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The origins and composition of seismic low-velocity heterogeneities atop the core-mantle boundary (CMB) remain poorly understood. It is also debated whether they are static features or whether they can be displaced and modified by mantle convection, although recent seismological and geodynamic evidence suggests the latter. In this work, we perform the first simultaneous analysis of SPdKS waves to characterize low-velocity heterogeneity and seismic anisotropy, which is evidence for deformation, at the base of the mantle. We find seismic velocity heterogeneity and seismic anisotropy that are co-located with, or adjacent to, each other. Our study region, the lowermost mantle beneath North America, has been shaped by long-term subduction. Through geodynamic modeling simulations, we show that the sinking of subducted slabs to the lowermost mantle can trigger formation of hot thermal anomalies near subducted slabs, where chemical heterogeneities can accumulate. The thermochemical anomalies can cause reduction of seismic velocity while the slab-induced flow can cause seismic anisotropy, potentially explaining our seismic observations. • We study the lowermost mantle structure and dynamics beneath North America. • We use SPdKS waves for a simultaneous analysis of heterogeneity and anisotropy. • Strong seismic anisotropy is near heterogeneity, likely induced by slab-driven flow. • The heterogeneities are likely continuously redistributed and/or reshaped by flow. [ABSTRACT FROM AUTHOR]

Abstract The seismically-observed large low shear velocity provinces in the Earth's lowermost mantle have been hypothesized to be caused by thermochemical piles of compositionally distinct, more-primitive material which may be remnants of Earth's early differentiation. However, one critical question is how the Earth's thermal evolution is affected by the long-term presence of the large-scale compositional heterogeneity in the lowermost mantle. Here, we perform geodynamical calculations to investigate the time evolution of the morphology of large-scale compositional heterogeneity and its influence on the Earth's long-term thermal evolution. Our results show that a global layer of intrinsically dense material in the lowermost mantle significantly suppresses the CMB heat flux, which leads to faster cooling of the background mantle relative to an isochemical mantle. As the background mantle cools, the intrinsically dense material is gradually pushed into isolated thermochemical piles by cold downwellings. The size of the piles also decreases with time due to entraining of pile material into the background mantle. The morphologic change of the accumulations of intrinsic dense material eventually causes a gradual increase of CMB heat flux, which significantly reduces the cooling rate of Earth's mantle. Highlights • The morphology of compositional heterogeneity on the CMB affects mantle temperature. • A global layer of intrinsic dense material on the CMB leads to rapid mantle cooling. • The intrinsic dense material forms into isolated piles as the mantle cools. • The change from dense global layer to isolated piles reduces mantle cooling rate. [ABSTRACT FROM AUTHOR]

About 40% of Earth's surface is covered by continental crust. It has been found that the global detrital zircon age distribution is dominated by multiple periodicities from <100 Myr to ∼800 Myr, suggesting that continental crust may have been produced periodically. However, what causes the periodic growth of continental crust remains unclear. Continental crust is produced mainly by arc magmatism at subduction zones and plume-induced magmatism. With 3D global mantle convection models, we investigate the long-term evolution of heat flux of mantle plumes and surface flow velocity, which can be respectively related to plume-induced magmatism and arc magmatism. About 10-30 plumes are present in our models, with significant spatial and temporal variations of heat flux for individual plumes. Both the temporal variations of the total plume heat flux and the root-mean-square (rms) of the surface velocity in our models have periodicities that decrease with the vigor of mantle convection. When extrapolated to conditions with Earth-like vigor of convection, the total plume heat flux and the rms of the surface velocity vary with periodic cycles of ∼90-200 Myr and ∼300 Myr, respectively. Therefore, the ∼90-200 Myr and the ∼300 Myr periodic cycles in the growth of continental crust as suggested by the global detrital zircon age distribution may be respectively caused by plume-induced magmatism and arc magmatism. • 3D global geodynamic models are used to study plume dynamics and surface motion. • There are large spatial and temporal variations of heat flux for individual plumes. • Total plume heat flux varies with periodicities of ∼90-200 Myr. • The average surface velocity varies with periodicity of ∼300 Myr. • Arc and plume-induced magnetism may cause periodic growth of continental crust. [ABSTRACT FROM AUTHOR]

Mantle plumes are thought to originate from thermal boundary layers such as Earth's core-mantle boundary (CMB), and may cause intraplate volcanism such as large igneous provinces (LIPs) on the Earth's surface. Previous studies showed that the original eruption sites of deep-sourced LIPs for the last 200 Myrs occur mostly above the margins of the seismically-observed large low shear velocity provinces (LLSVPs) in the lowermost mantle. However, the mechanism that leads to the distribution of the LIPs is not clear. The location of the LIPs is largely determined by the source location of mantle plumes, but the question is under what conditions mantle plumes form outside, at the edges, or above the middle of LLSVPs. Here, we perform 3D geodynamic calculations and theoretical analyses to study the plume source location in the lowermost mantle. We find that a factor of five decrease of thermal expansivity and a factor of two increase of thermal diffusivity from the surface to the CMB, which are consistent with mineral physics studies, significantly reduce the number of mantle plumes forming far outside of thermochemical piles (i.e., LLSVPs). An increase of mantle viscosity in the lowermost mantle also reduces number of plumes far outside of piles. In addition, we find that strong plumes preferentially form at/near the edges of piles and are generally hotter than that forming on top of piles, which may explain the observations that most LIPs occur above LLSVP margins. However, some plumes originated at pile edges can later appear above the middle of piles due to lateral movement of the plumes and piles and morphologic changes of the piles. ∼65–70% strong plumes are found within 10 degrees from pile edges in our models. Although plate motion exerts significant controls over the large-scale mantle convection in the lower mantle, mantle plume formation at the CMB remains largely controlled by thermal boundary layer instability which makes it difficult to predict geographic locations of most mantle plumes. However, all our models show consistently strong plumes originating from the lowermost mantle beneath Iceland, supporting a deep mantle plume origin of the Iceland volcanism. [ABSTRACT FROM AUTHOR]

• Thick subducted oceanic crust can accumulate above the CMB to form LLSVPs. • Accumulations of thick crust start with diffusive tops and stratified interiors. • The crustal accumulations gradually gain sharp tops and homogeneous interiors. • The LLSVPs may be produced in the early hotter Earth with thicker oceanic crust. • The LLSVPs may gradually gain sharp top boundaries and more homogenized interiors. Subducted oceanic crust is one of the major sources of compositional heterogeneity in Earth's mantle. It has been proposed that subducted oceanic crust may accumulate at the bottom of the Earth's mantle and cause the seismically-observed, Large Low Shear Velocity Provinces (LLSVPs). Testing this hypothesis requires a better understanding of the morphology of the crustal accumulations in the lowermost mantle. Here, through geodynamic modeling experiments, we find that thick subducted oceanic crust could accumulate into thermochemical piles with a height up to ∼1000 km above the core-mantle boundary (CMB), and the crustal accumulations typically have chemically fuzzy top boundaries and stratified interiors. As the oceanic crust thins with mantle cooling, it becomes more difficult to accumulate on the CMB, and the previous crustal accumulations gradually become smaller in size and gain sharp top boundaries and relatively homogeneous interiors. Our results suggest that if the present-day LLSVPs are mainly caused by the accumulations of subducted oceanic crust, they may be produced in the early hotter Earth when subducted oceanic crust may be thicker, and they may start with chemically fuzzy top boundaries and stratified interiors and gradually develop into chemically sharper top boundaries and more homogenized interiors. [ABSTRACT FROM AUTHOR]

Abstract The amount of water in the Earth's deep mantle is critical for the evolution of the Earth. Mineral physics studies revealed that the mantle transition zone can store several times the volume of water in an ocean. However, the actual water distribution in the transition zone remains enigmatic. We used the highest resolution images produced of scatterers in the North-American transition zone derived from teleseismic data recorded by the Earthscope Transportable Array. We find that the transition zone is filled with previously unrecognized small-scale heterogeneities that produce pervasive, negative polarity signals. Simulations demonstrated the images can be explained by low-velocity bodies shaped as distributed blobs or near vertical structures. We suggest these low-velocity bodies may be heterogeneously distributed, water-enriched subducted harzburgites produced through long-term accumulation. Highlights • Plane wave migration method imaged the transition zone at highest resolution ever. • Pervasive low-velocity small-scale heterogeneities of various geometry is observed. • Heterogeneously distributed water is inferred by simulation and geodynamic modeling. [ABSTRACT FROM AUTHOR]

Seismic tomography resolves anomalies interpreted as oceanic lithosphere subducted deep into Earth's lower mantle. However, the fate of the compositionally distinct oceanic crust that is part of the lithosphere is poorly constrained but provides important constraints on mixing processes and the recycling process in the deep Earth. We present high-resolution seismic array analyses of anomalous P-waves sampling the deep mantle, and deterministically locate heterogeneities in the lowermost 300 km of the mantle. Spectral analysis indicates that the dominant scale length of the heterogeneity is 4 to 7 km. The heterogeneity distribution varies laterally and radially and heterogeneities are more abundant near the margins of the lowermost mantle Large Low Velocity Provinces (LLVPs), consistent with mantle convection simulations that show elevated accumulations of deeply advected crustal material near the boundaries of thermo-chemical piles. The size and distribution of the observed heterogeneities is consistent with that expected for subducted oceanic crust. These results thus suggest the deep mantle contains an imprint of continued subduction of oceanic crust, stirred by mantle convection and modulated by long lasting thermo-chemical structures. The preferred location of the heterogeneity in the lowermost mantle is consistent with a thermo-chemical origin of the LLVPs. Our observations relate to the mixing behaviour of small length-scale heterogeneity in the deep Earth and indicate that compositional heterogeneities from the subduction process can survive for extended times in the lowermost mantle. [ABSTRACT FROM AUTHOR]