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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.

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.

The high water storage capacity of minerals in Earth's mantle transition zone (410- to 660-kilometer depth) implies the possibility of a deep H2O reservoir, which could cause dehydration melting of vertically flowing mantle. We examined the effects of downwelling from the transition zone into the lower mantle with high-pressure laboratory experiments, numerical modeling, and seismic P-to-S conversions recorded by a dense seismic array in North America. In experiments, the transition of hydrous ringwoodite to perovskite and (Mg,Fe)O produces intergranular melt. Detections of abrupt decreases in seismic velocity where downwelling mantle is inferred are consistent with partial melt below 660 kilometers. These results suggest hydration of a large region of the transition zone and that dehydration melting may act to trap H2O in the transition zone.

We report the first natural occurrence and single-crystal X-ray diffraction study of the Fe-analog of wadsleyite [a = 5.7485(4), b = 11.5761(9), c = 8.3630(7) Å, V = 556.52(7) Å3; space group Imma], spinelloid-structured Fe2SiO4, a missing phase among the predicted high-pressure polymorphs of ferroan olivine, with the composition (Fe1.102+Mg0.80Cr0.043+Mn0.022+Ca0.02Al0.02Na0.01)# 1S2.01(Si0. 97Al0.03)Σ1.00O4. The new mineral was approved by the International Mineralogical Association (No. 2018-102) and named asimowite in honor of Paul D. Asimow, the Eleanor and John R. McMillan Professor of Geology and Geochemistry at the California Institute of Technology. It was discovered in rare shock-melted silicate droplets embedded in Fe,Ni-metal in both the Suizhou L6 chondrite and the Quebrada Chimborazo (QC) 001 CB3.0 chondrite. Asimowite is rare, but the shock-melted silicate droplets are very frequent in both meteorites, and most of them contain Fe-rich wadsleyite (Fa30-45). Although the existence of such Fe-rich wadsleyite in shock veins may be due to the kinetic reasons, new theoretical and experimental studies of the stability of (Fe,Mg)2SiO4 at high temperature (>1800 K) and pressure are clearly needed. This may also have a significant impact on the temperature and chemical estimates of the mantle's transition zone in Earth.

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.

A biogeographical regionalization is a hierarchical system that categorizes geographical areas in terms of their biotas. I provide a general protocol to undertake biogeographical regionalizations, that consists of seven steps: (1) defining the study area; (2) assembling distributional data; (3) identifying natural areas; (4) discovering area relationships; (5) defining boundaries/transition zones; (6) regionalization and (7) area nomenclature. Natural biogeographical units are useful for people undertaking different types of analyses, like macroecologists, evolutionary biologists, systematists and conservationists. Biogeographical regionalizations may help biogeographers communicate more effectively between themselves and discover opportunities to work on common problems, contributing to the development of a truly integrative biogeography.

The recent cataloging of global microseisms benefits imaging of the Earth's interior with improved data coverage. Relying on the catalog, converted waves were extracted and employed to reveal variations of the mantle discontinuities beneath Japan. Here, we extract reflected body waves with merely 1 month of continuous recordings, leveraging the abundant microseisms in the southern hemisphere. We convert the extracted reflections to depth and find consistent features with previous studies, such as a depressed 660 km discontinuity due to the subducting Pacific slab. Moreover, we observe strong reflections associated with an elevated 410 km discontinuity westward of the slab, a feature predicted from thermo‐chemical models but not imaged previously. This work probes the complementary potentials of reflected waves from global microseisms and provides new tools for future investigation of the Earth's interior, especially in temporary deployments during which usable earthquakes as sources are often scarce.

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

Aim To define the major biogeographical regions and transition zones for freshwater fish species. Taxon Strictly freshwater species of actinopterygian fish (i.e. excluding marine and amphidromous fish families). Methods We based our bioregionalization on a global database of freshwater fish species occurrences in drainage basins, which, after filtering, includes 11,295 species in 2,581 basins. On the basis of this dataset, we generated a bipartite (basin‐species) network upon which we applied a hierarchical clustering algorithm (the Map Equation) to detect regions. We tested the robustness of regions with a sensitivity analysis. We identified transition zones between major regions with the participation coefficient, indicating the degree to which a basin has species from multiple regions. Results Our bioregionalization scheme showed two major supercontinental regions (Old World and New World, 50% species of the world and 99.96% endemics each). Nested within these two supercontinental regions lie six major regions (Nearctic, Neotropical, Palearctic, Ethiopian, Sino‐Oriental and Australian) with extremely high degrees of endemism (above 96% except for the Palearctic). Transition zones between regions were of limited extent compared to other groups of organisms. We identified numerous subregions with high diversity and endemism in tropical areas (e.g. Neotropical), and a few large subregions with low diversity and endemism at high latitudes (e.g. Palearctic). Main conclusions Our results suggest that regions of freshwater fish species were shaped by events of vicariance and geodispersal which were similar to other groups, but with freshwater‐specific processes of isolation that led to extremely high degrees of endemism (far exceeding endemism rates of other continental vertebrates), specific boundary locations and limited extents of transition zones. The identified bioregions and transition zones of freshwater fish species reflect the strong isolation of freshwater fish faunas for the past 10–20 million years. The extremely high endemism and diversity of freshwater fish fauna raises many questions about the biogeographical consequences of current introductions and extinctions.

The observed 2%–7% low‐shear velocity (VS) anomalies near the subducted slab at the bottom mantle transition zone (MTZ) indicate strong lateral heterogeneity, which is commonly attributed to subducted oceanic crust. However, davemaoite, a major constituent of the subducted oceanic crust, has been poorly constrained in its elasticity, hindering accurate velocity modeling and obscuring the origin of these low‐velocity features. Here we report single‐crystal elasticity of Ti‐bearing davemaoite with the composition of Ca(Si0.57Ti0.43)O3 under high pressure‐temperature and found that Ti incorporation significantly reduces velocities and alters the pressure dependence of the shear modulus. Further velocity modeling demonstrated that subducted crusts with varying Ti content have seismic signatures of 1.7(2)–6.8(5)% low‐VS at the bottom MTZ, consistent with the observed low‐VS structure in the region. These findings highlight the role of slab‐derived chemical heterogeneity in generating mantle seismic anomalies and provide new experimental constraints on the structure and dynamics of the deep Earth.