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Convective instabilities at various boundary layers in the earth’s mantle—including the core–mantle boundary, mantle transition zone and lithosphere-asthenosphere boundary— result in upwellings (mantle plumes) and downwellings (subducting slabs). While hotspot volcanism is traditionally linked to mantle plumes, their structure, origins, evolution, and death remain subjects of ongoing debate. Recent progress in seismic tomography has revealed a complex plumbing system connecting the core–mantle boundary and the surface. In particular, recent seismic imaging results suggest the presence of large-scale ponding zones between 660 km and ∼1000 km, associated with several mantle plumes around the globe. The broad upwellings originating from the CMB spread laterally beneath the 660 km seismic discontinuity, forming extensive ponding zones several thousand kilometers wide and extending up from an approximately 1000 km depth. Similar ponding zones are also observed for downwellings, with stagnant subducting slabs, within the 660–1000 km depth range. Here, we review evidence for wide ponding zones characterized by low seismic velocities and anomalous radial and azimuthal anisotropies in light of recent high-resolution regional studies below La Réunion Island in the Indian Ocean and below St Helena/Ascension in the southern Atlantic Ocean. We review and discuss possible interpretations of these structures, as well as possible mineralogical, geodynamic implications and outlook for further investigations aiming to improve our understanding of the mantle plumbing system.

The mantle transition zone represents an important layer in the interior of the Earth that is characterized by phase transformations of olivine polymorphs. Constraining the rheology difference between wadsleyite and ringwoodite is important in determining the viscosity contrast at a depth of 520 km. In this study, we perform a post-mortem by transmission electron microscopy of a wadsleyite + ringwoodite aggregate, deformed at high-pressure and high-temperature, in a deformation-DIA apparatus. From orientation maps acquired by scanning precession electron diffraction, we calculate local misorientations and misorientation-gradients, which are used as a proxy of plastic strain. We show that at 17.3 GPa, 1700 K, the plastic responses of wadsleyite and ringwoodite are comparable, although recovery by subgrain boundary migration is more easily activated in wadsleyite.

The causes and global distribution of intraplate volcanism remain poorly understood, particularly the occurrence of scattered magmatism unrelated to large igneous provinces (LIPs). In this study, high‐resolution numerical simulations are employed to examine the interaction between deep thermochemical mantle plumes and the mantle transition zone (MTZ) to clarify its role in plume ascent and surface magmatism. Results demonstrate that the MTZ exerts a significant control on plume behavior, with some plumes ascending directly while others stall and generate secondary upwellings (“baby plumes”), which may contribute to scattered, localized magmatism. The transition from direct ascent to stagnation of the primary (“parent”) thermochemical plume is influenced by temperature, plume volume, Clapeyron slopes, and compositional heterogeneities. Our results highlight the crucial role of the MTZ in how mantle plumes evolve and drive surface magmatism. This provides new insights into why some deep mantle plumes fail to generate LIPs, instead producing widely scattered volcanism.

Constraining the thermal and compositional state of the mantle is crucial for deciphering the formation and evolution of Mars. Mineral physics predicts that Mars’ deep mantle is demarcated by a seismic discontinuity arising from the pressureinduced phase transformation of the mineral olivine to its higher-pressure polymorphs, making the depth of this boundary sensitive to both mantle temperature and composition. Here, we report on the seismic detection of a midmantle discontinuity using the data collected by NASA’s InSight Mission to Mars that matches the expected depth and sharpness of the postolivine transition. In five teleseismic events, we observed triplicated P and S waves and constrained the depth of this discontinuity to be 1,006 ± 40 km by modeling the triplicated waveforms. From this depth range, we infer a mantle potential temperature of 1,605 ± 100 K, a result consistent with a crust that is 10 to 15 times more enriched in heat-producing elements than the underlying mantle. Our waveform fits to the data indicate a broad gradient across the boundary, implying that the Martian mantle is more enriched in iron compared to Earth. Through modeling of thermochemical evolution of Mars, we observe that only two out of the five proposed composition models are compatible with the observed boundary depth. Our geodynamic simulations suggest that the Martian mantle was relatively cold 4.5 Gyr ago (1,720 to 1,860 K) and are consistent with a present-day surface heat flow of 21 to 24 mW/m².

To understand the stability of hydrous phases in mafic oceanic crust under deep subduction conditions, high-pressure and high-temperature experiments were conducted on two hydrous basalts using a Kawai-type multi-anvil apparatus at 17-26 GPa and 800-1200°C. In contrast to previous studies on hydrous basalt that reported no hydrous phases in this pressure range, we found one or two hydrous phases in all run products at or below 1000°C. Three hydrous phases, including Fe-Ti oxyhydroxide, Al-rich phase D and Al-rich phase H, were present at the investigated P-T conditions. At T≤1000°C, Fe-Ti oxyhydroxide is stable at 17 GPa, Al-rich phase D is stable at 18-23 GPa, and Al-rich phase H is stable at 25-26 GPa. Our results, in combination with published data on the stability of hydrous phases at lower pressures, suggest that a continuous chain of hydrous phases may exist in subducting cold oceanic crust (≤1000°C): lawsonite (0-8 GPa), Fe-Ti oxyhydroxide (8-17 GPa), Al-rich phase D (18-23 GPa), and Al-rich phase H (>23 GPa). Therefore, in cold subduction zones, mafic oceanic crust, in addition to peridotite, may also carry a substantial amount of water into the mantle transition zone and the lower mantle.

Seismological estimates of the 400‐km and 670‐km mantle discontinuities (d400 and d670) are crucial for understanding the thermochemical structure and dynamics of the mantle transition zone (MTZ). However, artifacts from using ray theory and uncertainties in mantle velocity structure can affect topographic mapping of these boundaries. The artifacts are expected but have not been thoroughly explored. We focus on the mantle beneath North America, computing common‐conversion‐point (CCP) images using spectral‐element method synthetics for eight global and regional tomographic models. Our results reveal artificial undulations in d400 and d670 and variations in MTZ thickness arising from incorrect velocity corrections that are large compared to actual topography resolved using USArray data. Our synthetic experiments highlight that interpretation of d400 and d670 maps from CCP imaging and similar techniques should be done judiciously, due to substantial uncertainties introduced by velocity corrections.

Water in the mantle controls mantle convection and chemical transport processes within Earth's interior. However, whether the water in the lower mantle is primordial or recycled remains an open question. Here we report an electrical structure consisting of two stepped high‐conductivity anomalies crossing the 660‐km interface beneath northeastern Asia. The sheet‐like first‐step conductor locates in the mantle transition zone that can be best interpreted as hydrated stagnant Pacific slab, whereas the funnel‐like second‐step anomaly occurs in the uppermost lower mantle that is inferred to be a water reservoir replenished by the collapsed stagnant slab. The pivotal mechanism of this water transport process could be the dehydration reaction of dense hydrous magnesium silicates and the rehydration of stishovite and liebermannite in the transition zone. Our findings provide evidence for recycled lower‐mantle water reservoirs, and together with seismological and mineralogical studies, illustrate the deep‐mantle water plumbing system operated by cold slab subduction. Plain Language Summary Water plays a critical role in modulating the dynamics of Earth because it reduces mantle viscosity and favours extensive melting. Although the mantle transition zone is considered a barrier to the sinking of water into the lower mantle, subduction slabs are able to transfer water from the surface to the deep mantle down to the core‐mantle boundary. Here, we obtain a mantle electrical conductivity model converted from geomagnetic data in northeastern Asia. We identify two sheet‐like step‐wise conductive anomalies across the 660‐km discontinuity. We interpret the anomaly in the mantle transition zone as a water channel in terms of the hydrated stagnant Pacific slab, whereas the anomaly in the uppermost lower mantle is a water reservoir. We argue the dehydration of dense hydrous magnesium silicates and the rehydration of stishovite and liebermannite in the mantle transition zone are responsible for the water transport into the lower mantle across the mantle transition zone from the surface. Key Points An electrical conductivity model reveals a step‐like structure related with water crossing the 660‐km interface beneath northeastern Asia Water may store in the lower mantle mainly in the form of hydrated stishovite and liebermannite

To assess whether intraplate magmatism in eastern continental China records the subduction history of the Pacific plate, we plotted elemental compositions of the volcanic rocks in this region since the Early Cretaceous as a function of age. These rocks show distinct compositional transitions at ∼110 and ∼50 Ma, coinciding with the cessation of paleo‐Pacific plate subduction and initiation of present‐day western Pacific plate subduction, respectively. The dehydration of subducted oceanic slab in the mantle transition zone (MTZ) (410–660 km) governs key parameters such as overlying lithospheric thickness, melt extraction depth, partial melting extent, and asthenospheric mantle enrichment, ultimately controlling the compositions of mantle‐derived melts. Therefore, continental intraplate volcanism in regions with (present or past) subducted oceanic plates stagnant in the MTZ records the subduction history of adjacent oceanic plates.

We analyzed 49,592 teleseismic receiver functions (RFs) recorded by 278 CEArray stations to image the mantle transition zone (MTZ) beneath the South China Block to understand the origins of deep velocity anomalies and their potential links to subduction and intraplate volcanism. We employed a fast‐marching method and a high‐resolution 3‐D velocity model (FWEA18) derived from full waveform inversion in computing P‐to‐S conversion times to better image the 410‐ and 660‐km discontinuities. Our results indicate that the common‐conversion‐point stacking of RFs using 3‐D conversion times yielded better migration images of the two discontinuities. The images revealed a slightly depressed 410‐km with a few small uplifted patches, and showed that the 660‐km beneath the western Yangtze Craton is depressed by 10–25 km, which is likely caused by the stagnant Paleo‐Pacific slab. The 660‐km beneath the southern Cathaysia Block has a 5–15 km high plateau with a topographic low at its central part. The lateral dimension of the topographic low is ∼150 km and is located beneath the central Pearl River Mount Basin near Hong Kong. We speculate that the topographic low occurs within the Hainan plume with a temperature excess of ∼300–400 K and is caused by the garnet phase transition. The displaced deep plume enters the MTZ and spreads nearly horizontally at the base. The plume evolves into two channels with a minor one toward the northeast and a major one toward the southwest, which keep moving upward to the 410‐km. The southwest channel is likely the source that feeds the Hainan volcanoes. Plain Language Summary Using data from seismic stations in the South China Block, we investigated the mantle transition zone (MTZ) to understand the origins of deep velocity anomalies and their associations with subduction and intraplate volcanism. By applying advanced techniques and a ground‐truth reference model, we obtained clearer images of the 410‐ and 660‐km discontinuities. The images showed that the 410‐km discontinuity is slightly depressed with some small uplifted areas. Additionally, the 660‐km discontinuity beneath the western Yangtze Craton is depressed due to the presence of a stagnant slab from the ancient Pacific Ocean. In contrast, beneath the southern Cathaysia Block, the 660‐km discontinuity forms a high plateau with a central low area. This area located near Hong Kong may be related to a plume originating from the lower mantle. The plume, with elevated temperatures, enters the MTZ and spreads horizontally. It then evolves into two channels, with one moving toward the northeast and the other toward the southwest. The southwest channel likely supplies magma to the volcanoes in Hainan. These findings provide insights into the complex processes occurring deep within the Earth's mantle in the South China region. Key Points FWEA18 is used in migrating CEArray receiver functions to image the mantle transition zone beneath the South China Block The 660‐km is depressed ∼10–25 km by stagnant slabs beneath the northwestern part of the block The Hainan volcanoes are fed by a displaced lower mantle plume beneath the central Pearl River Mouth Basin

Great volumes of water are carried downward into the mantle transition zone (MTZ, 410–670 km depth) by subducting slabs. If this water is later drawn upward, the resulting mantle melting may generate continental intraplate volcanism (IPV). Despite water's importance, its amount and spatial distribution within the MTZ, and its impact on IPV, are poorly constrained. Here we use plate tectonic reconstructions to estimate the rates and positions of water injection into the MTZ by subducted slabs during the past 400 Myr. This allows us to construct global maps of heterogeneous MTZ hydration, which we then compare to IPV eruption locations from the past 200 Myr. We detect a statistically significant correlation between wet MTZ regions and IPV locations at the surface, but only if slabs sink faster than 1 cm/yr, water remains stored in the MTZ for periods of 30–100 Myr, and IPV eruptions occur 10–30 Myr later. We find that 42%–68% of continental IPV is underlain by wet MTZ, with greater fractions associated with longer MTZ residence time. Hydrous underpinning of continental IPV was highest during the Jurassic, when more extensive slab interaction with the MTZ hydrated a wider area of the MTZ. Since the Cretaceous, continents have been moving over the wet MTZ, increasing IPV possibilities. MTZ regions near the northern Pacific, southern Africa, and western Europe have remained dry by avoiding wet slabs. We suggest that subducted water shapes global patterns of intraplate volcanism, with hydrous upwellings rising from the MTZ to generate continental IPV above wet MTZ regions. Plain Language Summary Minerals within the Earth's interior may hold several oceans of water. Most of this water is stored within the mantle transition zone (MTZ), a layer that lies between 410 and 670 km in depth. It is carried there by subducted “slabs,” which are tectonic plates that have descended into the mantle. We used reconstructions of past plate motions to determine the locations and rates of water transport into the MTZ by slabs during the past 400 million years. This exercise allows us to construct maps of water storage within Earth's MTZ. These maps suggest that more than a third of the MTZ is likely to be hydrated today, and even greater areas were hydrated in the past. We also found that “intraplate” volcanism, which erupts away from tectonic plate boundaries, tends to preferentially occur above these “wet” areas of the MTZ, especially if water remains in the MTZ for long periods of time. Based on this correlation, we suggest that MTZ hydration exerts an important control on global patterns of intraplate volcanism. This occurs via hydrous upwellings that carry water and heat upward from the MTZ. Near the surface, these upwellings increase the tendency of rocks to melt and form magma that can erupt. Key Points We use tectonic reconstructions of subduction history to map the hydration state of the mantle transition zone (MTZ) for the past 400 Myr We identify a statistically significant correlation between hydrated MTZ and continental intraplate volcanism (IPV) on Earth's surface Hydrated MTZ can explain IPV if subducted water stalls in the MTZ for ∼100 Myr and hydrous upwelling induces sub‐lithospheric melting