660-km Discontinuity Research Articles

Slab stagnation vs. penetration of Nazca subduction inferred from shear wave reflectivity

The morphology and composition of subducted slabs in the upper mantle transition zone (400 to 700 km depth) have direct implications for the style of mantle convection. Here we investigate the seismic reflectivity structure at transition zone depths beneath South America by combining the arrival times and amplitudes of an updated data set of SS precursors. We observe broad regions with up to 20 km depressed 410- and 660-km discontinuities and diminished precursor amplitudes beneath the back-arc region of Nazca subduction, which are incompatible with temperature-dominated mineral phase boundary perturbations. A probabilistic inversion is applied to simultaneously determine mantle temperature and composition, and the outcomes suggest a mechanically mixed transition zone with basalt enrichment beneath the Amazon basin. This is corroborated by a novel amplitude-versus-offset inversion that measures the changes in density and shear velocity across the discontinuities. Our observations offer direct evidence for thermochemical anomalies within the mantle, which are largely associated with temporary slab stagnation. The slab-transition zone interactions suggest that the Nazca-South American convergence may represent a later-stage analog to the Pacific plate subduction in northeast Asia.

Change in carbonate budget and composition during subduction below metal saturation boundary

It is generally accepted that carbonates can be subducted to the mantle depths, where they are reduced with iron metal to produce a diamond. In this work, we found that this is not always the case. The mantle carbonates from inclusions in diamonds show a wide range of cation compositions (Mg, Fe, Ca, Na, and K). Here we studied the reaction kinetics of these carbonates with iron metal at 6–6.5 GPa and 1000–1500 °C. We found that the reduction of carbonate with Fe produces C-bearing species (Fe, Fe-C melt, Fe3C, Fe7C3, C) and wüstite containing Na2O, CaO, and MgO. The reaction rate constants (k = Δx2/2t) are log-linear relative to 1/T and their temperature dependences are determined to bekMgCO3 (m2/s) = 4.37 × 10−3 exp [−251 (kJ/mol)/RT]kCaMg(CO3)2 (m2/s) = 1.48 × 10−3 exp [−264 (kJ/mol)/RT]kCaCO3 (m2/s) = 3.06 × 10−5 exp [−245 (kJ/mol)/RT] andkNa2CO3 (m2/s) = 1.88 × 10−10 exp [−155 (kJ/mol)/RT].According to obtained results at least, 45–70 vol% of carbonates preserve during subduction down to the 660-km discontinuity if no melting occurs. The slab stagnation and warming, subsequent carbonate melting, and infiltration into the mantle saturated with iron metal are accompanied by a reduction of carbonate melt with Fe. The established sequence of reactivity of carbonates: FeCO3 ≥ MgCO3 > CaMg(CO3)2 > CaCO3 ≫ Na2CO3, where K2CO3 does not react at all with iron metal, implies that during reduction carbonate melt with Fe evolves toward alkali-rich. The above conclusions are consistent with the findings of carbonates in inclusions in diamonds from the lower mantle and high concentrations of alkalis, particularly K, in mantle carbonatite melts entrapped by diamonds from kimberlites and placers worldwide.

Influence of shear wave velocity heterogeneity on SH-wave reverberation imaging of the mantle transition zone

SUMMARY Long-period (T > 10 s) shear wave reflections between the surface and reflecting boundaries below seismic stations are useful for studying phase transitions in the mantle transition zone (MTZ) but shear-velocity heterogeneity and finite-frequency effects complicate the interpretation of waveform stacks. We follow up on a recent study by Shearer & Buehler (hereafter SB19) of the top-side shear wave reflection Ssds as a probe for mapping the depths of the 410-km and 660-km discontinuities beneath the USArray. Like SB19, we observe that the recorded Ss410s-S and Ss660s-S traveltime differences are longer at stations in the western United States than in the central-eastern United States. The 410-km and 660-km discontinuities are about 40–50 km deeper beneath the western United States than the central-eastern United States if Ss410s-S and Ss660s-S traveltime differences are transformed to depth using a common-reflection point (CRP) mapping approach based on a 1-D seismic model (PREM in our case). However, the east-to-west deepening of the MTZ disappears in the CRP image if we account for 3-D shear wave velocity variations in the mantle according to global tomography. In addition, from spectral-element method synthetics, we find that ray theory overpredicts the traveltime delays of the reverberations. Undulations of the 410-km and 660-km discontinuities are underestimated when their wavelengths are smaller than the Fresnel zones of the wave reverberations in the MTZ. Therefore, modelling of layering in the upper mantle must be based on 3-D reference structures and accurate calculations of reverberation traveltimes.

Eastward subduction of the Indian plate beneath the Indo-Myanmese arc revealed by teleseismic P-wave tomography

The deep structure of the eastward-subducting Indian plate can provide new information on the dynamics of the India-Eurasia collision. We collected and processed waveform data from temporary seismic arrays (networks) on the eastern Tibetan Plateau, seismic arrays in Northeast India and Myanmar, and permanent stations of the China Digital Seismic Network in Tibet, Gansu, Qinghai, Yunnan, and Sichuan. We combined these data with phase reports from observation stations of the International Seismological Center on the Indian plate and selected 124,808 high-quality P-wave relative travel-time residuals. Next, we used these data to invert the 3-D P-wave velocity structure of the upper mantle to a depth of 800 km beneath the eastern segment of the arcuate Himalayan orogen, at the southeastern margin of the Tibetan Plateau. The results reveal a high-angle, easterly dipping subducting plate extending more than 200 km beneath the Indo-Myanmese arc. The plate breaks off at roughly 96°E; its fragments have passed through the 410-km discontinuity (D410) into the mantle transition zone (MTZ). The MTZ beneath the Tengchong volcanic area contains a high-velocity anomaly, which does not exceed the Red River fault to the east. No other large-scale continuous subducted plates were observed in the MTZ. However, a horizontally spreading high-velocity anomaly was identified on the D410 in some regions. The anomaly may represent the negatively buoyant 90°E Ridge plate or a thickened and delaminated lithospheric block experiencing collision and compression at the southeastern margin of the Tibetan Plateau. The Tengchong volcano may originate from the mantle upwelling through the slab window formed by the break-off of the subducting Indian continental plate and oceanic plate in the upper mantle. Low-velocity upper mantle materials on the west side of the Indo-Myanmese arc may have supplemented materials to the Tengchong volcano.

Shear wave velocities across the olivine – wadsleyite – ringwoodite transitions and sharpness of the 410 km seismic discontinuity

The seismic signature of the 410-km seismic discontinuity is generally attributed to the olivine to wadsleyite polymorphic transformation. However, apparent discrepancies exist between seismic and experimental observations. Among those, the sharpness of the discontinuity as inferred from the reflectivity of seismic waves is difficult to reconcile with the gradual nature of the olivine to wadsleyite transformation predicted by phase equilibria. In this study, we explore the contribution of the phase transition kinetics to the sharpness of the discontinuity by performing X-ray diffraction and sound velocity measurements on (Mg,Fe)2SiO4 with an unprecedented time resolution as a function of the reaction progress. Our data document for the first time a transient velocity softening phenomenon and attenuation which we relate to the formation of a metastable spineloid phase. In the Earth's mantle this transformation mechanism would affect the elastic gradient within the olivine-wadsleyite two-phase loop, potentially creating a low-velocity layer; hence explaining the unique sharpness and reflectivity of the discontinuity.

Roles of partial hydration of mantle transition zone and wadsleyite–olivine phase transition in wet plume development: Origin of Quaternary intraplate volcanoes in Korea

The Quaternary intraplate volcanoes in Korea have a sparser distribution (~330–563 km) than that of Japanese arc volcanoes (~80 km), geochemistry indicating mixing of components from the stagnant slab and mantle transition zone, and a longer elapsed time (~10–20 Myr) of the Quaternary eruption after the stagnation of the Pacific Plate. Although volcanism can be explained by wet plumes originating from the hydrous layer of the stagnant slab, previous numerical studies could not consistently explain the spatiotemporal and geochemical observations because of the neglected partial hydration of the mantle transition zone and wadsleyite–olivine phase transition at the 410-km discontinuity, both of which affect the viscosity and buoyancy structures. Thus, we constructed a series of two-dimensional numerical models by considering the partial hydration of the mantle transition zone and phase transition at the 410-km discontinuity. The results showed that the wet plumes are retarded at the 410-km discontinuity by the phase transition and merge into batches forming hybrid plumes in the asthenosphere, explaining the mixing of components from the hydrous slab and mantle transition zone, evidenced by the geochemical observations. The spacing (~485 km) and elapsed time (~24 Myr) of the hybrid plumes beneath the lithosphere are consistent with the spatiotemporal observations. Furthermore, the degree of mixing of the components from the hydrous slab and mantle transition zone found in the hybrid plumes explains the spatial distribution of water in the mantle transition zone beneath Korea.

Imprints of subducted Palaeo-Tethys oceanic lithosphere on upper-mantle discontinuities and the formation of the Emeishan large igneous province

SUMMARYGenerally, the upwelling of a mantle plume is suggested to have led to the formation of the Emeishan large igneous province (ELIP). However, this idea has been challenged by recent geological and seismological studies. In this investigation, three profiles of velocity perturbations crossing the ELIP are drawn following previous tomographic studies, and common conversion point (CCP) stacking of receiver functions in the ELIP is carried out. A slab-like high-velocity structure is revealed, which may be associated with the lithosphere of the Palaeo-Tethys Ocean (LPTO) subducting toward the northeast. CCP stacking of receiver functions shows that the LPTO generated imprints on the 410- and 660-km discontinuities. A comprehensive analysis suggests that the LPTO may have induced large-scale mantle upwelling, which contributed to the formation of the ELIP.

The effects of discontinuity topography in the mantle transition zone on global geodynamic observables and mantle heterogeneity

SUMMARYDespite progress in tomographic imaging of Earth's interior, a number of critical questions regarding the large-scale structure and dynamics of the mantle remain outstanding. One of those questions is the impact of phase-boundary undulations on global imaging of mantle heterogeneity and on geodynamic (i.e. convection-related) observables. To address this issue, we developed a joint seismic-geodynamic-mineral physical tomographic inversion procedure that incorporates lateral variations in the depths of the 410- and 660-km discontinuities. This inversion includes S-wave traveltimes, SS precursors that are sensitive to transition-zone topography, geodynamic observables/data (free-air gravity, dynamic surface topography, horizontal divergence of tectonic plates and excess core-mantle boundary ellipticity) and mineral physical constraints on thermal heterogeneity. Compared to joint tomography models that do not include data sensitivity to phase-boundary undulations in the transition zone, the inclusion of 410- and 660-km topography strongly influences the inference of volumetric anomalies in a depth interval that encompasses the transition zone and mid-mantle. It is notable that joint tomography inversions, which include constraints on transition-zone discontinuity topography by seismic and geodynamic data, yield more pronounced density anomalies associated with subduction zones and hotspots. We also find that the inclusion of 410- and 660-km topography may improve the fit to the geodynamic observables, depending on the weights applied to seismic and geodynamic data in the inversions. As a consequence, we find that the amplitude of non-thermal density anomalies required to explain the geodynamic data decreases in most of the mantle. These findings underline the sensitivity of the joint inversions to the inclusion of transition-zone complexity (e.g. phase-boundary topography) and the implications for the inferred non-thermal density anomalies in these depth regions. Finally, we underline that our inferences of 410- and 660-km topography avoid a commonly employed approximation that represents the contribution of volumetric heterogeneity to SS-wave precursor data. Our results suggest that this previously employed correction, based on a priori estimates of upper-mantle heterogeneity, might be a significant source of error in estimating the 410- and 660-km topography.

Depressed 660-km discontinuity caused by akimotoite\u2013bridgmanite transition

The 660-kilometre seismic discontinuity is the boundary between the Earth’s lower mantle and transition zone and is commonly interpreted as being due to the dissociation of ringwoodite to bridgmanite plus ferropericlase (post-spinel transition)1–3. A distinct feature of the 660-kilometre discontinuity is its depression to 750 kilometres beneath subduction zones4–10. However, in situ X-ray diffraction studies using multi-anvil techniques have demonstrated negative but gentle Clapeyron slopes (that is, the ratio between pressure and temperature changes) of the post-spinel transition that do not allow a significant depression11–13. On the other hand, conventional high-pressure experiments face difficulties in accurate phase identification due to inevitable pressure changes during heating and the persistent presence of metastable phases1,3. Here we determine the post-spinel and akimotoite–bridgmanite transition boundaries by multi-anvil experiments using in situ X-ray diffraction, with the boundaries strictly based on the definition of phase equilibrium. The post-spinel boundary has almost no temperature dependence, whereas the akimotoite–bridgmanite transition has a very steep negative boundary slope at temperatures lower than ambient mantle geotherms. The large depressions of the 660-kilometre discontinuity in cold subduction zones are thus interpreted as the akimotoite–bridgmanite transition. The steep negative boundary of the akimotoite–bridgmanite transition will cause slab stagnation (a stalling of the slab’s descent) due to significant upward buoyancy14,15.

Receiver function mapping of the mantle transition zone beneath the Western Alps: New constraints on slab subduction and mantle upwelling

To better constrain the deep structure and dynamics of the Western Alps, we studied the mantle transition zone (MTZ) structure using P-wave receiver functions (RFs). We obtained a total of 24904 RFs from 1182 events collected by 307 stations in the Western Alps. To illustrate the influence of the heterogeneity on the upper mantle velocity, we used both IASP91 and three-dimensional (3-D) velocity models to perform RF time-to-depth migration. We documented an MTZ thickening of about 40 km under the Western Alps and most of the Po Plain due to the uplift associated with the 410-km discontinuity and the depression associated with the 660-km discontinuity. Based upon the close spatial connection between the thickened MTZ and the location of the subducted slabs, we proposed that the thick MTZ was due to the subduction of the Alpine slab through the upper MTZ and the presence of remnants of subducted oceanic lithosphere in the MTZ. The uplift associated with the 410-km discontinuity provided independent evidence of the subduction depth of the Western Alps slab. In the Alpine foreland in eastern France, we observed localized arc-shaped thinning of the MTZ caused by a 12 km depression of the 410-km discontinuity, which has not been previously reported. This depression indicated a temperature increase of 120 K in the upper MTZ, and we proposed that it was caused by a small-scale mantle upwelling. Hardly any uplift of the 660 km discontinuity was observed, suggesting that the thermal anomaly was unlikely to be the result of a mantle plume. We observed that the thinning area of the MTZ corresponded to the area with the highest uplift rate in the Western Alps, which may have indicated that the temperature increase caused by the mantle upwelling contributed to the topographic uplift.

A poorly mixed mantle transition zone and its thermal state inferred from seismic waves

The abrupt changes in mineralogical properties across the Earth’s mantle transition zone substantially impact convection and thermochemical fluxes between the upper and lower mantle. While the 410-km discontinuity at the top of the mantle transition zone is detected with all types of seismic waves, the 660-km boundary is mostly invisible to underside P-wave reflections (P660P). The cause for this observation is debated. The dissociation of ringwoodite and garnet into lower-mantle minerals both contribute to the ‘660’ visibility; only the garnet reaction favours material exchanges across the discontinuity. Here, we combine large datasets of SS and PP precursors, mineralogical modelling and data-mining techniques to obtain a global thermal map of the mantle transition zone, and explain the lack of P660P visibility. We find that its prevalent absence requires a chemically unequilibrated mantle, and its visibility in few locations is associated with potential temperatures greater than 1,800 K. Such temperatures occur in approximately 0.6% of Earth, indicating that the 660 is dominated by ringwoodite decomposition, which tends to impede mantle flow. We find broad regions with elevated temperatures beneath the Pacific surrounded by major volcanic hotspots, indicating plume retention and ponding of hot materials in the mantle transition zone. The mantle transition zone is poorly, mechanically mixed, and acts to impede mantle flow, according to seismic observations integrated with detailed mineral-physics models.

Depressed 660-km discontinuity caused by akimotoite-bridgmanite transition

Depressed 660-km discontinuity caused by akimotoite-bridgmanite transition

Constraining the 410-km discontinuity and slab structure in the Kuril subduction zone with triplication waveforms

SUMMARYThe detailed structure near the 410-km discontinuity provides key constraints of the dynamic interactions between the upper mantle and the lower mantle through the mantle transition zone (MTZ) via mass and heat exchange. Meanwhile, the temperature of the subducting slab, which can be derived from its fast wave speed perturbation, is critical for understanding the mantle dynamics in subduction zones where the slab enters the MTZ. Multipathing, i.e. triplicated, body waves that bottom near the MTZ carry rich information of the 410-km discontinuity structure and can be used to constrain the discontinuity depth and radial variations of wave speeds across it. In this study, we systematically analysed the trade-off between model parameters in triplication studies using synthetic examples. Specifically, we illustrated the necessity of using array-normalized amplitude. Two 1-D depth profiles of the wave speed below the Tatar Strait of Russia in the Kuril subduction zone are obtained. We have observed triplications due to both the 410-km discontinuity and the slab upper surface. And, seismic structures for these two interfaces are simultaneously inverted. Our derived 410-km discontinuity depths for the northern and southern regions are at 420$\pm $15 and 425$\pm $15 km, respectively, with no observable uplift. The slab upper surface is inverted to be located about 50–70 km below the 410-km discontinuity. This location is between the depths of the 1 and 2 per cent P-wave speed perturbation contours of a regional 3-D full-waveform inversion (FWI) model, but we found twice the wave speed perturbation amplitude. A wave speed increase of 3.9–4.6 per cent within the slab, compared to 2.0–2.4 per cent from the 3-D FWI model, is necessary to fit the waveforms with the shortest period of 2 s, indicating that high-frequency waves are required to accurately resolve the detailed structures near the MTZ.

FastTrip: A Fast MPI-Accelerated 1D Triplication Waveform Inversion Package for Constraining Mantle Transition Zone Discontinuities

AbstractThe 410- and 660-km discontinuities define the top and bottom of the mantle transition zone (MTZ). The properties of these mineralogical phase transformation interfaces provide critical constraints on the dynamics, temperature, and composition of the MTZ. Triplicated body waves that bottom near these discontinuities carry rich information about them. To streamline the modeling of upper-mantle triplications recorded at regional distances (13°–30°), we have developed a (Fast) Message Passing Interface (MPI)-accelerated 1D (Tr)iplication Waveform (I)nversion (P)ackage (FastTrip). With triplication waveform data as input, FastTrip uses a global search method to output a set of acceptable 1D velocity models. Quantitative estimation of the model uncertainties can be further derived based on the range of acceptable models. FastTrip supports central processing unit (CPU) parallel acceleration (15,000 models within 2 hr with 100 CPUs) and is portable to other inversion problems that can be described by a relatively small number of model parameters.

Segregated oceanic crust trapped at the bottom mantle transition zone revealed from ambient noise interferometry

The recycling of oceanic crust, with distinct isotopic and chemical signature from the pyrolite mantle, plays a critical role in the chemical evolution of the Earth with insights into mantle circulation. However, the role of the mantle transition zone during this recycling remains ambiguous. We here combine the unique resolution reflected body waves (P410P and P660P) retrieved from ambient noise interferometry with mineral physics modeling, to shed new light on transition zone physics. Our joint analysis reveals a generally sharp 660-km discontinuity and the existence of a localized accumulation of oceanic crust at the bottom mantle transition zone just ahead of the stagnant Pacific slab. The basalt accumulation is plausibly derived from the segregation of oceanic crust and depleted mantle of the adjacent stagnant slab. Our findings provide direct evidence of segregated oceanic crust trapped within the mantle transition zone and new insights into imperfect whole mantle circulation.

Geomagnetic evidence for a continuously connected plume conduit extending to at least the 660-km discontinuity below Hawaii

Do plumes exist? If so, do plume tails connect hotspots to the core-mantle boundary (CMB), or only to the 660-km discontinuity (between the upper and the lower mantle)? Are they continuous or fragmented? Here, we demonstrate quantitatively that the Hawaiian plume distorts 24-h-period magnetovariational data by providing a continuous electrical connection between the Earth's surface (via the ocean) and the 660-km mid-mantle discontinuity. We extend our analysis to predict how a plume ascending from the CMB should distort magnetic secular variation observed locally at the surface. Hence, the Hawaiian plume is demonstrated to be a continuous feature with a deep-seated (at least mid-mantle) source. Our analysis provides a novel technique for determining whether the Hawaiian plume originates at the mid-mantle transition zone or CMB subject to magnetic secular variation data being acquired locally, as well as allowing the depth of genesis of other mantle plumes to be constrained should further geomagnetic data become available in their vicinity.

3-D and 5-D reconstruction ofPreceiver functions via multichannel singular spectrum analysis

SUMMARYThe receiver function (RF) method is fundamental in assessing mantle seismic discontinuity depths and reflectivities. Most of the current approaches rely on phase equalization, though in many applications, high levels of incoherent noise, incomplete and irregular sampling customarily interfere with the analysis of weak secondary phases. In recent years, advancements in the field of multidimensional seismic data processing have triggered a shift in interest towards its application to RFs, specifically to single station gathers that depend on a single spatial dimension. Our work generalizes the application of singular spectrum analysis to RFs that rely on two and four spatial dimensions recorded by dense seismic arrays. We adopt a multidimensional signal processing approach known as multichannel singular spectrum analysis. We develop a strategy to assemble and enhance 3-D and 5-D seismic volumes via matrix rank reduction and a reinsertion algorithm to simultaneously suppress random noise, retrieve absent observations and boost identifiability of secondary conversions. We provide informative synthetic examples to gain insight into the effectiveness and limitations of our approach. In the real data example, we improve weak conversions from the mantle transition zone (MTZ) recorded by the USArray in the Yellowstone area. The reconstruction algorithm accurately recovers the timing and polarity of conversions associated with the 410-, 520- and 660-km seismic discontinuities. Our investigation shows that the simultaneous processing of several spatial variables expedites signal restoration, particularly in directions where large recording gaps exist due to a lack of earthquakes, which aids the mapping and identification of the MTZ interfaces. This study presents a theoretical/practical framework for the reconstruction of multidimensional RF data, and its full potential can be exploited with dense acquisition available from the emerging seismic nodal arrays to improve passive seismic imaging.

Pervasive low-velocity layer atop the 410-km discontinuity beneath the northwest Pacific subduction zone: Implications for rheology and geodynamics

Regional triplication waveforms of five intermediate-depth events are modeled to simultaneously obtain the compressional (P) and shear (SH) wave velocity structure beneath northwestern Pacific subduction zone. Both the P- and SH-wave velocity models for three different sub-regions show a low-velocity layer (LVL) with a thickness of ∼55-80 km lying above the 410-km discontinuity with a ∼900 km lateral extent from the Japan Sea to the northeastern Asian continental margin. With the dihedral angle approaching to zero around 400 km, a minute amount of melt atop the 410-km discontinuity caused by the hydrous slab might completely wet olivine grain boundaries and result in a low seismic velocity layer in this specific subduction zone. This mechanism suggests that the 410-LVL is a low viscosity zone that would partially decouple the upper mantle from the transition zone. We infer that the widespread 410-LVL provides evidence for a water-bearing mantle transition zone beneath the western Pacific subduction zone.

Elasticity of hydrous ringwoodite at mantle conditions: Implication for water distribution in the lowermost mantle transition zone

The mantle transition zone (MTZ) is potentially a geochemical water reservoir because of the high H2O solubility in its dominant minerals, wadsleyite and ringwoodite. Whether the MTZ is wet or dry fundamentally impacts our understanding of the deep-water distribution, geochemical recycling, and the pattern of mantle convection. However, the water content in the MTZ inferred from previous studies remains disputed. Seismic observations such as velocity anomalies were used to evaluate the water content in the MTZ, but the hydration effect on the velocities of MTZ minerals under appropriate pressure (P) and temperature (T) conditions is poorly constrained. Here we investigated the elastic properties and velocities of hydrous ringwoodite at high P-T conditions using first-principles calculations. Our results show that the hydration effects on elastic moduli and velocities of ringwoodite are significantly reduced by pressure but strongly enhanced by temperature. The incorporation of 1.0 wt% water into ringwoodite decreases the compressional and shear velocities of the pyrolitic mantle by −1.0% and −1.4% at the conditions of MTZ, respectively. Using results from seismic tomography and together with the topography of the 660-km discontinuity, we evaluate the global distribution of water in the lower MTZ. We find that about 80% of the MTZ can be explained by varying water content and temperature, however, the remaining 20% requires the presence of high-velocity heterogeneities such as harzburgite. Our models suggest an average water concentration of ∼0.2 wt% in the lower MTZ, with an interregional variation from 0 to 0.9 wt%. Together with our previous work, we conclude that the water concentration in the MTZ likely decreases with depth globally and the whole MTZ contains the equivalent of about one ocean mass of water.

Global variations of Earth's 520- and 560-km discontinuities

We investigate seismic discontinuities in the mantle transition zone (MTZ) by analyzing SS precursors recorded at global seismic stations. Our observations confirm the global existence of the 520-km discontinuity. Although substantial regional depth variations in the 520-km discontinuity are generally correlated with temperature in the mid-MTZ, they cannot be fully explained by the Clapeyron slope of the wadsleyite-ringwoodite phase transition, suggesting both thermal and compositional heterogeneities in the MTZ. A second discontinuity at ∼560-km depth, previously interpreted as splitting of the 520-km discontinuity, is most commonly detected in cold subduction zones and hot mantle regions. The depth separation between the 520- and 560-km discontinuities varies from ∼80 km in cold regions to ∼40 km in hot areas. The exsolution of calcium-perovskite (Ca-pv) from majorite garnet has been proposed to explain the velocity and density changes across the 560-km discontinuity. However, the gradual exsolution of perovskite and partitioning of Ca and Al between perovskite and garnet appear inconsistent with the relatively “sharp” discontinuity in seismic observations and thus need to be revisited in the future. Nevertheless, because the only known transition in major minerals at this depth in the MTZ is the formation of Ca-pv, the existence of the 560-km discontinuity may imply localized high calcium concentrations in the mid-MTZ possibly related to the recycling of oceanic crust.