The Earth's lowermost mantle large low velocity provinces are accompanied by small-scale ultralow velocity zones in localized regions on the core-mantle boundary. Large low velocity provinces are hypothesized to be caused by large-scale compositional heterogeneity (i.e., thermochemical piles). The origin of ultralow velocity zones, however, remains elusive. Here we perform three-dimensional geodynamical calculations to show that the current locations and shapes of ultralow velocity zones are related to their cause. We find that the hottest lowermost mantle regions are commonly located well within the interiors of thermochemical piles. In contrast, accumulations of ultradense compositionally distinct material occur as discontinuous patches along the margins of thermochemical piles and have asymmetrical cross-sectional shape. Furthermore, the lateral morphology of these patches provides insight into mantle flow directions and long-term stability. The global distribution and large variations of morphology of ultralow velocity zones validate a compositionally distinct origin for most ultralow velocity zones.

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. (C) 2018 Elsevier B.V. All rights reserved.

Some regions of the Earth's lowermost mantle exhibit anomalous seismic properties within a thin zone, less than tens of kilometers in thickness, that directly overlies the core-mantle boundary (CMB). These regions have been dubbed Ultra-Low Velocity Zones (ULVZs) due to their greater than 10% drop in seismic velocities. High resolution seismic array studies have found small, localized ULVZs (e.g., 10 km thick and 50-100 km wide) with a large increase in ULVZ density (similar to 10%) relative to the background mantle. Many studies note that ULVZ material may be chemically distinct, though P-to-S-wave velocity reductions are sometimes consistent with partial melting. The apparent absence of ULVZs in many regions of the CMB is consistent with having a distinct chemical signature, regardless of melt content. However, it is unknown how a small volume of very dense ULVZ material can be locally elevated, particularly in the presence of large-scale compositional reservoirs predicted by seismology, geochemistry, and geodynamics. We perform ultra-high resolution, kilometer-scale, thermochemical convection calculations for an entire mantle system containing three distinct compositional components in order to investigate how a ULVZ interacts with large-scale lower mantle compositional reservoirs. We demonstrate that convection can dynamically support small scale accumulations of dense ULVZ material, consistent with the size and density inferred from seismology. Furthermore, we show that ULVZs preferentially reside at the boundaries of large compositional reservoirs, which periodically break apart and merge together in response to changes in downwelling patterns. As they do. ULVZ material migrates and recollects in a systematic fashion. ULVZ material can become entrained in mantle plumes forming from reservoir boundaries, contributing to isotopic anomalies found in hotspot volcanism. Thus ULVZ detection helps to constrain large-scale mantle convection patterns, the locations of compositional reservoir boundaries, and the evolution of geochemical reservoirs. (C) 2010 Elsevier B.V. All rights reserved.

Seismic images of Earth's interior reveal two massive anomalous zones at the base of the mantle, above the core, where seismic waves travel slowly. The mantle materials that surround these anomalous regions are thought to be composed of cooler rocks associated with downward advection of former oceanic tectonic plates. However, the origin and composition of the anomalous provinces is uncertain. These zones have long been depicted as warmer-than-average mantle materials related to convective upwelling. Yet, they may also be chemically distinct from the surrounding mantle, and potentially partly composed of subducted or primordial material, and have therefore been termed thermochemical piles. From seismic, geochemical and mineral physics data, the emerging view is that these thermochemical piles appear denser than the surrounding mantle materials, are dynamically stable and long-lived, and are shaped by larger-scale mantle flow. Whether remnants of a primordial layer or later accumulations of more-dense materials, the composition of the piles is modified over time by stirring and by chemical reactions with material from the surrounding mantle, underlying core and potentially from volatile elements transported into the deep Earth by subducted plates. Upwelling mantle plumes may originate from the thermochemical piles, so the unusual chemical composition of the piles could be the source of distinct trace-element signatures observed in hotspot lavas.

Processes within the lowest several hundred kilometers of Earth's rocky mantle play a critical role in the evolution of the planet. Understanding Earth's lower mantle requires putting recent seismic and mineral physics discoveries into a self- consistent, geodynamically feasible context. Two nearly antipodal large low- shear- velocity provinces in the deep mantle likely represent chemically distinct and denser material. High- resolution seismological studies have revealed laterally varying seismic velocity discontinuities in the deepest few hundred kilometers, consistent with a phase transition from perovskite to post- perovskite. In the deepest tens of kilometers of the mantle, isolated pockets of ultralow seismic velocities may denote Earth's deepest magma chamber.

Seismic tomography has revealed two large low shear velocity provinces (LLSVPs) in the lowermost mantle beneath the central Pacific and Africa. The LLSVPs are further shown to be compositionally different from their surroundings. Among several hypotheses put forth in recent years to explain the cause of the LLSVPs, one postulates that they are thermochemical piles caused by accumulation of subducted oceanic crust at the core-mantle boundary (CMB). Mineral physics experiments indicate that oceanic crust becomes denser than the surrounding mantle at lower mantle pressures. In addition, seismic observations provide evidence of subducted slabs arriving at the CMB. However, a major question pertains to whether subducted oceanic crust can survive viscous stirring associated with mantle plumes and accumulate into piles with the same spatial scale as LLSVPs. We perform a set of high-resolution convection calculations to examine this hypothesis by investigating the interaction of thin oceanic crust (6km) with mantle plumes. Our results show that as subducted oceanic crust is swept toward upwelling plume regions, the majority of it is viscously stirred into the surrounding mantle. Only a small amount of oceanic crust may accumulate at the base of plumes, but it is consistently entrained away into the plume at a rate equal to or greater than it is accumulated. We find that it is difficult for subducted oceanic crust to accumulate into large thermochemical piles at the CMB.

Large low-velocity seismic anomalies have been detected in the Earth's lower mantle beneath Africa and the Pacific Ocean that are not easily explained by temperature variations alone. The African anomaly has been interpreted to be a northwest-southeast-trending structure with a sharp-edged linear, ridge-like morphology. The Pacific anomaly, on the other hand, appears to be more rounded in shape. Mantle models with heterogeneous composition have related these structures to dense thermochemical piles or superplumes. It has not been shown, however, that such models can lead to thermochemical structures that satisfy the geometrical constraints, as inferred from seismological observations. We present numerical models of thermochemical convection in a three-dimensional spherical geometry using plate velocities inferred for the past 119 million years. We show that Earth's subduction history can lead to thermochemical structures similar in shape to the observed large, lower-mantle velocity anomalies. We find that subduction history tends to focus dense material into a ridge-like pile beneath Africa and a relatively more-rounded pile under the Pacific Ocean, consistent with seismic observations

Numerous seismic studies reveal the presence of two large, low velocity anomalies beneath Africa and the central Pacific. Efforts to characterize these anomalies have yielded a variety of interpretations over the years, both isochemical and thermochemical. Previous interpretations have included large, isochemical superplumes, clusters of smaller thermal plumes, and doming thermochemical superplumes. A conceptual mantle model that is presently growing favor involves long-lived thermochemical piles. In anticipation that nutation studies will provide better topographic constraints on the core-mantle boundary (CMB) in the future, we examine the effects of thermochemical piles at this boundary. In this study, we perform numerical modeling of thermochemical and isochemical convection as a function of convective vigor and temperature-dependent viscosity to predict the topography at Earth's CMB. Our results show that in isochemical convection, downwellings always lead to negative (depressed) CMB topography, while upwellings cause positive (elevated) topography, consistent with previous studies. However, in thermochemical convection, CMB topography and its relationship to downwellings and thermochemical piles are significantly affected by temperature-dependent viscosity. For isoviscous or weakly temperature-dependent viscosity, the piles cause negative CMB topography, while positive topography is often below cold downwellings. However, for realistic, more strongly temperature-dependent viscosity, the most negative CMB topography is below downwellings, while the topography below the piles is often slightly more positive and/or flat, similar to upwellings in isochemical models. We also show that although thermochemical piles are intrinsically more dense, the large thermal buoyancy associated with them leads to overall relative buoyancy that is on par with slabs. Consequently, thermochemical models lead to an overall reduction in magnitude of CMB topography with respect to isochemical models. Published by Elsevier B.V.

[1] The resolution operator R is a critical accompaniment to tomographic models of the mantle. R facilitates the comparison between conceptual three-dimensional velocity models and tomographic models because it can filter these theoretical models to the spatial resolution of the tomographic model. We compute R for the tomographic model S20RTS ( Ritsema et al., 1999, 2004) and two companion models that are based on the same data but derived with different norm damping values. The three models explain ( within measurement uncertainty) S-SKS and S-SKKS travel times equally well. To demonstrate how artifacts distort tomographic images and complicate model interpretation, we apply R to ( 1) a thermochemical and ( 2) an isochemical model of convection in the mantle that feature different patterns of shear velocity heterogeneity in the deep mantle if we assume that shear velocity heterogeneity is caused by temperature variations only. R suppresses short-wavelength structures, removes strong velocity gradients, and introduces artificial stretching and tilting of velocity anomalies. Temperature anomalies in the thermochemical model resemble the spatial extent of low seismic velocity anomalies and the shear velocity spectrum in the D'' region better than the isochemical model. However, the thermochemical model overpredicts the amplitude of shear velocity variation and places the African and Pacific anomalies imperfectly. We suspect that inaccurate velocity scaling laws and uncertain initial conditions control these mismatches. Extensive hypothesis testing is required to identify successful models.

Recent seismological observations reveal the presence of seismic anisotropy in localized regions at the base of the mantle within an otherwise isotropic lower mantle. These regions can be placed in a tectonic context, corresponding to locations of paleosubduction and plume upwelling. This project works toward determining whether the observed seismic anisotropy may be explained by the development of a mineral fabric by lattice-preferred orientation (LPO). Numerical modeling is used to explore whether the conditions at the base of upwelling and downwelling regions are consistent with those required for fabric development. Specifically, we examine whether dislocation creep dominates these regions within a background mantle that flows primarily by diffusion creep. The key to our study is the use of a composite rheology that includes both mechanisms of diffusion and dislocation creep and is based on mineral physics experiments. Results show that it is possible to produce a localization of dislocation creep near slabs within a background mantle dominated by diffusion creep. In contrast, upwelling regions are characterized by a domination of diffusion creep. These results indicate that LPO may be the cause of lowermost mantle seismic anisotropy near paleoslabs, but other mechanisms such as shape-preferred orientation may be required to produce the anisotropy observed near upwellings.

No abstract is available for this article.