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The core-mantle boundary (CMB) is the most abrupt internal discontinuity in the Earth, marking the solid-fluid boundary between mantle and outer core that strongly affects the dynamics of the Earth's interior. However, good agreement between models of CMB topographic variations is still lacking. This is probably due to difficulties relating to observations on seismograms and to the lack of good models of lowermost mantle velocity structure. Using spectral-element synthetic seismograms and adjoint methods, we perform traveltime analyses of seismic waves interacting with the CMB. We focus on reflected and refracted P and S waves. We select some of the most important and routinely used seismic phases: ScS, SKS, SKKS, PcP, PKP, PKKP and PcS, given their path through mantle and core and their interaction with the CMB. These seismic waves have been widely deployed by seismologists trying to image CMB topography and lowermost mantle structure. To analyse the reliability of measuring their traveltimes to infer CMB topography, we perform experiments in two ways. First, we compute synthetic seismograms with a dominant period of T ≈ 11s, for computational efficiency, using existing models of CMB topography. We compare traveltime perturbations measured by cross-correlation on the synthetics to those predicted using ray theory. We find deviations from a perfect agreement between ray theoretical predictions of time shifts and those measured on synthetics with and without CMB topography. Second, we calculate Fréchet sensitivity kernels of traveltimes with respect to shear and compressional wave speeds. We also explicitly compute boundary sensitivities with respect to the CMB interface. We observe that the overall sensitivity of the traveltimes is mostly due to volumetric velocity structure and that imprints of CMB on traveltimes are less pronounced. Our study explains the observed difficulties relating to inferring CMB topography using traveltimes and provides a suite of finite frequency sensitivity kernels computed with the adjoint method. The kernels allow us to qualitatively explain the behaviour of measured traveltimes and understand the trade-off between velocity and CMB topography. They can also serve as reference of finite frequency effects on traveltimes of observed seismic phases. From our analyses we conclude that: i) traveltime anomalies measured on Swaves are more in accord with ray theoretical predictions, ii) PcP, PKP, ScS and SKS phases have more pronounced sensitivity to the boundary and iii) separating the greater effects of velocity from those due to the boundary structure is difficult, as they intricately affect the traveltime. We propose that jointly inverting for CMB topography and lowermost mantle velocity structure using full-waveform synthetics and adjoint sensitivity kernels can progress our understanding of deep Earth structure and finite frequency effects on observed waveforms., Geophysical Journal International, 232 (2), ISSN:0956-540X, ISSN:1365-246X

The temperature at Earth’s core-mantle boundary (CMB) is a key parameter to understand the dynamics of our planet’s interior. However, it remains poorly known, with current estimate ranging from about 3000 K to 4500 K and more. Here, we introduce a new approach based on joint measurements of seismic shear-wave velocity,VS, and quality factor,QS, in the lowermost mantle. Lateral changes in bothVSandQSabove the CMB provide constraints on lateral temperature anomalies with respect to a reference temperature,Tref, defined as the average temperature in the layer immediately above the CMB. The request that, at a given location, temperature anomalies inferred independently fromVSandQSshould be equal gives a constraint onTref. CorrectingTreffor radial adiabatic and super-adiabatic increases in temperature gives an estimate of the CMB temperature,TCMB. This approach further relies on the fact thatVS-anomalies are affected by the distribution of post-perovskite (pPv) phase. As a result, the inferredTrefis linked to the temperatureTpPvat which the transition from bridgmanite to pPv occurs close to the CMB. A preliminary application toVSandQSmeasured beneath Central America and the Northern Pacific suggest that forTpPv= 3500 K,TCMBlies in the range 3,470–3880 K with a 95% likelihood. Additional measurements in various regions, together with a better knowledge ofTpPv, are however needed to determine a precise value ofTCMBwith our method.

SUMMARY Stoneley modes are a special subset of normal modes whose energy is confined along the core–mantle boundary (CMB). As such, they offer a unique glimpse into Earth structure at the base of the mantle. They are often observed through coupling with mantle modes due to rotation, ellipticity and lateral heterogeneity, though they can be detected without such coupling. In this study, we explore the relative sensitivities of seismic spectra of two low-frequency Stoneley modes to several factors, taking as reference the fully coupled computation up to 3 mHz in model S20RTS. The factors considered are (i) theoretical, by exploring the extent to which various coupling approximations can accurately reproduce reference spectra and (ii) model-based, by exploring how various Earth parameters such as CMB topography, attenuation and S- and P-wave structures, and the seismic source solution may influence the spectra. We find that mode-pair coupling is insufficiently accurate, but coupling modes within a range of ±0.1 mHz produces acceptable spectra, compared to full coupling. This has important implications for splitting function measurements, which are computed under the assumption of isolated modes or at best, mode-pair or group coupling. We find that uncertainties in the P-wave velocity mantle model dominate compared to other model parameters. In addition, we also test several hypothetical models of mantle density structure against real data. These tests indicate that, with the low-frequency Stoneley mode spectral data considered here, it is difficult to make any firm statement on whether the large-low-shear-velocity-provinces are denser or lighter than their surroundings. We conclude that better constraints on long wavelength elastic mantle structure, particularly P-wave velocity, need to be obtained, before making further statements on deep mantle density heterogeneity. In particular, a dense anomaly confined to a thin layer at the base of the mantle (less than ∼100–200 km) may not be resolvable using the two Stoneley modes tested here, while the ability of higher frequency Stoneley modes to resolve it requires further investigations.

The core-mantle boundary (CMB) is a complex and poorly understood region of the Earth. The existence of melt or partial melt at the CMB is strongly debated with evidence coming from a range of sources. While partial melt has been inferred in specific locations using seismology, often as an explanation for ultra-low velocity zones (ULVZs), no global layer of melt has been observed despite evidence that such melt should accumulate at the CMB.Using high frequency synthetic seismograms we have systematically examined the effect of a one-dimensional melt layer at the CMB on the global seismic wavefield. We find that PKKP diffracted waves are an extremely sensitive, robust and previously underutilised seismic phase for studying the CMB and, in synthetic testing, could resolve very thin melt layers. We have constructed a global data set of PKKP diffracted observations to attempt to observe or exclude melt at the CMB. We find that within the bounds of our data, we are unable to robustly exclude or observe a melt layer at the CMB on the order of single kilometres thick. The presence of a thin layer of unobservable melt at the CMB would have profound impacts for the internal dynamics of the Earth.

SUMMARY Seismic observations indicate that the lowermost mantle above the core–mantle boundary (CMB) is strongly heterogeneous. Body waves reveal a variety of ultra-low velocity zones (ULVZs), which extend not more than 100 km above the CMB and have shear velocity reductions of up to 30 per cent. While the nature and origin of these ULVZs remain uncertain, some have suggested they are evidence of partial melting at the base of mantle plumes. Here we use coupled geodynamic/thermodynamic modelling to explore the hypothesis that present-day deep mantle melting creates ULVZs and introduces compositional heterogeneity in the mantle. Our models explore the generation and migration of melt in a deforming and compacting host rock at the base of a plume in the lowermost mantle. We test whether the balance of gravitational and viscous forces can generate partially molten zones that are consistent with the seismic observations. We find that for a wide range of plausible melt densities, permeabilities and viscosities, lower mantle melt is too dense to be stirred into convective flow and instead sinks down to form a completely molten layer, which is inconsistent with observations of ULVZs. Only if melt is less dense or at most ca. 1 per cent more dense than the solid, or if melt pockets are trapped within the solid, can melt remain suspended in the partial melt zone. In these cases, seismic velocities would be reduced in a cone at the base of the plume. Generally, we find partial melt alone does not explain the observed ULVZ morphologies and solid-state compositional variation is required to explain the anomalies. Our findings provide a framework for testing whether seismically observed ULVZ shapes are consistent with a partial melt origin, which is an important step towards constraining the nature of the heterogeneities in the lowermost mantle and their influence on the thermal, compositional and dynamic evolution of the Earth.

The dynamic topography of the core-mantle boundary (CMB) provides important constraints on dynamic processes in the mantle and core. However, inferences on CMB topography are complicated by the une...

The pyrite-type FeO2 and FeO2H were synthesized at the pressure-temperature conditions relevant to Earth’s deep lower mantle. Through the water-iron reaction, the pyrite-phase is a good candidate to explain the chemical heterogeneities and seismological anomalies at the bottom of the mantle. The solid solution of pyrite-type FeO2 and FeO2H, namely the FeO2Hx (0 ​≤ ​x ​≤ ​1), is particularly interesting and introduces puzzling chemical states for both the O and H atoms in the deep mantle. While the role of H in the FeO2–FeO2H system has been primarily investigated, discrepancies remain. In this work, we summarize recent progress on the pyrite-phase, including FeO2, FeO2H, and FeO2Hx, which is critical for understanding the water cycling, redox equilibria, and compositional heterogenicities in the deep lower mantle.

The South Atlantic Anomaly (SAA) is a region at Earth’s surface where the intensity of the magnetic field is particularly low. Accurate characterization of the SAA is important for both fundamental understanding of core dynamics and the geodynamo as well as societal issues such as the erosion of instruments at surface observatories and onboard spacecrafts. Here, we propose new measures to better characterize the SAA area and center, accounting for surface intensity changes outside the SAA region and shape anisotropy. Applying our characterization to a geomagnetic field model covering the historical era, we find that the SAA area and center are more time dependent, including episodes of steady area, eastward drift and rapid southward drift. We interpret these special events in terms of the secular vari‑ation of relevant large‑scale geomagnetic flux patches on the core–mantle boundary. Our characterization may be used as a constraint on Earth‑like numerical dynamo models.

We have constrained a small-scale, dome-shaped low-velocity structure near the core-mantle boundary (CMB) of Earth beneath Perm (the Perm anomaly) using travel-time analysis and three-dimensional (3-D) forward waveform modeling of seismic data sampling of the mantle. The best-fitting dome-shaped model centers at 60.0°E, 50.5°N, and has a height of 400 km and a radius that increases from 200 km at the top to 450 km at the CMB. Its velocity reduction varies from 0% at the top to –3.0% at 240km above the CMB to –3.5% at the CMB. A surrounding 240-km-thick high-velocity D'' structure has also been detected. The Perm anomaly may represent a stable small-scale chemical pile in the lowermost mantle, although the hypothesis of a developing mantle plume cannot be ruled out.

A striking feature of many natural magnetic fields generated by dynamo action is the occurrence of polarity reversals. Paleomagnetic measurements revealed that the Earth’s magnetic field has been characterised by few hundred stochastic polarity switches during its history. The rate of reversals changes in time, maybe obeying some underlying regular pattern. While chaotic dynamical systems can describe the short-term behaviour of the switches of the Earth’s magnetic polarity, modelling the long-term variations of the reversal rate is somewhat problematic, as they occur on timescales of tens to hundreds of millions of years, of the order of mantle convection timescales. By investigating data of geomagnetic reversal rates, we find the presence of cycles with variable frequency and show that the transition towards periods where reversals do not occur for tens of million years (superchrons) can be described by a second-order phase transition that we interpret to be driven by variations of the heat flux at the core-mantle boundary (CMB). The model allows us to extract from the reversal sequence quantitative information on the susceptibility of the reversal rate caused by changes in the CMB heat flux amplitude, thus providing direct information on the deep inner layers of the Earth.

SUMMARY Body waves can be extracted from correlation functions computed from seismic records even at teleseismic distances. Here we use P and PcP waves from the secondary microseism frequency band that are propagating between Europe and the Eastern United States to image the core–mantle boundary (CMB) and D” structure beneath the North Atlantic. This study presents the first 3-D image of the lower mantle obtained from ocean-generated microseism data. Robustness of our results is evaluated by comparing images produced by propagation in both directions. Our observations reveal complex patterns of lateral and vertical variations of P-wave reflectivity with a particularly strong anomaly extending upward in the lower mantle up to 2600 km deep. We compare these results with synthetic data and associate this anomaly to a Vp velocity increase above the CMB. Our image aims at promoting the study of the lower mantle with microseism noise excitations.

SUMMARY Antipodal diffracted, compressional wave (Pdiff) data analysed diametrically opposite three large earthquakes have uniformly sampled 99% of the laterally heterogeneous zone above Earth's core–mantle boundary (D″ in seismic nomenclature). These antipodal data offer a fundamental gross Earth datum—a robust, global constraint on the average compressional velocity at the base of the mantle. We use for the first time the seismic phase PKPAB as a reference, which travels an identical mantle path as Pdiff, thereby cancelling common mantle heterogeneity. Differential traveltimes between Pdiff, PKPAB and PKIKP are measured, appropriately making allowance for the phase shifts acquired in propagation. We have independently confirmed the $\pi /4$ polar phase shift of Pdiff at the antipode. The global mean PKPAB − Pdiff time is 136.5 ± 0.6 s. The global mean apparent velocity (13.05 km s−1) and ray parameter (4.65 ± 0.01 s deg−1) are within the margin of error of prior Pdiff studies—which were dominated by Northern Hemisphere paths—indicating that complementary, southern hemisphere paths have a comparable, mean Pdiff apparent velocity. The seismic velocity constraints afforded by antipodal Pdiff and PKPAB suggest that the heterogeneous processes already observed in D″ may be broadly ascribed where D″ coverage has been lacking or poorly resolved.

Earth���s core-mantle boundary marks the transition from the rocky mantle to the liquid outer core and exhibits the most extreme velocity jump in the Earth���s radial seismic structure. In recent decades, seismological studies demonstrate that the seismic structure close to this boundary is far more complicated than a one-dimensional velocity model could describe. In particular, anomalous heterogeneities such as Large Low-Shear-Velocity Provinces (LLSVPs) and Ultra-Low Velocity Zones (ULVZs) have been reported in this region. We analyse seismic diffraction in different aspects including real data observations, numerical synthetics, asymptotic theory, and use it as a tool to resolve the fine-scale structure close to the core- mantle boundary. We present seismic observations of rare core-diffracted signals sensitive to the interior of the Hawaiian ULVZ at the highest frequency yet observed. This signal indicates a pronounced vertical structure on the scale of kilometres inside the ULVZ. We also explored a novel data set revealing a new ULVZ close to the area beneath Macdonald Island. These new observations reinforce the proposed correlation between the ULVZs and surface hotspots. Utilizing the latest computational advances in 3D synthetic waveform modelling, we also compare the waveforms of ULVZ models in different dimensions and demonstrate the consistency and nuance among the waveform in these modellings. We also discuss a new theoretical framework for modelling diffracted waves asymptotically, which extends surface wave ray theory to core-diffracted waves. To complement the discussion of the dynamics at the core-mantle boundary, we also analyse thermal boundary layer theory, where we validate the scaling of the core-mantle heat flux and the thermal boundary temperature under a strong temperature-dependent viscosity and compositional heterogeneity., This work is supported by the European Research Council (ERC) under the European Union���s Horizon 2020 research and innovation programme (grant agreement No.804071-ZoomDeep).

In the wake of its accretion, the Moon was likely partially or fully molten, forming a lunar magma ocean (LMO). A fully molten Moon may crystallize neutrally buoyant olivine, whose accumulation may form a barrier leading to a separation into two independently evolving melt reservoirs. This study investigates the crystallization of such a putative basal lunar magma ocean, ranging from the core mantle boundary (with radius r = 380 km) to the level of neutral olivine buoyancy (r = 600 km). Consecutive crystallization experiments determine liquidus temperatures and crystallize 10–30 wt% minerals, before conceptually segregating these according to their buoyancy, and then stepping to a new bulk composition that corresponds to the residual melt after removal of the cumulate minerals. The first olivine to crystallize from a Taylor Whole Moon composition (twm, XMg = Mg/(Mg + Fe2+) = 0.83) has XMg = 0.94 and is neutrally buoyant at 3.8 GPa according to the melt density model of Lange and Carmichael (1990). Crystallization begins at the core mantle boundary, but early olivine floats and re-dissolves until the magma ocean cools to the liquidus temperature at the depth of neutral buoyancy (1850 °C). At this point a > 500 km thick olivine-only layer could form, mainly fed from the upper magma shell. The crystallization sequence in the basal magma ocean is olivine-only at 1850–1675 °C, 0–26 pcs (wt% percent solidified) → olivine + opx (to 1600 °C, 42 pcs) → opx + cpx + garnet (to 1580 °C, 60 pcs) → cpx + garnet (to 1520 °C, 73 pcs) → cpx + garnet + olivine leaving 11 wt% residual melt at 1450 °C. Cooling this last melt to 1300 °C leads to 80% garnet + cpx + olivine + Ti-spinel, the residual liquid corresponding to 2.2% of the basal magma ocean. Olivine, opx and cpx remain buoyant over the entire crystallization interval and various pyroxenite layers are added to the olivine layer. Garnet and the final cotectic cpx + garnet + olivine + FeTi-oxide assemblage instead form a 70 km thick basal layer on the core-mantle boundary. Such a layer provides a gravitationally stable high-density lowermost lunar mantle, which would concur with a recent re-analysis of the lunar seismic data. The lowest temperature experiment at 1300 °C, not much above the 1250 °C proposed for the core-mantle boundary from inversion of geophysical data, had about 20% of a highly evolved, dense Fe-rich melt. It is feasible that this melt has remained in its liquid state to present day, providing an explanation for the proposed low vp and vs layer and high dissipation just above the core-mantle boundary., Icarus, 371, ISSN:0019-1035

The Earth’s magnetic field is measured on and above the crust, while the turbulent dynamo in the outer core produces magnetic field values at the core–mantle boundary (CMB). The connection between the two sets of values is usually assumed to be independent of the electrical conductivity in the mantle. However, the turbulent magnetofluid in the Earth’s outer core produces a time-varying magnetic field that must induce currents in the lower mantle as it emerges, since the mantle is observed to be electrically conductive. Here, we develop a model to assess the possible effects of mantle electrical conductivity on the magnetic field values at the CMB. This model uses a new method for mapping the geomagnetic field from the Earth’s surface to the CMB. Since numerical and theoretical results suggest that the turbulent magnetic field in the outer core as it approaches the CMB is mostly parallel to this boundary, we assume that this property exists and set the normal component of the model magnetic field to zero at the CMB. This leads to a modification of the Mauersberger–Lowes spectrum at the CMB so that it is no longer flat, i.e., the modified spectrum depends on mantle conductance. We examined several cases in which mantle conductance ranges from low to high in order to gauge how CMB magnetic field strength and mantle ohmic heat generation may vary.

The eccentric dipole (ED) is the next approach of the geomagnetic field after the generally used geocentric dipole. Here, we analyzed the evolution of the ED during extreme events, such as the Matuyama-Brunhes polarity transition (~780 ka), the Laschamp (~41 ka) and Mono Lake (~34 ka) excursions, and during the time of two anomalous features of the geomagnetic field observed during the Holocene: the Levantine Iron Age Anomaly (LIAA, ~1000 BC) and the South Atlantic Anomaly (SAA, analyzed from ~700 AD to present day). The analysis was carried out using the paleoreconstructions that cover the time of the mentioned events (IMMAB4, IMOLEe, LSMOD.2, SHAWQ-Iron Age, and SHAWQ2k). We found that the ED moves around the meridian plane of 0–180° during the reversal and the excursions, it moves towards the region of the LIAA, and it moves away from the SAA. To investigate what information can be extracted from its evolution, we designed a simple model based on 360-point dipoles evenly distributed in a ring close to the inner core boundary that can be reversed and their magnitude changed. We tried to reproduce with our simple model the observed evolution of the ED, and the total field energy at the Earth’s surface. We observed that the modeled ED moves away from the region where we set the dipoles to reverse. If we consider that the ring dipoles could be related to convective columns in the outer core of the Earth, our simple model would indicate the potential of the displacement of the ED to give information about the regions in the outer core where changes start for polarity transitions and for the generation of important anomalies of the geomagnetic field. According to our simple model, the regions in which the most important events of the Holocene occur, or in which the last polarity reversal or excursion begin, are related to the regions of the Core Mantle Boundary (CMB), where the heat flux is low.

The core-mantle boundary (CMB) is the most extreme boundary within the Earth where the liquid, iron-rich outer core interacts with the rocky, silicate mantle. The nature of the lowermost mantle atop the CMB, and its role in mantle dynamics, is not completely understood. Various regional studies have documented significant heterogeneities at different spatial scales. While there is a consensus on the long scale-length structure of the inferred S-wave speed tomograms, there are also notable differences stemming from different imaging methods and datasets. Here we aim to overcome over-smoothing and avoid over-fitting data for the case where the spatial coverage is sparse and the inverse problem ill-posed. Here we present an S-wave tomography model at global scale for the Lowermost Mantle (LM) using the Hierarchical Trans-dimensional Bayesian Inversion (HTDBI) framework, LM-HTDBI. Our HTDBI analysis of ScS-S travel times includes uncertainty, and the complexity of the model is deduced from the data itself through an implicit parameterization of the model space. Our comprehensive resolution estimates indicate that short-scale anomalies are significant and resolvable features of the lowermost mantle regardless of the chosen mantle-model reference to correct the travel times above the D’’ layer. The recovered morphology of the Large-Low-Shear-wave Velocity Provinces (LLSVPs) is complex, featuring small high-velocity patches among low-velocity domains. Instead of two large, unified, and smooth LLSVPs, the newly obtained images suggest that their margins are not uniformly flat.

The relationship between cyclic processes in the core and lithosphere in the Middle Jurassic–Paleogene (167–22 Ma) was investigated. The dynamics of cyclic processes was associated with paleointensity behavior and sea level. It is shown that variations in paleointensity and ocean level are two broadband oscillatory processes. The characteristic times of variations in geomagnetic and eustatic processes changed in different geologic ages. We discovered synchronization of variations in geomagnetic and eustatic processes that occurred in the Cretaceous–Early Paleogene (125–40 Ma) based on a spectral analysis of paleointensity and sea level data. The characteristic times of coherent variations in paleointensity and global ocean level ranged from 1 to 16 Ma. Possible reasons for the synchronization of geomagnetic and lithospheric processes have been identified. Features of geomagnetic and lithospheric processes in the interval of 125–40 Ma, expressed as sharp increases in the amplitudes of paleointensity variations and activation of magmatic effusive processes, are considered.

This database includes Isotropic lattice thermal conductivities of MgSiO3 post-perovskite under the lowermost mantle conditions. The conductivity was calculated based on the ab initio anharmonic lattice dynamics with solving the linearized phonon Boltzmann transport equation.

Earth’s outer core is generally thought to be a well-mixed liquid consisting mostly of iron and a small amount of lighter elements. Recent seismic studies using SmKS waves show that the top a few hundred kilometers of the outer core possess a P-wave velocity slightly lower than the PREM model, which cannot be explained by self-compression of a chemically homogeneous outer core. We investigated the SmKS waveforms of a deep earthquake occurring beneath South America recorded by a large and dense seismic array in China, and measured the differential arrival times of the SmKS pairs. We found significant waveform distortion of the SmKS caused by postcritical refraction and reflection at the core-mantle boundary. This waveform distortion can introduce significant bias to the measured differential times, leading to incorrect estimate of P-wave velocity of the outer core. Whether stable stratification is occurring in outer core or not requires further seismic investigations.

The weak intrinsic magnetic field of Mercury is intimately tied to the structure and cooling history of its metallic core. Recent constraints about the planet's internal structure suggest the presence of a FeS layer overlying a silicon-bearing core. We performed 4-electrode resistivity experiments on core analogues up to 10 GPa and over wide temperature ranges in order to investigate the insulating properties of core materials. Our results show that the FeS layer is liquid and insulating, and that the electrical resistivity of a miscible Fe-Si(-S) core is comparable to the one of an immiscible Fe-S, Fe-Si core. The difference in electrical resistivity between the FeS-rich layer and the underlying Fe-Si(-S) core is at least 1 log unit at pressure and temperature conditions relevant to Mercury's interior. Estimates of the lower bound of thermal conductivity for FeS and Fe-Si(-S) materials are calculated using the Wiedemann-Franz law. A thick (>40 km) FeS-rich shell is expected to maintain high temperatures across the core, and if temperature in this layer departs from an adiabat, then this might affect the core cooling rate. The presence of a liquid and insulating shell is not inconsistent with a thermally stratified core in Mercury and is likely to impact the generation and sustainability of a magnetic field.

Earth’s magnetic field is generated by turbulent motion in its fluid outer core. Although the bulk of the outer core is vigorously convecting and well mixed, some seismic, geomagnetic and geodynamic evidence suggests that a global stably stratified layer exists at the top of Earth’s core. Such a layer would strongly influence thermal, chemical and momentum exchange across the core–mantle boundary and thus have important implications for the dynamics and evolution of the core. Here we argue that the relevant scenario is not global stratification, but rather regional stratification arising solely from the lateral variations in heat flux at the core–mantle boundary. Using our extensive suite of numerical simulations of the dynamics of the fluid core with heterogeneous core–mantle boundary heat flux, we predict that thermal regional inversion layers extend hundreds of kilometres into the core under anomalously hot regions of the lowermost mantle. Although the majority of the outermost core remains actively convecting, sufficiently large and strong regional inversion layers produce a one-dimensional temperature profile that mimics a globally stratified layer below the core–mantle boundary—an apparent thermal stratification despite the average heat flux across the core–mantle boundary being strongly superadiabatic.

SUMMARY Temperature anomalies in Earth’s liquid core reflect the vigour of convection and the nature and extent of thermal core–mantle coupling. Numerical simulations suggest that longitudinal temperature anomalies forced by lateral heat flow variations at the core–mantle boundary (CMB) can greatly exceed the anomalies that arise in homogeneous convection (i.e. with no boundary forcing) and may even penetrate all the way to the inner core boundary. However, it is not clear whether these simulations access the relevant regime for convection in Earth’s core, which is characterized by rapid rotation (low Ekman number E) and strong driving (high Rayleigh number Ra). We access this regime using numerical simulations of non-magnetic rotating convection with imposed heat flow variations at the outer boundary (OB) and investigate the amplitude and spatial pattern of thermal anomalies, focusing on the inner and outer boundaries. The 108 simulations cover the parameter range 10−4 ≤ E ≤ 10−6 and Ra = 1−800 times the critical value. At each Ra and E we consider two heat flow patterns—one derived from seismic tomography and the hemispheric $Y_1^1$ spherical harmonic pattern—with amplitudes measured by the parameter q⋆ = 2.3, 5 as well as the case of homogeneous convection. At the OB the forcing produces strong longitudinal temperature variations that peak in the equatorial region. Scaling relations suggest that the longitudinal variations are weakly dependent on E and Ra and are much stronger than in homogeneous convection, reaching O(1) K at core conditions if q⋆ ≈ 35. At the inner boundary, latitudinal and longitudinal temperature variations depend weakly on Ra and q⋆ and decrease strongly with E, becoming practically indistinguishable between homogeneous and heterogeneous cases at E = 10−6. Interpreted at core conditions our results suggest that heat flow variations on the CMB are unlikely to explain the large-scale variations observed by seismology at the top of the inner core.

Geophysical observations suggest that the transition zone is wet locally. Continental and oceanic sediment components together with the basaltic and peridotitic components might be transported and accumulated in the transition zone. Low-velocity anomalies at the upper mantle–transition zone boundary might be caused by the existence of dense hydrous magmas. Water can be carried farther into the lower mantle by the slabs. The anomalous Q and shear wave regions locating at the uppermost part of the lower mantle could be caused by the existence of fluid or wet magmas in this region because of the water-solubility contrast between the minerals in the transition zone and those in the lower mantle. δ-H solid solution AlO2H–MgSiO4H2 carries water into the lower mantle. Hydrogen-bond symmetrization exists in high-pressure hydrous phases and thus they are stable at the high pressures of the lower mantle. Thus, the δ-H solid solution in subducting slabs carries water farther into the bottom of the lower mantle. Pyrite FeO2Hx is formed due to a reaction between the core and hydrated slabs. This phase could be a candidate for the anomalous regions at the core–mantle boundary.

In response to the primary electric field originated by the fluid-core-dynamo, electric charges accumulate at the boundaries of a solid conducting D″ layer due to the poor circuitry conditions until the irrotational primary electric field is neutralized. The neutralization undoes the internal induction excited by the primary electric field and, as a result, eliminates the secondary toroidal magnetic field inside the D″. The primary magnetic field generated by the fluid-core-dynamo gives rise to a sustainable secondary poloidal magnetic field inside the conducting D″ layer. We introduce the general kinematic boundary conditions that are valid with different induction processes and parameter jumps across the core mantle boundary. We find by dimensional analysis that the strength of the secondary poloidal magnetic field is much weaker, by a factor of ∼10−2, than the irrotational primary. These findings indicate that the D″ is in effect an insulator for the geodynamo regardless of its material composition. Any mechanism for the core-mantle magnetic coupling should take this fact into consideration. Keywords: D′′ layer, Magnetic induction, Boundary conditions

The Earth's core is composed mainly of iron. Since the liquid core coexists with solid at the inner core boundary (ICB), the melting point of iron at 330 GPa offers a key constraint on core temperatures. However, previous results using a laser-heated diamond-anvil cell (DAC) have been largely inconsistent with each other, likely because of an intrinsic large temperature gradient and its temporal fluctuation. Here we employed an internal-resistance-heated DAC and determined the melting temperature of pure iron up to 290 GPa, for the first time above 200 GPa by static compression experiments. A small extrapolation of the present experimental results yields a melting point of 5500 ± 220 K at the ICB, higher than 4850 ± 200 K reported by previous laser-heated DAC by Boehler (1993) but is lower than 6230 ± 500 K by Anzellini et al. (2013) . Accounting for the melting temperature depression due to core-alloying elements, the upper bounds for the temperature at the ICB and the core–mantle boundary (CMB) are estimated to be 5120 ± 390 K and 3760 ± 290 K, respectively. Such low present-day CMB temperature suggests that the lowermost mantle has avoided global melting, at least since early Proterozoic Eon.

The core–mantle boundary is the only interface where the metallic core and the silicate mantle interact physically and chemically. Many geophysical anomalies such as low shear velocity and high electrical conductivity have been observed at the bottom of the mantle. Perturbations in the Earth's rotation rate at decadal time periods require the existence of a thin conductive layer with a conductance of 108 S. Substantial additions of molten iron from the outer core into the mantle may produce these geophysical anomalies. Although iron enrichment by penetration has only been observed in (Mg,Fe)O, the second dominant mineral in the lower mantle, the penetration process leading to iron enrichment in the silicate mantle has not been experimentally confirmed. In this study, high-pressure and high-temperature experiments were conducted to investigate the penetration of molten iron alloy into lower mantle phases; postspinel, polycrystalline bridgmanite and polycrystalline (Mg,Fe)O. At the interface between (Mg,Fe)O aggregate and molten iron alloy, liquid metal penetrated the (Mg,Fe)O aggregate along grain boundaries and formed a thin layer containing metal-rich blobs. In contrast, no penetration of molten iron alloy was observed at the interface between molten iron alloy and silicate phases. Penetration of liquid iron alloy into the (Mg,Fe)O aggregate is caused by the capillarity phenomenon or Mullins–Sekerka instability. Neither mechanism occurs at the boundary of pure polycrystalline MgO, indicating that the FeO in (Mg,Fe)O plays an essential role in this phenomenon. Infiltration of molten iron alloy along grain boundaries (capillarity phenomenon) is the dominant process and precedes penetration due to the Mullins–Sekerka instability. The capillarity phenomenon is governed by the balance of forces between surface tension and gravity. In the case where the ultralow velocity zone (ULVZ) with a low shear velocity is composed of Fe-rich (Mg,Fe)O, the maximum penetration distance of molten iron alloy by capillary rise is limited to 20 m. The addition of iron-rich melt to the base of the mantle is therefore unlikely to be the main cause of the high conductance of the CMB region predicted from decadal variation of the length of day. Furthermore, the absence of molten iron alloy penetration into silicate phases does not allow an extensive modification of the chemical composition of the mantle by core–mantle interaction.

It is suggested that ultra-low velocity zones (ULVZs) are often located near the boundary of large low shear velocity zones (LLSVPs). Previous studies have provided evidence of ULVZs residing along the western and northern boundary of the Pacific LLSVP. However, so far there is no detailed report on ULVZs along the eastern boundary of the Pacific LLSVP. In this study, we collected PKP precursor waveforms from earthquakes in South America that are recorded by seismic arrays in Australia. These precursors show clear and coherent phases. Slowness and migration analysis of these precursors indicate that they originate from scatterers in the lowermost mantle east of the Pacific LLSVP. Waveform modeling suggests that these seismic scatterers are localized ULVZs, with P-wave velocity reductions of ∼3–10% and thickness of several tens of kilometers. The dimension of the ULVZs and the geological correlation of the ULVZs with the Galapagos hotspot, suggest that these ULVZs are mainly due to compositionally distinct heterogeneities, although partial melting cannot be completely ruled out. The distribution of both coherent and incoherent PKP precursors also suggests vigorous small-scale mantle convections near the edges of the LLSVP.

Qualitative and quantitative analysis of seismic waveforms sensitive to the core–mantle boundary (CMB) region reveal the presence of ultralow-velocity zones (ULVZs) that have a strong decrease in compressional (P) and shear (S) wave velocity, and an increase in density within thin structures. However, understanding their physical origin and relation to the other large-scale structures in the lowermost mantle are limited due to an incomplete mapping of ULVZs at the CMB. The SKS and SPdKS seismic waveforms is routinely used to infer ULVZ presence, but has thus far only been used in a limited epicentral distance range. As the SKS/SPdKS wavefield interacts with a ULVZ it generates additional seismic arrivals, thus increasing the complexity of the recorded wavefield. Here, we explore utilization of the multi-scale sample entropy method to search for ULVZ structures. We investigate the feasibility of this approach through analysis of synthetic seismograms computed for PREM, 1-, 2.5-, and 3-D ULVZs as well as heterogeneous structures with a strong increase in velocity in the lowermost mantle in 1- and 2.5-D. We find that the sample entropy technique may be useful across a wide range of epicentral distances from 100° to 130°. Such an analysis, when applied to real waveforms, could provide coverage of roughly 85% by surface area of the CMB.

The lowermost mantle right above the core-mantle boundary shows a complex and heterogeneous landscape containing multiple poorly understood seismic features visible across a wide range of length scales. The smallest, but most extreme, heterogeneities yet observed are 'Ultra-Low Velocity Zones' (ULVZ), several of which have recently been linked to the base of mantle plumes. We exploit seismic shear waves that diffract along the core-mantle boundary to provide new insight into these enigmatic structures. We demonstrate that these waves have a strong frequency-dependent sensitivity to structure at different length scales above the core-mantle boundary, similar to the dispersive characteristics of surface waves. We measure a rare core-diffracted signal refracted by a ULVZ at the base of the Hawaiian mantle plume at unprecedented high frequencies. This signal shows remarkably longer time delays compared to lower frequencies, indicating extreme internal variability within the Hawaiian ULVZ. Utilizing the latest computational advances in 3D synthetic waveform modeling, we are able to model this high frequency signal and constrain high-resolution structure on the scale of kilometers at the core-mantle boundary, for the first time. Results reveal that the lowermost part of the Hawaiian ULVZ is extremely reduced in shear wave velocity, by up to -40%. This new observation suggests a chemically distinct ULVZ with increasing iron content towards the core-mantle boundary, which has implications for Earth’s early evolutionary history and core-mantle interaction.

Geomagnetic jerks are sudden changes in the geomagnetic field secular variation related to changes in outer core flow patterns. Finding geophysical phenomena related to geomagnetic jerks provides a vital contribution to better understand the geomagnetic field behavior. Here, we link the geomagnetic jerks occurrence with one of the most relevant features of the geomagnetic field nowadays, the South Atlantic Anomaly (SAA), which is due to the presence of reversed flux patches (RFPs) at the Core-Mantle Boundary (CMB). Our results show that minima of acceleration of the areal extent of SAA calculated using the CHAOS-7 model (CHAOS-7.2 release) coincide with the occurrence of geomagnetic jerks for the last 2 decades. In addition, a new pulse in the secular acceleration of the radial component of the geomagnetic field has been observed at the CMB, with a maximum in 2016.2 and a minimum in 2017.5. This fact, along with the minimum observed in 2017.8 in the acceleration of the areal extent of SAA, could point to a new geomagnetic jerk. We have also analyzed the acceleration of the areal extent of South American and African RFPs at the CMB related to the presence of the SAA at surface and have registered minima in the same periods when they are observed in the SAA at surface. This reinforces the link found and would indicate that physical processes that produce the RFPs, and in turn the SAA evolution, contribute to the core dynamics at the origin of jerks.

Water is transported to Earth's interior in lithospheric slabs at subduction zones. Shallow dehydration fuels hydrous island arc magmatism but some water is transported deeper in cool slab mantle. Further dehydration at ∼700 km may limit deeper transport but hydrated phases in slab crust have considerable capacity for transporting water to the core-mantle boundary. Quantifying how much remains the challenge.

The locations of ultralow‐velocity zones (ULVZs) at the core‐mantle boundary (CMB) have been linked to a variety of features including hot spot volcanoes and large low‐velocity province (LLVP) boundaries, yet only a small portion of the CMB region has been probed for ULVZ existence. Here we present a new map of lower mantle heterogeneity locations using a global collection of highly anomalous SPdKS recordings based on a dataset of more than 58,000 radial component seismograms, which sample 56.9% of the CMB by surface area. The inference of heterogeneity location using the SPdKS seismic phase is challenging due to source‐versus receiver‐side ambiguity. Due to this ambiguity, we conducted an inversion using the principle of parsimony. The inversion is conducted using a genetic algorithm which is repeated several thousand times in order to construct heterogeneity probability maps. This analysis reveals that at probabilities urn:x-wiley:15252027:media:ggge22404:ggge22404-math-00010.5, 0.25, and 0.125 up to 1.3%, 8.2%, or 19.7% of the CMB may contain ULVZ‐like heterogeneities. These heterogeneities exist in all lower mantle settings, including both high‐ and low‐velocity regions. Additionally, we present evidence that the Samoan ULVZ may be twice as large as previously estimated, and also present evidence for the existence of additional mega‐sized ULVZs, such as a newly discovered ULVZ located to the east of the Philippines. We provide new evidence for the ULVZ east of the Philippines through an analysis of ScP records.

The core-mantle boundary (CMB) at ~2900 km depth and ~135 GPa is the interface between the solid silicate mantle and the liquid metallic outer core. The temperature at CMB (TCMB) is a key parameter in assessing the heat flow out of the core and is thus important for our understanding of the geodynamo and mantle convection. The upper bound on TCMB is constrained by the solidus temperature (onset of melting) of pyrolite, a chemical proxy of the mantle rock, at ~135 GPa because the lowermost mantle is predominantly solid. Previous laser-heated diamond anvil cell (LH DAC) studies reported pyrolite solidus temperatures of 3430–4180 K (±

While the main causes of the temporal gravity variations observed by the Gravity Recovery and Climate Experiment (GRACE) space mission result from water mass redistributions occurring at the surface of the Earth in response to climatic and anthropogenic forces (e.g., changes in land hydrology, ocean mass, and mass of glaciers and ice sheets), solid Earth’s mass redistributions were also recorded by these observations. This is the case, in particular, for the glacial isostatic adjustment (GIA) or the viscous response of the mantle to the last deglaciation. However, it has only recently been shown that the gravity data also contain the signature of flows inside the outer core and their effects on the core–mantle boundary (CMB). Detecting deep Earth’s processes in GRACE observations offers an exciting opportunity to provide additional insight into the dynamics of the core–mantle interface. Here, we present one aspect of the GRACEFUL (GRavimetry, mAgnetism and CorE Flow) project, i.e., the possibility to use gravity field data for understanding the dynamic processes inside the fluid core and core–mantle boundary of the Earth, beside that offered by the geomagnetic field variations.

The interplay between stress and chemical processes is a fundamental aspect of how rocks evolve, relevant for understanding fracturing due to metamorphic volume change, deformation by pressure solution and diffusion creep, and the effects of stress on mineral reactions in crust and mantle. There is no agreed microscale theory for how stress and chemistry interact, so here I review support from eight different types of the experiment for a relationship between stress and chemistry which is specific to individual interfaces: (chemical potential) = (Helmholtz free energy) + (normal stress at interface) × (molar volume). The experiments encompass temperatures from -100 to 1300 degrees C and pressures from 1 bar to 1.8 GPa. The equation applies to boundaries with fluid and to incoherent solid–solid boundaries. It is broadly in accord with experiments that describe the behaviours of free and stressed crystal faces next to solutions, that document flow laws for pressure solution and diffusion creep, that address polymorphic transformations under stress, and that investigate volume changes in solid-state reactions. The accord is not in all cases quantitative, but the equation is still used to assist the explanation. An implication is that the chemical potential varies depending on the interface, so there is no unique driving force for reaction in stressed systems. Instead, the overall evolution will be determined by combinations of reaction pathways and kinetic factors. The equation described here should be a foundation for grain-scale models, which are a prerequisite for predicting larger scale Earth behaviour when stress and chemical processes interact. It is relevant for all depths in the Earth from the uppermost crust (pressure solution in basin compaction, creep on faults), reactive fluid flow systems (serpentinisation), the deeper crust (orogenic metamorphism), the upper mantle (diffusion creep), the transition zone (phase changes in stressed subducting slabs) to the lower mantle and core mantle boundary (diffusion creep).

The North Atlantic Igneous Province (NAIP) is unique in that it presents a complete magmatic history from ~62 Ma to today. This provides an unrivalled opportunity to study the geochemical and thermal evolution of a mantle plume through time. Despite the wealth of geochemical data available for Iceland, our knowledge of mantle heterogeneity beneath Iceland during the mid-Miocene remains sketchy. The oldest basalts (15-16 Ma) on Iceland are preserved in several ~1 km thick sequences in Vestfirdir in Northwest Iceland. Each sequence contains a hiatus in volcanism, marked by a laterite-lignite horizon, that is likely the result of the relocation of a spreading axis. The basalt geochemistry from above and below this horizon provide an insight into the heterogeneity of the Iceland mantle plume during the mid-Miocene. The Vestfirdir basalts are also characterised by the highest 3He/4He (~40 Ra) on Iceland requiring the involvement of deep mantle. Incompatible trace element and Nd-Sr-Pb isotope data indicate that there are two enriched components (NWE1 and NWE2) and one depleted component (NWD1) within the mantle beneath Iceland during the Miocene. Both enriched components have low 143Nd/144Nd and high 87Sr/86Sr, that are comparable with the most enriched modern Iceland basalts. The two components are differentiated by the relative enrichment in the more incompatible trace elements and their Pb-isotope compositions. Component NWE1 is similar to the modern Iceland mantle plume composition, while NWE2 is closer in composition to modern depleted Iceland rift-zone basalts. The depleted component identified in the Vestfirdir basalts (NWD1) is distinct from modern North Atlantic N-MORB and the depleted component in the neovolcanic rift zones by, for instance, higher incompatible trace element concentrations and a more negative ∆207Pb composition. These observations allow the identification of a depleted mantle component present at 15-16 Ma that is no longer sampled in the modern Iceland plume setting and argue for a distinct compositional change in the mantle beneath Iceland over the last 15-16 Myr. Existing and new 3He/4He and magmatic temperature measurements help understand how primordial helium and heat in mantle plumes vary with time and provide insight into the nature of the deep mantle and the source of primordial terrestrial volatiles. The highest 3He/4He (42 RA) recorded in Vestfirdir is midway between the starting plume (50 RA) and modern Iceland values (34 RA), demonstrating that 3He/4He appears to have steadily decreased with time. The high-3He/4He Vestfirdir basalts have a large range of incompatible trace element and Sr-Nd-Pb isotope ratios, as also seen in the PIP picrites, implying that there is no compositionally-unique mantle source for the high 3He/4He component in the Iceland plume. Additionally, the uncontaminated Vestfirdir basalts that haven’t been erupted through continental crust also have Pb-isotope values that rule out the need for a primordial, isolated mantle reservoir formed ~4.55-4.45 Ga. The new Al-in-olivine data from high-3He/4He Vestfirdir basalts yield crystallisation temperatures significantly less than that of proto-Iceland plume and modern Iceland picrites. The apparent ~40°C increase of the Iceland plume temperature in the last 15 million years contrasts strongly with the decrease of 3He/4He. This suggests that the heat and primordial 3He are decoupled in the Iceland plume. This may reflect the different sources (i.e. heat from the core, 3He from the deep mantle) or alternatively different diffusion rates of heat and helium across the core-mantle boundary.

Subduction of oceanic lithosphere transports surface water into the mantle where it can have remarkable effects, but how much can be cycled down into the deep mantle, and potentially to the core, remains ambiguous. Recent studies show that dense SiO2 in the form of stishovite, a major phase in subducted oceanic crust at depths greater than ~300 km, has the potential to host and carry water into the lower mantle. We investigate the hydration of stishovite and its higher-pressure polymorphs, CaCl2-type SiO2 and seifertite, in experiments at pressures of 44–152 GPa and temperatures of ~1380–3300 K. We quantify the water storage capacity of these dense SiO2 phases at high pressure and find that water stabilizes CaCl2-type SiO2 to pressures beyond the base of the mantle. We parametrize the P-T dependence of water capacity and model H2O storage in SiO2 along a lower mantle geotherm. Dehydration of slab mantle in cooler slabs in the transition zone can release fluids that hydrate stishovite in oceanic crust. Hydrous SiO2 phases are stable along a geotherm and progressively dehydrate with depth, potentially causing partial melting or silica enrichment in the lower mantle. Oceanic crust can transport ~0.2 wt% water to the core-mantle boundary region where, upon heating, it can initiate partial melting and react with the core to produce iron hydrides, providing plausible explanations for ultra-low velocity regions at the base of the mantle.