alignment with the shear direction, and the [100] axes were subnormally aligned with the shear plane. At a shear strain between 2< γ <4, the (010) poles of epidote were in subparallel alignment with the shear direction, and the [100] axes were subnormally aligned with the shear plane. At a shear strain where γ > 4, the alignment of the (010) epidote poles had altered from subparallel to subnormal to the shear plane, while the [001] axes were in subparallel alignment with the shear direction. The experimental results indicate that the magnitude of shear strain and rheological contrast between component minerals plays an important role in the formation of LPOs for glaucophane and epidote.

Cascadia low frequency earthquakes at the base of an overpressured subduction shear zone

In subduction zones, landward dipping regions of low shear wave velocity and elevated Poisson’s ratio, which can extend to at least 120 km depth, are interpreted to be all or part of the subducting igneous oceanic crust. This crust is considered to be overpressured, because fluids within it are trapped beneath an impermeable seal along the overlying inter-plate boundary. Here we show that during slow slip on the plate boundary beneath southern Vancouver Island, low frequency earthquakes occur immediately below both the landward dipping region of high Poisson’s ratio and a 6–10 km thick shear zone revealed by seismic reflections. The plate boundary here either corresponds to the low frequency earthquakes or to the anomalous elastic properties in the lower 3–5 km of the shear zone immediately above them. This zone of high Poisson’s ratio, which approximately coincides with an electrically conductive layer, can be explained by slab-derived fluids trapped at near-lithostatic pore pressures.

Regions of the subducting oceanic crust are often considered to be overpressured, owing to fluid trapped beneath an impermeable seal along the overlying inter-plate boundary. Here, the authors show that slow slip earthquakes at the Cascadia subduction zone occur immediately below a 6-10 km-thick shear zone, in which slab-derived fluids are likely trapped at near-lithostatic pore pressures.

Microstructural Evolution of Amphibole Peridotites in Åheim, Norway, and the Implications for Seismic Anisotropy in the Mantle Wedge

The microstructure of amphibole peridotites from Åheim, Norway were analyzed to understand the evolution of the lattice-preferred orientation (LPO) of olivine throughout the Scandian Orogeny and its implication for the seismic anisotropy of the subduction zone. The Åheim peridotites had a porphyroclastic texture and some samples contained an abundant amount of hydrous minerals such as tremolite. Detailed microstructural analysis on the Åheim peridotites revealed multiple stages of deformation. The coarse grains showed an A-type LPO of olivine, which can be interpreted as the initial stage of deformation. The spinel-bearing samples showed a mixture of B-type and C-type LPOs of olivine, which is considered to represent the deformation under water-rich conditions. The recrystallized fine-grained olivine displays a B-type LPO, which can be interpreted as the final stage of deformation. Microstructures and water content of olivine indicate that the dominant deformation mechanism of olivine showing a B-type LPO is a dislocation creep under water-rich condition. The observation of the B-type LPO of olivine is important for an interpretation of trench-parallel seismic anisotropy in the mantle wedge. The calculated seismic anisotropy of the tremolite showed that tremolite can contribute to the trench-parallel seismic anisotropy in the mantle wedge.

Effects of Corneal Hydration on Brillouin Microscopy In Vivo

Purpose

To investigate how corneal hydration affects the Brillouin frequency of corneal stroma.

Methods

From a simple analytical model considering the volume fraction of water in corneal stroma, we derived the dependence of Brillouin frequency on hydration and hydration-induced corneal thickness variation. The Brillouin frequencies of fresh ex vivo porcine corneas were measured as their hydration was varied in dextran solution and water. Healthy volunteers (8 eyes) were scanned in vivo repeatedly over the course of 9 hours, and the diurnal variations of Brillouin frequency and central corneal thickness (CCT) were measured.

Results

The measured dependence of Brillouin frequency on hydration, both ex vivo and in vivo, agreed well with the theoretical prediction. The Brillouin frequencies of human corneas scanned immediately after waking were on average ∼25 MHz lower than their daytime average values. For stabilized corneas, the typical variation of Brillouin frequency was ± 7.2 MHz. With respect to CCT increase or swelling, the Brillouin frequency decreased with a slope of −1.06 MHz/μm in vivo.

Conclusions

The ex vivo and in vivo data agree with our theoretical model and support that the effect of corneal hydration on Brillouin frequency comes predominantly from the dependence of the tissue compressibility on the water. Corneal hydration correlates negatively with the Brillouin frequency. During daytime activities, the influence of physiological hydration changes in human corneas is < ± 10 MHz. The sensitivity to hydration may potentially be useful in detecting abnormal hydration change in patients with endothelial disorders.

Shear heating reconciles thermal models with the metamorphic rock record of subduction

Some commonly referenced thermal-mechanical models of current subduction zones imply temperatures that are 100-500 °C colder at 30-80-km depth than pressure-temperature conditions determined thermobarometrically from exhumed metamorphic rocks. Accurately inferring subduction zone thermal structure, whether from models or rocks, is crucial for predicting metamorphic reactions and associated fluid release, subarc melting conditions, rheologies, and fault-slip phenomena. Here, we compile surface heat flow data from subduction zones worldwide and show that values are higher than can be explained for a frictionless subduction interface often assumed for modeling. An additional heat source--likely shear heating--is required to explain these forearc heat flow values. A friction coefficient of at least 0.03 and possibly as high as 0.1 in some cases explains these data, and we recommend a provisional average value of 0.05 ± 0.015 for modeling. Even small coefficients of friction can contribute several hundred degrees of heating at depths of 30-80 km. Adding such shear stresses to thermal models quantitatively reproduces the pressure-temperature conditions recorded by exhumed metamorphic rocks. Comparatively higher temperatures generally drive rock dehydration and densification, so, at a given depth, hotter rocks are denser than colder rocks, and harder to exhume through buoyancy mechanisms. Consequently--conversely to previous proposals--exhumed metamorphic rocks might overrepresent old-cold subduction where rocks at the slab interface are wetter and more buoyant than in young-hot subduction zones.

Carbon Cycling and Habitability of Earth-Sized Stagnant Lid Planets

Models of thermal evolution, crustal production, and CO₂ cycling are used to constrain the prospects for habitability of rocky planets, with Earth-like size and composition, in the stagnant lid regime. Specifically, we determine the conditions under which such planets can maintain rates of CO₂ degassing large enough to prevent global surface glaciation but small enough so as not to exceed the upper limit on weathering rates provided by the supply of fresh rock, a situation which would lead to runaway atmospheric CO₂ accumulation and an inhospitably hot climate. The models show that stagnant lid planets with initial radiogenic heating rates of 100-250 TW, and with total CO₂ budgets ranging from ∼10^-2 to 1 times Earth's estimated CO₂ budget, can maintain volcanic outgassing rates suitable for habitability for ≈1-5 Gyr; larger CO₂ budgets result in uninhabitably hot climates, while smaller budgets result in global glaciation. High radiogenic heat production rates favor habitability by sustaining volcanism and CO₂ outgassing longer. Thus, the results suggest that plate tectonics may not be required for establishing a long-term carbon cycle and maintaining a stable, habitable climate. The model is necessarily highly simplified, as the uncertainties with exoplanet thermal evolution and outgassing are large. Nevertheless, the results provide some first-order guidance for future exoplanet missions, by predicting the age at which habitability becomes unlikely for a stagnant lid planet as a function of initial radiogenic heat budget. This prediction is powerful because both planet heat budget and age can potentially be constrained from stellar observations. Key Words: Exoplanets-Habitability-Stagnant lid tectonics-Carbon cycle-Volcanism. Astrobiology 18, 873-896.

Mineral dissolution and reprecipitation mediated by an amorphous phase

Fluid-mediated mineral dissolution and reprecipitation processes are the most common mineral reaction mechanism in the solid Earth and are fundamental for the Earth's internal dynamics. Element exchange during such mineral reactions is commonly thought to occur via aqueous solutions with the mineral solubility in the coexisting fluid being a rate limiting factor. Here we show in high-pressure/low temperature rocks that element transfer during mineral dissolution and reprecipitation can occur in an alkali-Al-Si-rich amorphous material that forms directly by depolymerization of the crystal lattice and is thermodynamically decoupled from aqueous solutions. Depolymerization starts along grain boundaries and crystal lattice defects that serve as element exchange pathways and are sites of porosity formation. The resulting amorphous material occupies large volumes in an interconnected porosity network. Precipitation of product minerals occurs directly by repolymerization of the amorphous material at the product surface. This mechanism allows for significantly higher element transport and mineral reaction rates than aqueous solutions with major implications for the role of mineral reactions in the dynamic Earth.

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.

Seismogenesis of dual subduction beneath Kanto, central Japan controlled by fluid release

Dual subduction represents an unusual case of subduction where one oceanic plate subducts on top of another, creating a highly complex tectonic setting. Because of the complex interaction between the two subducted plates, the origin of seismicity in such region is still not fully understood. Here we investigate the thermal structure of dual subduction beneath Kanto, central Japan formed as a consequence of a unique case of triple trench junction. Using high-resolution three-dimensional thermo-mechanical models tailored for the specific dual subduction settings beneath Kanto, we show that, compared with single-plate subduction systems, subduction of double slabs produces a strong variation of mantle flow, thermal and fluid release pattern that strongly controls the regional seismicity distribution. Here the deepening of seismicity in the Pacific slab located under the Philippine Sea slab is explained by delaying at greater depths (~150 km depth) of the eclogitization front in this region. On the other hand, the shallower seismicity observed in the Philippine Sea slab is related to a young and warm plate subduction and probably to the presence of a hot mantle flow traveling underneath the slab and then moving upward on top of the slab.

Pressure induced elastic softening in framework aluminosilicate- albite (NaAlSi3O8)

Albite (NaAlSi₃O₈) is an aluminosilicate mineral. Its crystal structure consists of 3-D framework of Al and Si tetrahedral units. We have used Density Functional Theory to investigate the high-pressure behavior of the crystal structure and how it affects the elasticity of albite. Our results indicate elastic softening between 6–8 GPa. This is observed in all the individual elastic stiffness components. Our analysis indicates that the softening is due to the response of the three-dimensional tetrahedral framework, in particular by the pressure dependent changes in the tetrahedral tilts. At pressure <6 GPa, the PAW-GGA can be described by a Birch-Murnaghan equation of state with  = 687.4 ų,  = 51.7 GPa, and  = 4.7. The shear modulus and its pressure derivative are  = 33.7 GPa, and  = 2.9. At 1 bar, the azimuthal compressional and shear wave anisotropy  = 42.8%, and  = 50.1%. We also investigate the densification of albite to a mixture of jadeite and quartz. The transformation is likely to cause a discontinuity in density, compressional, and shear wave velocity across the crust and mantle. This could partially account for the Mohorovicic discontinuity in thickened continental crustal regions.

Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone

It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed.

Dehydration of chlorite explains anomalously high electrical conductivity in the mantle wedges

Mantle wedge regions in subduction zone settings show anomalously high electrical conductivity (~1 S/m) that has often been attributed to the presence of aqueous fluids released by slab dehydration. Laboratory-based measurements of the electrical conductivity of hydrous phases and aqueous fluids are significantly lower and cannot readily explain the geophysically observed anomalously high electrical conductivity. The released aqueous fluid also rehydrates the mantle wedge and stabilizes a suite of hydrous phases, including serpentine and chlorite. In this present study, we have measured the electrical conductivity of a natural chlorite at pressures and temperatures relevant for the subduction zone setting. In our experiment, we observe two distinct conductivity enhancements when chlorite is heated to temperatures beyond its thermodynamic stability field. The initial increase in electrical conductivity to ~3 × 10(-3) S/m can be attributed to chlorite dehydration and the release of aqueous fluids. This is followed by a unique, subsequent enhancement of electrical conductivity of up to 7 × 10(-1) S/m. This is related to the growth of an interconnected network of a highly conductive and chemically impure magnetite mineral phase. Thus, the dehydration of chlorite and associated processes are likely to be crucial in explaining the anomalously high electrical conductivity observed in mantle wedges. Chlorite dehydration in the mantle wedge provides an additional source of aqueous fluid above the slab and could also be responsible for the fixed depth (120 ± 40 km) of melting at the top of the subducting slab beneath the subduction-related volcanic arc front.

Subducting slab ultra-slow velocity layer coincident with silent earthquakes in southern Mexico

Great earthquakes have repeatedly occurred on the plate interface in a few shallow-dipping subduction zones where the subducting and overriding plates are strongly locked. Silent earthquakes (or slow slip events) were recently discovered at the down-dip extension of the locked zone and interact with the earthquake cycle. Here, we show that locally observed converted SP arrivals and teleseismic underside reflections that sample the top of the subducting plate in southern Mexico reveal that the ultra-slow velocity layer (USL) varies spatially (3 to 5 kilometers, with an S-wave velocity of approximately 2.0 to 2.7 kilometers per second). Most slow slip patches coincide with the presence of the USL, and they are bounded by the absence of the USL. The extent of the USL delineates the zone of transitional frictional behavior.