Region Of Partial Melt Research Articles

Thermochemical Structure and Melting Distribution of the Upper Mantle Beneath Intraplate Volcanic Areas in Eastern South China Block

AbstractThe Eastern South China Block (SCB) has experienced complex Mesozoic‐Cenozoic tectonism and intraplate volcanism. However, due to a lack of exhaustive exploration of the upper mantle's thermochemical structure, it is difficult to determine the extent of the lithospheric modification and the mechanisms by which the volcanism generate. Here, we jointly invert Rayleigh wave dispersion, surface heat flow, geoid height, and elevation data to provide a comprehensive thermal and compositional structure of the upper mantle beneath eastern SCB and infer regions of partial melting. Our model reveals widespread lithospheric thinning in the eastern SCB and large variations of lithospheric composition with a more fertile eastern Lower Yangtze lithosphere than the lithosphere elsewhere, suggesting the lithosphere of the eastern Lower Yangtze is more severely modified than the rest of the SCB. Moreover, two high‐temperature anomalies are revealed: one beneath the eastern Lower Yangtze and the other beneath the Pearl River Delta region, associated with the Pacific plate subduction and Hainan plume, respectively. The high‐degree partial melting (∼6%) in the asthenosphere beneath the Lower Yangtze is responsible for the strong ongoing lithospheric modification and the young intraplate volcanism in the Nvshan and Subei areas. Small‐scale upper mantle convections triggered by the large mantle upwellings created a low value of ∼3% melts, possibly responsible for the intraplate volcanism in the coastal CB and less severe lithospheric modification. We demonstrate that the lithospheric thickness and its thermochemical state are the key factors that influence the composition and evolution of intraplate volcanism in the eastern SCB.

A crustal LVZ in Iceland revealed by ambient noise multimodal surface wave tomography

The crustal low-velocity zone (LVZ), an important anomaly found in some regional structures of Iceland, is still absent in the Icelandic average velocity structure due to limitations of tomography methods. Using stations from the HOTSPOT experiment and other supplemental stations throughout Iceland, we apply the frequency-Bessel transform method (F-J method) to extract the first two mode dispersion curves from ambient noise data. We obtain an average S-wave velocity (Vs) model of Iceland down to 120 km depth, where two LVZs at depths of 12–22 km and below 55 km are found. The shallow LVZ, whose rationalities are justified using theoretical dispersion curves of certain models to recover themselves, may improve the understanding of the Icelandic average crust. Furthermore, our model shows better representativeness by comparing travel time residuals of the primary wave between observed and synthetic data predicted using different average velocity models. Based on the variations of the Vs gradient, the Icelandic crust with an average thickness of 32 km is divided into the upper crust (0–10 km), middle crust (10–22 km), and lower crust (22–32 km). The asthenosphere starts from the deeper LVZ at 55 km depth, potentially indicating the relatively concentrated melt in this depth range. In this study, crustal LVZs are revealed both in a volcanic active zone and a non-volcanic zone, which may also suggest the LVZ in the average model has more complex origins than the high-temperature zone beneath the central volcanoes. The prevalent thick-cold crustal model of Iceland, considered to rule out the existence of a broad region of partial melt in the crust, also strengthens the possibility of diverse origins. The variations in petrology may also contribute to the crustal LVZ in the average model.

Lithospheric Structures beneath the Tibetan Plateau Constrained by Sn Propagation Efficiency

Abstract The uppermost mantle is a crucial region linking the lower crust and lithospheric mantle and carries important dynamic and structural information (e.g., upwelling and delamination) on interactions between the crust and mantle. An intriguing observation of an inefficient Sn wave zone beneath the Tibetan Plateau was reported but had unclear spatial distributions. By analyzing the Sn wave propagation efficiency from 34,917 Sn wave arrivals, we provide strong constraints on the lateral and depth variations in the inefficient Sn wave zone in the Tibetan Plateau. Our results show that with increasing frequencies, the inefficient Sn wave zone becomes larger, expanding to the eastern Songpan-Ganze terrane and then to the Sanjiang area and the southern margin of the Qilian orogenic belt. The northern and eastern boundaries of the inefficient Sn zone, particularly a small inefficient zone in the Qilian orogenic belt (37°N, 102°E), are clearly depicted, indicating the widespread distribution of the seismic attenuation structure or partial melt in the uppermost mantle. Moreover, we provide the depth variations in the inefficient Sn wave zone by simulating 31,538 3D raypaths with the fast marching method. We therefore constrain the depth range of the partial melt region in the uppermost mantle beneath the Tibetan Plateau. The seismic attenuation structure in the northern-central Qiangtang terrane and western Songpan-Ganze terrane extends to 130 km from the bottom of the crust, whereas that in the eastern Songpan-Ganze terrane and Sanjiang area is thinner and mainly concentrated from 90 to 130 km. Combining this information with the temporal and spatial variations in potassic-ultrapotassic magmatic rocks, it is suggested that asthenospheric materials might upwell and flow laterally beneath the Tibetan Plateau. This behavior provides a thermal driving force for the uplift of the plateau and the deformation of the crust-mantle structure.

Lithosphere–asthenosphere interactions beneath northeast China and the origin of its intraplate volcanism

AbstractNortheast China hosts one of the largest Cenozoic intraplate volcanic regions in the world. However, the mechanisms that generate the volcanism, its spatial-temporal distribution, and compositional signatures remain highly debated due to the lack of high-resolution images of the mantle's thermochemical structure. We jointly inverted new surface-wave dispersion data, surface heat flow, geoid height, and elevation data to image the fine-scale thermal and compositional structures beneath northeast China and infer regions of partial melting in the mantle. Our model reveals a complex circulation pattern in the asthenosphere and a highly variable lithospheric structure. Combining predictions from our model with independent geochemical data from recent basaltic volcanism, we demonstrate that the generation, location, and composition of intraplate volcanism in this region are controlled by the interaction between shallow asthenospheric circulation and lithospheric thickness. The modeling approach and correlations between basaltic composition and mantle state identified in our study are globally applicable to assessing mantle conditions over time in other continental regions.

Crustal evolution leading to successive rhyolitic supereruptions in the Jemez Mountains volcanic field, New Mexico, USA

The magmatic systems that feed supereruptions result from high magma supply from depth that sustains large regions of partial melt in the shallow crust. However, many questions remain around the processes and timescales over which super-sized magmatic systems develop. We present new zircon age, trace elemental and isotopic data from the Jemez Mountains volcanic field (JMVF) to reveal contrasting magmatic processes that led to successive rhyolitic supereruptions at 1.60 and 1.25 Ma. Before the supereruptions, zircons from lavas erupted over ~8.4 Myr have predominantly unimodal age distributions, close to their respective eruption ages. This highlights that distinct magma domains gradually formed in the JMVF crust. Zircon crystal inheritance shows that magma from the first supereruption at 1.60 Ma included at least three of these precursor plutons and basement rock that were partially molten by enhanced magma supply from ~2 Ma. In contrast, absence of inherited zircon and a change in zircon chemistry in ignimbrite from the second supereruption at 1.25 Ma indicates a thermal resetting of the remnants of the old magma reservoir and rapid construction of a new super-sized magma body. The sudden change in eruptive behaviour in the young JMVF reflects both thermal and chemical maturation of the crust, as well as elevated magma supply and partial melting of precursor plutons. Our study highlights that although crustal conditioning may occur during gradual magmatism over millions of years, the assembly of super-sized magma bodies occurs over much faster timescales and may occur in succession due to thermal adjustment following evacuation.

High-pressure elastic properties of dolomite melt supporting carbonate-induced melting in deep upper mantle

Deeply subducted carbonates likely cause low-degree melting of the upper mantle and thus play an important role in the deep carbon cycle. However, direct seismic detection of carbonate-induced partial melts in the Earth's interior is hindered by our poor knowledge on the elastic properties of carbonate melts. Here we report the first experimentally determined sound velocity and density data on dolomite melt up to 5.9 GPa and 2046 K by in-situ ultrasonic and sink-float techniques, respectively, as well as first-principles molecular dynamics simulations of dolomite melt up to 16 GPa and 3000 K. Using our new elasticity data, the calculated VP/VS ratio of the deep upper mantle (∼180-330 km) with a small amount of carbonate-rich melt provides a natural explanation for the elevated VP/VS ratio of the upper mantle from global seismic observations, supporting the pervasive presence of a low-degree carbonate-rich partial melt (∼0.05%) that is consistent with the volatile-induced or redox-regulated initial melting in the upper mantle as argued by petrologic studies. This carbonate-rich partial melt region implies a global average carbon (C) concentration of 80-140 ppm. by weight in the deep upper mantle source region, consistent with the mantle carbon content determined from geochemical studies.

Asthenospheric buoyancy and the origin of high-relief topography along the Cascadia forearc

The forearc topography of the Cascadia subduction zone varies systematically along-strike, with high-relief in the north and south (the Olympic and Klamath ranges) separated by relatively low relief in its central region. This systematic topographic variability reflects the long-term pattern of uplift and erosion, however, the underlying cause of uplift and the mechanisms by which current topography is supported are unclear. Here, we synthesize results from seismic imaging, geodetic (decadal) and geomorphic (millennial) uplift rates, erosion rates, topographic analysis, and characteristics of the megathrust interface, with a mechanical model to infer that buoyancy in the subslab asthenosphere influences the development and longevity of Cascadia's forearc topography. The Cascadia margin can be divided into three broad segments, with the northern and southern segments characterized by rapid geodetic and geomorphic uplift rates, rapid erosion rates, high coseismic subsidence of great megathrust ruptures, shallower slab dip angles, and increased plate locking compared to the central segment. Tomographic images show low-velocity anomalies beneath the slab in the northern and southern segments, which are inferred to be regions of partial melt, resulting from localized upwellings, and regions of positive mantle buoyancy. Modeling suggests that these buoyant regions can locally increase the total shear force at the megathrust by either shallowing the slab dip — thereby increasing the area of the seismogenic zone — or increasing mechanical plate coupling along the megathrust, by increasing normal stress and/or the effective coefficient of friction. We propose that: 1) sub-slab buoyancy influences topographic development by modulating along-strike patterns of strain within the over-riding plate during the seismic cycle, 2) Permanent development of surface topography occurs due to unrecovered strain over thousands of seismic cycles, and 3) variations in Cascadia's forearc topography are laterally supported by changes in the total shear force at the megathrust interface. Using independent estimates for slab dip and plate locking, we predict the first-order variations in Cascadia forearc topography, with local maxima in the north and south as observed. Given our results, variations in subslab buoyancy may be critical to explaining forearc topography in other subduction systems.

Experimental Study of the Peridotite–Basalt–Fluid System: Phase Relations at Subcritical and Supercritical Р-Т Conditions

The paper reports experimental data on the partial melting of hydrous peridotite and basalt at pressures up to 4 GPa and temperatures up to 1400°С, as well as peridotite–basalt association in the presence of alkaline water–carbonate fluid as an experimental model of mantle reservoir with subducted oceanic protoliths. At partial melting of the hydrous peridotite at 3.7–4.0 GPa and 1000–1300°С, critical relations were observed over the entire studied pressure and temperature interval. At partial melting of hydrous basalt, critical relations between silicate melt and aqueous fluid were recorded at 1000°С and 3.7 GPa. The Na-alkaline silicate melt coexists with garnetite at 1100°С and with clinopyroxenite at 1150 and 1300°С. The peridotite–basalt–alkaline-aqueous–carbonate fluid at 4 GPa and 1400°C shows signs of critical relations between a carbonated silicate melt and a fluid. Reaction relations in the residual peridotite minerals with replacements of olivine ← orythopyroxene ← clinopyroxene ← potassic amphibole-type testify to a high chemical activity of the supercritical liquid. Experimental results suggest the existence of regions of partial melting (asthenospheric lenses) in the fluid-bearing upper mantle under supercritical conditions. These lenses contain near-solidus highly reactive supercritical liquids enriched in incompatible elements. Mantle reservoirs with supercritical liquids are geochemically similar to undepleted mantle and could serve as sources of magmas enriched in incompatible elements. The modal and cryptic upper mantle metasomatism under the effect of supercritical liquids leads to the refertilization of peridotite via enrichment of residual minerals in incompatible elements.

Amphibious surface-wave phase-velocity measurements of the Cascadia subduction zone

SUMMARY A new amphibious seismic data set from the Cascadia subduction zone is used to characterize the lithosphere structure from the Juan de Fuca ridge to the Cascades backarc. These seismic data are allowing the imaging of an entire tectonic plate from its creation at the ridge through the onset of the subduction to beyond the volcanic arc, along the entire strike of the Cascadia subduction zone. We develop a tilt and compliance correction procedure for ocean-bottom seismometers that employs automated quality control to calculate robust station noise properties. To elucidate crust and upper-mantle structure, we present shoreline-crossing Rayleigh-wave phase-velocity maps for the Cascadia subduction zone, calculated from earthquake data from 20 to 160 s period and from ambient-noise correlations from 9 to 20 s period. We interpret the phase-velocity maps in terms of the tectonics associated with the Juan de Fuca plate history and the Cascadia subduction system. We find that thermal oceanic plate cooling models cannot explain velocity anomalies observed beneath the Juan de Fuca plate. Instead, they may be explained by a ≤1 per cent partial melt region beneath the ridge and are spatially collocated with patches of hydration and increased faulting in the crust and upper mantle near the deformation front. In the forearc, slow velocities appear to be more prevalent in areas that experienced high slip in past Cascadia megathrust earthquakes and generally occur updip of the highest-density tremor regions and locations of intraplate earthquakes. Beneath the volcanic arc, the slowest phase velocities correlate with regions of highest magma production volume.

Thermal and petrologic constraints on lower crustal melt accumulation under the Salton Sea Geothermal Field

In the Salton Sea region of southern California (USA), concurrent magmatism, extension, subsidence, and sedimentation over the past 0.5 to 1.0 Ma have led to the creation of the Salton Sea Geothermal Field (SSGF)—the second largest and hottest geothermal system in the continental United States—and the small-volume rhyolite eruptions that created the Salton Buttes. In this study, we determine the flux of mantle-derived basaltic magma that would be required to produce the elevated average heat flow and sustain the magmatic roots of rhyolite volcanism observed at the surface of the Salton Sea region. We use a 2D thermal model to show that a lower-crustal, partially molten mush containing <20–40% interstitial melt develops over a ∼105-yr timescale for basalt fluxes of 0.008 to 0.010 m3/m2/yr (∼0.0008 to ∼0.001 km/3yr injection rate) given extension rates at or below the current value of ∼0.01 m/yr (Brothers et al., 2009). These regions of partial melt are a natural consequence of a thermal regime that scales with average surface heat flow in the Salton Trough, and are consistent with seismic observations. Our results indicate limited melting and assimilation of pre-existing rocks in the lower crust. Instead, we find that basalt fractionation in the lower crust produces derivative melts of andesitic to dacitic composition. Such melts are then expected to ascend and accumulate in the upper crust, where they further evolve to give rise to small-volume rhyolite eruptions (Salton Buttes) and fuel local spikes in surface heat flux as currently seen in the SSGF. Such upper crustal magma evolution, with limited assimilation of hydrothermally altered material, is required to explain the slight decrease in δ18O values of zircons (and melts) that have been measured in these rhyolites.

Variable thermal loading and flexural uplift along the Transantarctic Mountains, Antarctica

The high elevations of the Transantarctic Mountains (TAMs) have been suggested to be flexural in origin, but to date, the thermal contribution to uplift has yet to be quantified. Here, we present new P- and S-wave tomography images of the upper-mantle seismic structure beneath the central to northern TAMs, which reveal two, focused low-velocity anomalies beneath Ross Island and Terra Nova Bay that laterally extend beneath the TAMs front and that are connected by a low-velocity region constrained offshore within the Victoria Land Basin. The focused low velocities are interpreted as shallow regions of partial melt, connected by a broad region of slow (warm) upper mantle associated with Cenozoic extension along the Terror Rift. Thermal loading constraints based on our tomographic results are used to update flexural uplift models for the TAMs. Our findings confirm that thermal buoyancy is a principal component leading to the uplift of the TAMs but suggest that the thermal loading is variable along the TAMs front.

Direct fabrication of unsupported inclined aluminum pillars based on uniform micro droplets deposition

In order to investigate forming directly complex parts without support materials or structures by uniform micro droplets deposition technique, the present work focus on fabricating the unsupported inclined aluminum pillars through offset deposition. An experimental system is developed to produce and deposit uniform molten aluminum droplets. A model is introduced to describe the inclined angle of the droplet deposition at different offset ratios. A one dimensional heat transfer model is proposed to help select the initial temperature parameters of the impinging droplet and the previous solidified droplet to ensure that the fusion occurs. No melting, partial melting and excessive melting region at different offset ratios are determined. The correspondence between offset ratio and inclined angle is considered to be a simple cosine function, and the hypothesis is verified by experiments. The influence of deposition error on an inclined angle of pillars is studied. Internal microstructure of droplet fusion is observed in order to ensure good metallurgical bonding. All of these studies show the feasibility of fabricating directly unsupported inclined aluminum pillars in the limited angle range by using uniform micro droplets.

The Effect of Aluminum Oxide Additives on the Phase Equilibrium in Borosilicate Systems and Crystallization of Borosilicate Melts

The effect of aluminum oxide additives on the phase relations in borosilicate glass based on the system Na2O–B2O3–SiO2 was investigated by means of the thermodynamic modeling. The phase equilibrium realized in this system was obtained using software package "FactSage" (version 6.4). The effect of aluminum oxide additives on the composition of the crystalline phases and residual melt formed during the cooling of borosilicate melts has been studied. The analysis of the diagrams has allowed establishing the regions of partial and complete melting of the substance depending on the chemical composition. The analysis allows determining the temperature of the melting process of a glassy matrix material. The combination of stable borate, silicate, mixed borosilicate, aluminosilicate and boroaluminate phases were determined for a wide range of compositions and temperatures. Boundaries of the regions formation of these phases depending on the composition and the temperature were determined. It allows assessing the prospects for absorption of elements in radioactive waste immobilization. The modeling of the crystallization melts formed in the studied systems was performed.

Three-dimensional conductivity model of crust and uppermost mantle at the northern Trans North China Orogen: Evidence for a mantle source of Datong volcanoes

While the Eastern Block of North China Craton (NCC) had experienced significant lithospheric destruction in the Mesozoic, the Western Block of NCC and the Trans North China Orogen (TNCO) have undergone localized lithospheric modification since the Cenozoic. The northern TNCO is highlighted by the Cenozoic magmatic activities including Hannuoba basalts and Datong volcanoes and is a seismically active region. In this study 3-D electrical conductivity model of the crust and uppermost mantle is derived by the 3-D inversion technique using data from 72 broadband magnetotelluric (MT) stations. The final model shows that a 15 km thick resistive layer of about 3000 Ω m dominates the upper crust, which may represent the intact Archean and Paleoproterozoic terrains. Whereas in the mid-crust there are marked high conductivity anomalies of about 10 Ω m beneath Shanxi rifting basin, which may result from the interconnected saline fluid of 0.2% to 6% volume fraction. The most important finding is that one significant conductor extended into the mantle is located between Hannuoba field and Datong volcanoes and it connects with the mid-crust conductor beneath the Datong volcanoes. We suggest that this could be the mantle source (partial melting region) for the Quaternary volcanic activities of Datong volcanoes and the melt fraction is estimated as 6.6%. Its location inside the Western Block suggests that the volcanic activities at Datong volcanoes are irrelevant to the tectonic process to the east of TNCO. It is likely to be related to the mantle flows from the Tibetan Plateau around the Ordos block which converges at the northeastern corner of the Ordos block and local upward flow along the slope of the thinning lithosphere resulted in decompression partial melting and the melt percolated upward through the crust to feed the lava eruptions at the Datong volcanoes to the east. Finally, large crustal earthquakes in this region are generally located in resistive zones with high conductivity anomalies beneath. The fluids brought up by hot mantle material may play an important role in fault strength weakening and rupture nucleation at mid-crust depths.

Geophysics of the Taupo Volcanic Zone: A review of recent developments

Since a previous review in 1995, improved geophysical networks and techniques have substantially improved our understanding of the Taupo Volcanic Zone (henceforth TVZ), and particularly why its central portion has such high outputs of magma and heat, much greater than found in similar arc environments elsewhere. Recent studies of the seismicity under New Zealand have revealed a concentration of intermediate-depth (150–220km) earthquakes immediately under this central portion of the TVZ. A low QP region above these earthquakes is interpreted as a high-temperature region of partial melt, supplying the active area above. The underlying reason for this zone of activity is still a matter of debate.Crustal seismic studies of the TVZ show that from Lake Taupo northwards, the seismicity is largely restricted to the top 8km of the crust, indicating a brittle–ductile transition at about this level. At the southern end of the zone, somewhat deeper seismicity becomes common. Recent detailed seismic studies covering the area from Taupo to Rotorua show that there are significant variations in seismic properties both across and along the strike of the TVZ.Continuous GPS measurements of deformation in the TVZ show that although the total deformation across the TVZ matches the extension expected from modelling of paleomagnetic data, there are large variations between different sites within the TVZ, with individual GPS sites displaying sudden jumps and periods of faster or slower change superimposed on the long-term trend. Some of these local movements may be due to cooling magma bodies.Magnetotelluric methods and 3-D modelling codes have significantly improved the resolution of deep electrical resistivity measurements. It is now possible to see conductivity anomalies to greater depths, to see the underlying feeding zones of the geothermal fields, as well as the conductivity anomalies in and immediately under the fields.Finally, recent modelling studies of the fluid convection under the geothermal fields of the TVZ indicate that they are driven by hot areas at least as deep as the brittle–ductile depth under the fields, rather than by a uniform heat source with the field spacing controlled by the shape of convection cells.

Prospects of applying diamond-like coatings for parts of friction units operating in a corrosion-abrasive medium

The results of studying the microstructure and microhardness of ring lugs on discs made of a steel–copper pseudoalloy after laser thermal treatment using a fiber laser with a power of 1 kW are given. It is revealed that the partial melting region in which melting occurs in the volumes of the low-melting component (copper) in the initial structure and contacting segments of steel matrix is formed in the material in addition to the total melting zone. Then the quenching zone from the solid state follows, in which the maximal hardness up to 1000 HV is attained for best samples in the volume of martensite, which is formed in perlite colonies of the initial steel–copper material.

Rayleigh wave dispersion measurements reveal low‐velocity zones beneath the new crust in the Gulf of California

AbstractRayleigh wave tomography provides images of the shallow mantle shear wave velocity structure beneath the Gulf of California. Low‐velocity zones (LVZs) are found on axis between 26 and 50 km depth beneath the Guaymas Basin but mostly off axis under the other rift basins, with the largest feature underlying the Ballenas Transform Fault. We interpret the broadly distributed LVZs as regions of partial melting in a solid mantle matrix. The pathway for melt migration and focusing is more complex than an axis‐centered source aligned above a deeper region of mantle melt and likely reflects the magmatic evolution of rift segments. We also consider the existence of solid lower continental crust in the Gulf north of the Guaymas Basin, where the association of the LVZs with asthenospheric upwelling suggests lateral flow assisted by a heat source. These results provide key constraints for numerical models of mantle upwelling and melt focusing in this young oblique rift.

Biotic vs. abiotic Earth: A model for mantle hydration and continental coverage

The origin and evolution of life has undoubtedly had a major impact on the evolution of Earth's oceans and atmosphere. Recent studies have suggested that bioactivity may have had an even deeper impact and may have caused a change in the redox-state of the mantle and provided a path for the formation of continents. We here present a numerical model that assumes that bioactivity increases the continental weathering rate and that relates the sedimentation rate to the growth of continents and to the hydration of the mantle using elements of plate tectonics and mantle convection. The link between these factors is provided by assuming that an increase of the thickness of the sedimentary layer of low permeability on top of a subducting oceanic slab will reduce its dewatering upon subduction. This in turn leads to a greater availability of water in the source region of andesitic partial melt, resulting in an enhanced rate of continental crust production, and to an increased regassing rate of the mantle. The mantle in turn responds by reducing the mantle viscosity while increasing the convective circulation rate, degassing rate and plate speed. We use parameters that are observed for the present Earth and gauge uncertain parameters such that the present day continental surface area and mantle water concentration can be obtained. Our steady state results show two stable fixed points in a phase plane defined by the fractional continental surface area and the water concentration in the mantle, one of them pertaining to a wet mantle and the continental surface area of the present day Earth, and the other to a dry mantle and a small continental surface area. When the sedimentation rate is reduced, both fixed points move and the area of attraction of the latter fixed point increases systematically. We conclude that if the presence of life has increased the continental weathering rate, as is widely believed, and led to the observables of a wet mantle and a continental surface coverage of roughly 40%, an abiotic Earth would likely have evolved toward a dry mantle with a small continental surface area instead.

Effect of dynamic melting on acoustic velocities in a partially molten peridotite

The mechanical effect of materials with a zero shear modulus aggregated with solids with non-zero shear moduli has been the primary motivation for why partially molten rocks should have lowered shear wave velocities. We propose that elastic softening associated with the process of melting must also be considered. In this view, seismic waves, propagating in regions of partial melting, perturb the thermodynamic state and drive solid to liquid and liquid to solid, thereby lowering the effective elastic moduli. We present measurements of Young’s modulus for a mantle peridotite, KLB1, at LVZ pressures and temperatures and seismic frequency that support this model. The model predicts that the softening of elastic modulus is controlled by the pressure dependence of melt fraction, ∂F/∂P, and not the percentage of melt present. In contrast to previous models, the P-wave velocity is decreased by nearly the same percentage as the S-wave velocity.

A Simulation Study on the Thermal Structure of Manila Trench Subduction Zone

AbstractNumerical simulation study of thermal structure is a supplement to field observations and an important method to verify geodynamics models. In this study, based on geological and geophysical conditions, we construct two thermal models across the East Volcano Chain (EVC) and the West Volcano Chain (WVC) along the Manila trench subduction zone and do the thermal simulations using finite‐element numerical method. During these simulations, we study the thermal structure of the subduction zones affected by the frictional and shear heating and simulate the thermal structure of the subduction zones beneath the seismic lines at EVC and WVC. We also employ the results of rock experiments to predict the regions of dehydration and partial melting in the subducting slab. The simulation results show that, at 100 km depth, the temperature of slab interface with frictional and shear heating is 865 °C, about 95 °C warmer than the temperature without frictional and shear heating. Considering the frictional and shear heating, the temperatures at the slab interface beneath the seismic lines of EVC and WVC are 865 °C and 895 °C respectively, and that at the base of subducting oceanic crust are 560 °C and 605 °C. At slab surface, the depth of incipient mineral dehydration is less than 50 km. Partial melting and dehydration may mainly occur at about 100 km depth, which is well associated with the observed location of the volcanic activity.