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We present observations on a new precursory phase of seismic waves scattered in the deep Earth. This phase arrives prior to the PKPab wave at epicentral distances larger than 155°, and we call it PKPab precursor. We show that the presence of the PKPab precursor is a necessary consequence of scattering in D″, which is the commonly accepted cause of the PKPdf precursor at distances smaller than 145°. PKPdf waves that propagate through the inner core should arrive before the PKPab precursor but those, are strongly attenuated in the inner core at frequencies between 4 Hz and 8 Hz used here, making the PKPab precursor the earliest teleseismic signal at distances larger than 155°. Calculated PKPab precursor sensitivity kernel shows that this phase is mostly sensitive to scattering along the closest PKPbc path between source and receiver. It can thus help to constrain the lateral distribution of heterogeneity along D″. Plain Language Summary: A new discovered seismic signal recorded far away from earthquakes, by stations on the other side of Earth, will help to study the properties of the core–mantle boundary. We use high frequencies at which seismic waves do not propagate through the Earth's inner core but are instead propagated around it by deflection at heterogeneity located along the core–mantle boundary. Key Points: PKP precursor observed at distance beyond 155°D″ scattering of teleseismic waves at 6 HzRadiative transfer simulation used to locate regions of heterogeneity [ABSTRACT FROM AUTHOR]

The noble gas isotope systematics of ocean island basalts suggest the existence of primordial mantle signatures in the deep mantle. Yet, the isotopic compositions of lithophile elements (Sr, Nd, Hf) in these lavas require derivation from a mantle source that is geochemically depleted by melt extraction rather than primitive. Here, this apparent contradiction is resolved by employing a compilation of the Sr, Nd, and Hf isotope composition of kimberlites--volcanic rocks that originate at great depth beneath continents. This compilation includes kimberlites as old as 2.06 billion years and shows that kimberlites do not derive from a primitive mantle source but sample the same geochemically depleted component (where geochemical depletion refers to ancient melt extraction) common to most oceanic island basalts, previously called PREMA (prevalent mantle) or FOZO (focal zone). Extrapolation of the Nd and Hf isotopic compositions of the kimberlite source to the age of Earth formation yields a 143Nd/144Nd-176Hf/177Hf composition within error of chondrite meteorites, which include the likely parent bodies of Earth. This supports a hypothesis where the source of kimberlites and ocean island basalts contains a long-lived component that formed by melt extraction from a domain with chondritic 143Nd/ 144Nd and 176Hf/177Hf shortly after Earth accretion. The geographic distribution of kimberlites containing the PREMA component suggests that these remnants of early Earth differentiation are located in large seismically anomalous regions corresponding to thermochemical piles above the core-mantle boundary. PREMA could have been stored in these structures for most of Earth's history, partially shielded from convective homogenization. [ABSTRACT FROM AUTHOR]

The inner core's (IC) axial torsional libration (ATL) is a manifestation of the mantle‐inner core gravitational rotational normal mode. Its natural period is critically determined by the interior sectoral quadrupole q22(Vex) (of mass distribution of spherical‐harmonic degree 2 and order 2) belonging to the mantle plus the core‐mantle boundary. We derive the (multipole) formalism to calculate q22(Vex) based on two physical postulations: (i) Postulation I that equates the observed 6‐year variation in Earth's length‐of‐day to ATL as laid out in Chao (2017, https://doi.org/10.1002/2016JB013515); (ii) Postulation II requiring that the shape of IC, thanks to its finite viscosity, conforms to the gravitational equipotential surface in the long term. q22(Vex) being a complex value, its phase relation with ATL asserts that the major axis of the IC aligns with that of q22(Vex) subject to the lowest‐energy state. The magnitude of q22(Vex) thus evaluated by Earth parameters is |q22(Vex)|≈0.78–1.71kgm−3 (the two endmember values corresponding respectively to IC's nominal polar moment of inertia vs. the effective value under 100% tangent‐cylinder coupling). The corresponding equatorial elliptical shape of the IC under Postulation II has peak‐to‐peak topography of 75–164 m. The acquired q22(Vex) can serve as source information for inversion on, among others, the density anomaly of the large low shear velocity provinces constructs in the lower mantle. Key Points: Equatorial ellipticity of inner core (IC) under gravitational equilibrium is derived in terms of spherical‐harmonic density multipolesInterior sectoral quadrupole determined by equating IC's axial libration to 6‐year oscillation found in Earth's length‐of‐dayThe obtained quadrupole is useful for inferring large low shear velocity provinces‐like density anomaly in lower mantle + core‐mantle boundary [ABSTRACT FROM AUTHOR]

Lithological heterogeneity is a widely accepted feature of the Earth's mantle, with recycled crustal material accounting for a significant part of heterogeneity in ocean island basalt (OIB) geochemistry. Fe isotopes have been used to link geochemical heterogeneity in OIB sources to distinct mantle lithologies due to their mineral-specific equilibrium fractionation effects, or their composition, such as incorporation of kinetically-fractionated core liquids entrained from the core-mantle boundary. Here we present Fe isotope data for Samoan shield, and Azores volcanoes, together with a combined phase-equilibria and equilibrium mineral-melt isotope fractionation model. These OIB lavas allow us to study the roles of core-derived and recycled mantle components in generating heavy δ 57Fe melts. Heavy δ 57Fe correlates with radiogenic isotope signatures of enrichment by a crustal component and not with Fe/Mn or any indicator of core involvement. However, single-stage melting of a MORB-like eclogitic pyroxenite cannot generate the heavy δ 57Fe seen in Pitcairn, Azores, and rejuvenated Samoa lavas. Melts of a reaction-zone pyroxenite (commonly suggested to form part of the OIB source), derived from eclogite melts hybridised with peridotite, also fail to generate the heaviest Fe isotopic compositions seen in OIB. Instead, the generation of heavy δ 57Fe melts in OIB requires: (1) processes that make subducted eclogite isotopically heavier than its pristine precursor MORB (e.g., hydrothermal alteration, metamorphism, sediment input); (2) lithospheric processing, such as remobilisation of previously frozen small-degree melts, or a contribution from lithospheric material metasomatised by silicate melts; and/or (3) melting conditions that limit the dilution of melts with heavy δ 57Fe by ambient lower δ 57Fe materials. No single process we consider can generate the heavy δ 57Fe seen in the Azores, Pitcairn, and rejuvenated Samoan lavas. Therefore, it cannot be assumed that a pyroxenite lithology derived from recycled crustal material is the sole producer of heavy δ 57Fe melts in OIB, nor can these signatures be related to contributions from the Earth's core. Instead, the observation of heavy δ 57Fe OIB melts cannot be ascribed to a unique source or process. This ambiguity reflects the multitude of processes operating from the generation of recycled lithologies through to their mantle melting: from MORB generation, its low temperature alteration, through mantle heterogeneity development and lithospheric processing, to eruption at ocean islands. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are carried out on polycystalline periclase (MgO) to determine the structure and diffusivity at grain boundaries for pressures and temperatures relevant to Earth's mantle. As temperature increases, the grain boundary structure becomes more disordered, with more ions having incomplete coordination and the system occupying regions of the energy landscape with shallower energy mimima. In contrast, as pressure increases the grain boundary structure becomes more ordered. The grain boundary diffusivity as a function of temperature and pressure can be understood in terms of these structural changes. At atmospheric pressure, the grain boundary diffusion coefficients for Mg and O extrapolate with increasing temperature to the values for the melt, indicating that the dynamics in the grain boundary are similar to those of a supercooled liquid. Just as in a supercooled liquid, diffusion in the grain boundary slows down with decreasing temperature for two reasons: there is less energy to surmount energy barriers, and the barriers are larger due to the more ordered structure. As pressure increases from zero pressure, the diffusivities first decrease sharply, due to the increase in energy barriers associated with the more ordered system, and then more gradually as pressure increases beyond ∼4 GPa. At conditions relevant to Earth's core-mantle boundary region (135 GPa, 3500 °C), no diffusion is observed on the timescale of ∼50 ns, and the diffusion coefficients are thus constrained to be <2 × 10−13 m2/s. This upper limit is considerably smaller than values obtained from experimental measurements at lower pressures and temperatures, suggesting that grain boundary transport at the core-mantle boundary is less efficient than has been inferred previously. [ABSTRACT FROM AUTHOR]

This paper provides a hypothesis to account for the two Large Low-Shear-Velocity Provinces (LLSVPs) and discusses evidence supporting this hypothesis. These features are located in the lower mantle just above the core-mantle boundary and display strikingly low seismic-shear wave speed relative to adjacent rock. They are nearly antipodal to each other, with one lying roughly beneath Africa and the other beneath the central Pacific Ocean. While secular literature does not have an explanation for how these structures arose or even why they display such low seismic speeds, we postulate that LLSVPs are a direct consequence of catastrophic plate tectonic activity during the Genesis Flood. This paper posits that LLSVPs correspond to hot lower-mantle rock that was forcibly shoved aside by subducted lithospheric slab material as it reached the core-mantle boundary during the Flood. The large contrast in seismic speed between the LLSVP material and the surrounding rock is due primarily to the large difference in temperature. The apparent paradox of why these two LLSVPs, if their low seismic speed is due to high temperature and reduced density, did not rise to the surface millions of years ago is resolved by the realities that the Flood occurred only a few thousand years ago and that mean mantle viscosity returned quickly to its present value at the end of the cataclysm. Numerical simulations with the mantle dynamics code, terra, support this scenario. [ABSTRACT FROM AUTHOR]

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 ≥0.5, 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. Plain Language Summary: Past deep‐Earth studies have identified thin zones of reduced seismic velocities located atop the core‐mantle boundary. These zones, referred to as ultralow‐velocity zones or ULVZs, have been related to several important features of the Earth's interior such as mantle plumes, continent‐sized low‐velocity provinces, and large igneous provinces. Nonetheless, we do not know what ULVZs are made of, whether they only exist in specific regions of the core‐mantle boundary (CMB), how they link to other mantle properties, or whether all ULVZs are the same. We have assembled a large dataset of seismic waveforms that are sensitive to deep mantle heterogeneity and have identified the waveforms with features indicative of heterogeneous mantle properties, such as ULVZ presence, and search for the simplest distribution of lower mantle heterogeneities. We find that as much as 19.7% of the CMB area must have anomalous seismic properties to explain our dataset. We also use a type of seismic wave reflected off the CMB to verify our initial results in a newly discovered heterogeneous region that lies on the CMB to the east of the Philippines. Our new heterogeneity map provides greater coverage than previous maps and suggests that several mega‐sized ULVZs may exist. Key Points: We inverted for ULVZ location from highly anomalous SPdKS data using the principle of parsimonyAs much as 19.7% of the CMB by surface area may contain ULVZ‐like heterogeneitySeveral mega‐ULVZs may exist in the deep Earth [ABSTRACT FROM AUTHOR]

The group IIF iron meteorites and Eagle Station pallasites (PES) have highly siderophile element abundances (HSE; Re, Os, Ir, Ru, Pt, and Pd) of metal that are consistent with formation in planetesimal cores by fractional crystallization with minor to major solid metal–liquid metal mixing. Modeling of HSE abundances of the IIF irons indicates a complex formation history that included the mixing of primitive and evolved solid and liquid metals. By contrast, modeling of HSE abundances of PES metal suggests these meteorites formed mainly as equilibrium solids from a common liquid. Abundances of some of the siderophile elements in the IIF irons and PES are permissive of a common core origin; however, the abundances of W and Ni indicate the PES ultimately formed on a more oxidized body. The PES most likely formed by the injection of olivine present at the core–mantle boundary into a metallic core liquid as a result of impact. The core then crystallized inward, trapping the olivine. [ABSTRACT FROM AUTHOR]

• The modified Lie-Hori method is useful to solve Hamiltonian dissipative problems. • That is exemplified by computing the nutations of a 2-layer Earth with dissipations. • Analytical and numerical results are compared with previous approximate solution. • Corrections to previous solution reach the microarcsecond level for some periods. • That is about the current level of determination of amplitudes from observations. In this research, a modification of the Lie-Hori perturbation method developed by the authors in a recent investigation is used to compute the forced nutations of a non-rigid Earth model, including dissipative processes at the core-mantle boundary. The study is tackled within the Hamiltonian formalism of a two-layer Earth, where the viscous and electromagnetic couplings between mantle and core are introduced via generalized forces. The modified Lie-Hori method is introduced within the framework of the generalized Hamiltonian formalism. It, therefore, allows for calculating first-order perturbations in both conservative and non-conservative systems, while the classical Lie-Hori procedure is not designed to include generalized forces in the kernel to account for dissipative processes. Unlike other methods, this one presents the advantage of keeping the same dimensionality of the original problem, avoiding the doubling of the dimension of the phase space. With this mathematical refinement, differences in the derived nutation amplitudes at the microarcsecond level have been found when compared with the former, first approximation for dissipative systems based on damped oscillators —the only existing previous solution. Those figures are of relevance according to recent recommendations of the International Astronomical Union (IAU) and the International Association of Geodesy (IAG) based on the final report of the Joint Working Group on Theory of Earth rotation and validation. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

We developed a thermodynamic model to explore the joint solubility of Mg, Si, and O in liquid Fe on the basis of high‐pressure metal‐silicate partitioning data in the literature, with more Mg kept in the metal when Si and O are present. With <1.7 ± 0.5 wt% Mg, the metal in the young Earth's core retains Mg as the core evolves and crystallizes SiO2. Higher Mg concentrations require either the late addition of metal that equilibrated with silicate in a super‐liquidus magma ocean or the assimilation of silicate into the core at the time of a giant impact. Above 1.7 wt% Mg, (Mg,Fe)‐silicate melts also exsolve from the core and transfer core‐hosted elements to the mantle. Fractional crystallization of the core‐derived silicate melts in the core or at the core‐mantle boundary could, additionally, yield a persistent molten silicate layer that could also contribute to ultralow velocity zone formation in the lowermost mantle. Plain Language Summary: Some think that crystallization of oxide in the liquid core, either MgO or SiO2, could have been responsible for driving core convection and geodynamo since early Earth times, at least until iron started crystallizing the inner core. However, the joint solubility of Mg, Si, and O in liquid Fe has not been explored yet. Here we developed a thermodynamic model to examine the exsolution of MgO and SiO2 from liquid Fe at core‐mantle boundary (CMB) pressure based on earlier experimental data on metal‐silicate partitioning. Our main findings are that (1) more Mg is retained in the presence of Si and O in metal; (2) with <1.7 wt% Mg, the core expels solid SiO2 but not MgO; and (3) with higher Mg, magnesium silicate melt also exolves and transfers core‐hosted elements to the mantle. The seismologically observed molten silicate layer above the CMB may have been originally derived from the core. Key Points: Mg uptake in liquid iron is controlled by the liquid's Si content; silicate Mg is reduced by Si in the metalIf the core later expelled Mg ingested while accreting as (Mg,Fe)‐silicate, it could transfer siderophile trace elements back to the mantleULVZs at the base of the mantle could be related to fractional crystallization of (Mg,Fe) silicate expelled by the core [ABSTRACT FROM AUTHOR]

Lunar mare basalts represent flood volcanism between ~4.0 and 1.2 Ga, therefore, providing insights into the thermal and volcanic history of the Moon. The present study investigates the spectral and chemical characteristics as well as ages of the nearside mare basaltic units from the Grimaldi basin, namely Mare Grimaldi and Mare Riccioli, using a wealth of orbital remote sensing data. This study delineated distinct basaltic units of varying albedo, mineralogy, and titanium contents in both Mare Grimaldi and Mare Riccioli. The crater size–frequency distribution technique revealed that at least two phases of basaltic magmatism spanning ~3.5 to 1.5 Ga (Late Imbrian–Eratosthenian) have occurred in the Grimaldi basin. High‐Ti olivine basalts dated at 2.05 Ga are found to be surrounded by the Late Imbrian (~3.47 Ga) low‐ to intermediate‐Ti basalts in Mare Grimaldi. Low‐ to intermediate‐Ti basalts observed in Mare Riccioli date back to two different volcanic events at ~3.5 Ga and ~3.2 billion years, while patches of basalts having remarkably higher titanium content within the Mare Riccioli record the youngest age of ~1.5 Ga. The chemical trend of the pyroxenes from distinct basaltic units also revealed that multiple events of volcanism have occurred in the Grimaldi basin. The high‐Ti basalts in the Mare Grimaldi crystallized from an Fe‐enriched late‐stage magma while the low‐Ti basalts crystallized from an Mg‐ and Ca‐rich initial magma that experienced an ultra‐late stage quenching. The low‐ to intermediate‐Ti basaltic magma erupted in both the units was derived by partial melting of early cumulate materials from the hybrid source region in the post‐overturn upper mantle and made its way to the surface through dikes that propagated by excess pressures accumulated in the diapirs stalled at the base of the crust due to buoyancy trap. The high‐Ti magma erupted in the Mare Grimaldi was generated by a hot plume ascended from deeper clinopyroxene–ilmenite‐rich cumulate layer near the core–mantle boundary. However, the Eratosthenian (~1.5 Ga) intermediate‐Ti volcanic activity in the Mare Riccioli rather sourced from the ilmenite–clinopyroxene cumulate materials that remained in the upper mantle after mantle overturn. The new results suggest that volcanism had not ceased in the Grimaldi basin at 3.27 Ga, rather it was active and fed by different mantle sources until 1.5 Ga for a period spanning ~2 billion years. [ABSTRACT FROM AUTHOR]

One of the central challenges in accurately estimating the mantle melting temperature is the sensitivity of the probe for detecting a small amount of melt at the solidus. To address this, we used a multichannel collimator to enhance the diffuse X‐ray scattering from a small amount of melt and probed an eutectic pyrolitic composition to increase the amount of melt at the solidus. Our in situ detection of diffuse scattering from the pyrolitic melt determined an anhydrous melting temperature of 3,302 ± 100 K at 119 ± 6 GPa and 3,430 ± 130 K at the core‐mantle boundary (CMB) conditions, as the upper bound temperature. Our CMB temperature is approximately 700 K lower than the previous estimates, implying much faster secular cooling and higher concentrations of S, C, O, and/or H in the region, and nonlinear, advocating the basal magma ocean hypothesis. Plain Language Summary: The heat stored in the Earth's deep interior has been the primary fuel for a range of global processes from mantle convection to surface tectonics, but quantitative estimation of the heat remains uncertain. The melting temperature of mantle materials is one of the key parameters to understanding the thermal evolution and present‐day state of the Earth's interior, but it has been poorly constrained, with recent measurement discrepancies as large as 600 K. Here, we report melting temperatures of mantle compositions measured over a wide range of pressures expected for the lower mantle. We investigated a eutectic mantle composition using multichannel collimator filtered X‐ray diffraction in combination with the laser‐heated diamond‐anvil cell. Fitting our melting data over the range of 46–145 GPa led to a solidus temperature of 3,430 ± 130 K at the core‐mantle boundary. This temperature is approximately 700 K lower than the previous estimates, implying much faster secular cooling at the lower mantle than previously believed. Furthermore, our solidus curve constrained for a wide pressure range is strongly nonlinear and thus supports the basal magma ocean hypothesis. Key Points: Combined usage of multichannel collimator and laser‐heated diamond‐anvil cell improved detection of silicate meltThis led to the lowest solidus temperature of anhydrous pyrolite, 3,430 ± 130 K, at the core‐mantle boundary conditionsWe advocate Fe‐enriched provinces at the core‐mantle boundary to be originated from the magma ocean [ABSTRACT FROM AUTHOR]

Water transportation to the deep lower mantle via plate subduction may induce a reaction between water and iron at the core‐mantle boundary. Recent experimental studies suggest that such a reaction may generate FeO2Hx‐rich domains, which can explain the seismic structures of the ultralow velocity zone in this region. In this study, the chemical reaction between metallic iron and a limited water supply at ~120 GPa was investigated using time‐resolved in situ synchrotron X‐ray diffraction measurements in combination with the laser‐heated diamond anvil cell technique. Contrary to the results of earlier studies, the formation of FeO instead of FeO2Hx without intermediate phases was observed. Considering the unlimited availability of iron in the core and the limited water supply resulting from mantle downflow, the FeO‐rich layers consisted of Fe‐bearing ferropericlase and postperovskite, which must have locally cumulated at the bottom of the mantle simultaneously with hydrogen incorporation into the core. Plain Language Summary: Water strongly influences the structure, dynamics, and evolution of the deep Earth. Recent experimental studies suggest that hydrous phases play an important role as carriers of surface water to the deep mantle via the subduction of oceanic plates. Such deep‐water subduction processes may allow the surface water to reach the bottom of the mantle, where the mantle minerals are in direct contact with the iron at the core. Thus, the purpose of this study was to provide understanding regarding the behavior of water when it meets iron at the core‐mantle boundary. To investigate the reaction between water and iron at high pressures and temperatures, experiments were performed using in situ X‐ray diffraction measurements in combination with the diamond‐anvil cell technique. The results obtained confirmed the formation of FeO during the reaction. Thus, the deep water cycle may produce FeO‐rich layers at the core‐mantle boundary, which may explain the seismic characteristics of the bottom of the mantle and at the top of the outer core. Key Points: The reaction between iron and a limited supply of water at ~120 GPa was studied via in situ X‐ray diffraction measurementsThe deep water cycle may produce local FeO‐rich layers at the bottom of the mantleExcess FeO contents may explain the seismic characteristics at the core‐mantle boundary [ABSTRACT FROM AUTHOR]

The geomagnetic field presents several stationary features that are thought to be linked to inhomogeneities at the core–mantle boundary. Particularly important stationary structures of the geomagnetic field are the flux lobes, which appear in pairs in mid- to high mid- to high latitudes. A recently discovered stratified layer at the top of the Earth's core poses important constraints on the dynamics at this layer and on the interaction of the core dynamics and the base of the mantle. In this article, we introduce the linear and nonlinear theories of magnetic Rossby waves in a thin shell at the top of the Earth's core. We study the nonlinear interaction of these waves in the presence of prescribed forcings at the base of the mantle of both a thermal and a topographic nature. We show that the combined effects of forcing and nonlinear interaction can lead the wave phases to be locked around a particular geographical longitude, generating a quasi- stationary flow pattern with a significant meridional component. The solutions of the system are shown to be analogous to atmospheric blocking phenomena. Therefore, we argue that persistent and long-lived structures of the geomagnetic field, such as the geomagnetic lobes, might be associated with a blocking at the top of the Earth's core due to nonlinear stationary waves. [ABSTRACT FROM AUTHOR]

The seismic structure of the Martian interior can shed light on the formation and dynamic evolution of the planet and our solar system. The deployment of the seismograph carried by the InSight mission provides a means to study Martian internal structure. We used ambient noise autocorrelation to analyze the available vertical component seismic data to recover the reflectivity beneath the Insight lander. We identify the noise that is approximately periodic with the Martian sol as daily lander operations and the diurnal variation in Martian weather and tides. To investigate the seismic discontinuities at different depths, the autocorrelograms are filtered and stacked into different frequency bands. We observe prominent reflection signals probably corresponding to the Martian Moho, the olivine‐wadsleyite transition in the mantle, and the core‐mantle boundary in the stacked autocorrelograms. We estimate the depths of these boundaries as ~35, 1,110–1,170, and 1,520–1,600 km, consistent with other estimates. Plain Language Summary: Knowledge of the Martian interior informs theories for the formation and dynamic evolution of another terrestrial planet, hence providing information on the history of the solar system. On Earth, subsurface structure is discovered by analysis of seismic signals recorded by large seismograph arrays deployed worldwide. The InSight lander carried one seismic station to Mars at the end of 2018, providing the opportunity to investigate the internal structure of Mars. Here we autocorrelated ambient noise from the available seismic data to investigate the subsurface discontinuities of Mars. In the raw seismic data, we observe the long‐period signals with a period of ~1 Martian sol (~3% longer than an Earth day), which are related to the diurnal variation in weather and tides. After the removal of instrument response, remaining high‐amplitude peaks are likely caused by the daily operations of the InSight lander. We preprocessed the raw data and used different frequency bands to detect discontinuities at different depths. We identify prominent signals in the stacked autocorrelation reflectivity as likely originating from the Martian Moho, the olivine‐wadsleyite transition in the Martian mantle, and the core‐mantle boundary. These results are consistent with other observations and measurements. Key Points: Autocorrelation analysis of SEIS data shows signals probably from the Martian Moho, olivine‐wadsleyite transition, and core‐mantle boundaryDepth conversion of these signals gives discontinuity depths consistent with estimates made with other methodsA high crustal Vp/Vs ratio suggests the composition of Martian crust is composed of basaltic and andesitic rocks [ABSTRACT FROM AUTHOR]

Seismic and geomagnetic observations have been used to argue both for and against a global stratified layer at the top of Earth's outer core. Recently, we used numerical models of turbulent thermal convection to show that imposed lateral variations in core‐mantle boundary (CMB) heat flow can give rise to regional lenses of stratified fluid at the top of the core while the bulk of the core remains actively convecting. Here, we develop theoretical scaling laws to extrapolate the properties of regional stratified lenses measured in simulations to the conditions of Earth's core. We estimate that regional stratified lenses in Earth's core have thicknesses of up to a few hundred kilometres and Brunt‐Väisälä frequencies of hours, consistent with independent observational constraints. The location, thickness, and strength of the stratified regions would change over geological time scales in response to the slowly evolving CMB heat flux heterogeneity imposed by mantle convection. Key Points: CMB heat flux heterogeneity results in regional lenses of stratified fluid at the top of the coreWe develop scaling laws for the strength and thickness of these lensesExtrapolations to Earth‐like conditions predict lenses a few hundred kilometres thick [ABSTRACT FROM AUTHOR]

Computational models of mantle convection must accurately represent curved boundaries and the associated boundary conditions within a 3-D spherical shell, bounded by Earth's surface and the core-mantle boundary. This is also true for comparable models in a simplified 2-D cylindrical geometry. It is of fundamental importance that the codes underlying these models are carefully verified prior to their application in a geodynamical context, for which comparisons against analytical solutions are an indispensable tool. However, analytical solutions for the Stokes equations in these geometries, based upon simple source terms that adhere to natural boundary conditions, are often complex and difficult to derive. In this paper, we present the analytical solutions for a smooth polynomial source and a delta-function forcing, in combination with free-slip and zero-slip boundary conditions, for both 2-D cylindrical and 3-D spherical shell domains. We study the convergence of the Taylor Hood (P2-P1) discretisation with respect to these solutions, within the finite element computational modelling framework Fluidity, and discuss an issue of suboptimal convergence in the presence of discontinuities. To facilitate the verification of numerical codes across the wider community, we provide a python package, Assess, that evaluates the analytical solutions at arbitrary points of the domain. [ABSTRACT FROM AUTHOR]

We explore the partitioning behavior of hydrogen between coexisting metal and silicate melts at conditions of the magma ocean and the current core–mantle boundary with the help of density functional theory molecular dynamics. We perform simulations with the two‐phase and thermodynamic integration methods. We find that hydrogen is weakly siderophile at low pressure (20 GPa and 2,500 K), and becomes much more strongly so with pressure, suggesting that hydrogen is transported to the core in a significant amount during core segregation and is stable there. Based on our results, the core likely contains ~1 wt% H, assuming single‐stage formation and equilibration at 40 GPa. Our two‐phase simulations further suggest that silicon is entrained in the core‐forming metal, while magnesium remains in the silicate phase. This preferred incorporation of silicon hints at an explanation for the elevated Mg/Si ratio of the bulk silicate Earth relative to chondritic compositions. Plain Language Summary: Hydrogen is the most abundant element in the universe and may be present in the Earth's core together with its main constituent, iron. Despite its importance, the amount of hydrogen in the core has been poorly understood as it is extremely difficult to measure the hydrogen content in iron metal from experiments at high pressure and temperature. Here, we use advanced quantum mechanical simulations on silicate and metallic melts showing that hydrogen becomes increasingly more incorporated in metal over silicate at high pressure and temperature, conditions under which Earth's core formed. Therefore, hydrogen can be present in the core in high abundance. We further show that silicon is also transported to the core to a smaller extent, while magnesium remains in the mantle material. Key Points: We perform density functional theory molecular dynamics simulations to study hydrogen partitioning between liquid metal and silicateHydrogen is predicted to be weakly siderophile at low pressure (~20 GPa) and becomes significantly more siderophile at higher pressuresCore segregating from the magma ocean may have sequestered significant amounts of H and, to a lesser extent, Si, but little or no Mg [ABSTRACT FROM AUTHOR]

Earth's magnetic field is presently characterized by a large and growing anomaly in the South Atlantic Ocean. The question of whether this region of Earth's surface is preferentially subject to enhanced geomagnetic variability on geological timescales has major implications for core dynamics, core-mantle interaction, and the possibility of an imminent magnetic polarity reversal. Here we present paleomagnetic data from Saint Helena, a volcanic island ideally suited for testing the hypothesis that geomagnetic field behavior is anomalous in the South Atlantic on timescales of millions of years. Our results, supported by positive baked contact and reversal tests, produce a mean direction approximating that expected from a geocentric axial dipole for the interval 8 to 11 million years ago, but with very large associated directional dispersion. These findings indicate that, on geological timescales, geomagnetic secular variation is persistently enhanced in the vicinity of Saint Helena. This, in turn, supports the South Atlantic as a locus of unusual geomagnetic behavior arising from core-mantle interaction, while also appearing to reduce the likelihood that the present-day regional anomaly is a precursor to a global polarity reversal. [ABSTRACT FROM AUTHOR]

The existence of a stably stratified layer underneath the core–mantle boundary (CMB) has been recently revived by corroborating evidences coming from seismic studies, mineral physics and thermal evolution models. Such a layer could find its physical origination either in compositional stratification due to the accumulation of light elements at the top or the core or in thermal stratification due to the heat flux becoming locally subadiabatic. The exact properties of this stably stratified layer, namely its size |$\mathcal {H}_s$| and the degree of its stratification characterized by the Brunt–Väisälä frequency N , are however uncertain and highly debated. A stable layer underneath the CMB can have crucial dynamical impacts on the geodynamo. Because of the inhibition of the convective motions, a stable layer is expected to primarily act as a low-pass filter on the magnetic field, smoothing out the rapidly varying and small-scale features by skin effect. To investigate this effect more systematically, we compute 70 global geodynamo models varying the size of the stably stratified layer from 0 to 300 km and its amplitude from N /Ω = 0 to N /Ω ≃ 50, Ω being the rotation rate. We show that the penetration of the convective flow in the stably stratified layer is controlled by the typical size of the convective eddies and by the local variations of the ratio N /Ω. Using quantitative measures of the degree of morphological semblance between the magnetic field obtained in numerical models and the geomagnetic field at the CMB, we establish an upper bound for the stable layer thickness |$\mathcal {H}_s\lt (N/\Omega)^{-1} \mathcal {L}_s$|⁠ , |$\mathcal {L}_s$| being the horizontal size of the convective flow at the base of the stable layer. This defines a strong geomagnetic constraint on the properties of a stably stratified layer beneath the CMB. Unless unaccounted double-diffusive effects could drastically modify the dynamics of the stable layer, our numerical geodynamo models hence favour no stable stratification atop the core. [ABSTRACT FROM AUTHOR]

Observations of changes in the geomagnetic field provide unique information about processes in the outer core where the field is generated. Recent geomagnetic field reconstructions based on palaeomagnetic data show persistent westward drift at high northern latitudes at the core–mantle boundary (CMB) over the past 4000 yr, as well as intermittent occurrence of high-latitude weak or reverse flux patches. To further investigate these features, we analysed time-longitude plots of a processed version of the geomagnetic field model pfm9k.1a, filtered to remove quasi-stationary features of the field. Our results suggest that westward drift at both high northern and southern latitudes of the CMB have been a persistent feature of the field over the past 9000 yr. In the Northern Hemisphere we detect two distinct signals with drift rates of 0.09° and 0.25° yr−1 and dominant zonal wavenumbers of m  = 2 and 1, respectively. Comparisons with other geomagnetic field models support these observations but also highlight the importance of sedimentary data that provide crucial information on high-latitude geomagnetic field variations. The two distinct drift signals detected in the Northern Hemisphere can largely be decomposed into two westward propagating waveforms. We show that constructive interference between these two waveforms accurately predicts both the location and timing of previously observed high-latitude weak/reverse flux patches over the past 3–4 millennia. In addition, we also show that the 1125-yr periodicity signal inferred from the waveform interference correlates positively with variations in the dipole tilt over the same time period. The two identified drift signals may partially be explained by the westward motion of high-latitude convection rolls. However, the dispersion relation might also imply that part of the drift signal could be caused by magnetic Rossby waves riding on the mean background flow. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Nickel is the second most abundant element in the Earth's core. However, the properties of Fe‐Ni alloys are still poorly constrained under planetary cores conditions, in particular concerning the effect of Ni on the melting curve of Fe. Here we show that Ni alloying up to 36 wt% does not affect the melting curve of Fe up to 100 GPa. However, Ni strongly modifies the hexagonal‐closed‐packed/face‐centered‐cubic (hcp/fcc) phase boundary, pushing the hcp/fcc/liquid triple point of Fe‐20wt%Ni to higher pressures and temperatures. Our results allow constraining the triple point for Fe‐10wt%Ni, a composition relevant for the Earth interior, and point out a decrease of the melting temperature at core‐mantle boundary by 400 K with respect to pure Fe. A lower amount of light elements than previously predicted is thus required to reduce the crystallization temperature of core materials below that of a peridotitic lower mantle, in better agreement with geochemical observations. Plain Language Summary: The Earth's core is believed to be composed of Fe alloyed with Ni and several lighter elements. In this paper, we investigate the effect of Ni alloying on the Fe phase diagram. The main effect of Ni addition is to enlarge the pressure/temperature stability domain of the face‐centered‐cubic (fcc) phase with respect to the hexagonal‐closed‐packed (hcp) phase and to shift the hcp/fcc/liquid triple point to higher pressures and temperatures. This implies a depression of the melting curve of Fe‐Ni alloys by around 400 K at mantle boundary conditions, at Ni concentrations pertinent for the Earth interior. This consequently decreases the temperature of the liquidus for Fe alloys constituting the Earth's core, in turn implying a reduced amount of light elements than previously predicted. Key Points: Melting curve and phase diagram of Fe‐20wt%Ni and Fe‐36wt%Ni have been investigated by in situ X‐ray absorption up to 120 GPa and 3500 KNi alloying shifts the hcp/fcc/liquid triple point to higher pressures and temperaturesThe triple point for Fe‐10wt%Ni is predicted to be around 135 GPa and 3800 K fixing new benchmarks for the Earth's core composition [ABSTRACT FROM AUTHOR]

The subsolidus phase diagram of silica in the 80–220 GPa pressure range was determined by density functional theory (DFT). The transition pressures calculated using the generalized gradient approximation (GGA) in the static limit (at 0 K, without zero point vibrational energy) for the β-stishovite (CaCl2- structure) to seifertite and the seifertite to pyrite-type transitions are 95 and 213 GPa, respectively. These are in good agreement with those calculated using hybrid functionals, giving transition pressures of 96 and 215 GPa. This indicates that previous local density approximation (LDA) results underestimate the transition pressure by 10–15 GPa. Density functional perturbation theory calculations, carried out using GGA within the quasi-harmonic approximations, give Clapeyron slopes of 5.4 and –2.8 MPa/K for the β-stishovite to seifertite and seifertite to pyrite-type transitions, respectively. This suggests that the seifertite-forming transition occurs at 109 GPa (470 km above the core-mantle boundary, CMB) at an ambient mantle geotherm, whereas the pyrite-type transition occurs at 200 GPa (620 km below the CMB) at 4700 K, which is close to the core adiabat. We also calculate the equation of state and show that the stability of seifertite in the lowermost mantle contributes negative buoyancy to recycled oceanic crust, although not as much as in some previous studies. Nevertheless, the increased density of seifertite over β-stishovite may lead to layers with elevated proportions of basaltic material within the large low S-wave velocity provinces. The seifertite to pyrite-type silica transition in the outer core will affect the silica liquidus surface in the system Fe-Si-O and forms a basis for further investigations of silica crystallization in the protocore. [ABSTRACT FROM AUTHOR]

Garnet is one of the most robust and ubiquitous minerals that record element zoning during crustal metamorphism. In addition to major elements, zoning in trace elements can provide a wealth of information to document the changing conditions of garnet growth and modification. However, mapping trace elements at low concentrations, over large areas and with high resolution has remained a major challenge. We present a comprehensive investigation of the TE distribution in garnet from three Alpine samples that underwent a complex evolution at different metamorphic conditions. The TE distribution in garnet grains is mapped in 2D in thin section with a novel approach using laser ablation inductively coupled plasma time of flight mass spectrometry (LA-ICP-TOFMS) to achieve a lateral resolution of 5 µm and limits of detection for the heavy rare earth elements (REE) down to 0.2 µg/g. Comparison with major element zoning measured by electron probe microanalysis and trace elements measured by conventional LA-ICPMS spot analysis testifies to the accuracy of the measurements. Garnet in an amphibolite-facies metapelite from Campolungo, Central Alps, that recorded metamorphism to 600 °C preserves Y + REE trace element zoning that closely matches that of Ca. In this sample, there is no notable diffusive modification for trace elements. Y + REE zoning is dominated by Rayleigh fractionation in the core and by the sporadic breakdown of accessory phases producing annuli in the rim of the garnet. A granulite-facies garnet from Malenco, Eastern Central Alps, formed during subsolidus heating, followed by peritectic melting reactions up to temperatures of 800–850 °C. Major and trace element zoning are decoupled indicating diffusional resetting of major elements, whereas trace elements still largely document the growth history. Enrichment of trace elements in the garnet mantle may be related to the consumption of biotite (V, Cr) and the dissolution of zircon (Zr) and monazite (Y + REE) in the melt. Diffusion of Y + HREE at the core–mantle boundary occurred over a length scale of ~ 200 µm. Garnet in an eclogite from the Sesia Zone, Western Alps (P ~ 2 GPa, T ~ 600 °C), displays pronounced fluid-related veinlets, visible in FeO, MgO and MnO, which cross-cut the primary growth zoning. Surprisingly, complex Y + REE and Cr zoning is not affected by the veinlets, indicating that they did not form by a crack-seal mechanism but are rather related to a selective replacement process. The trace element maps provide a detailed insight into the growth and modification of garnet and thus allow assessment of equilibrium versus disequilibrium processes, and assist in determination of P–T conditions, garnet dating, diffusion modelling as well as documenting fluid-induced modifications. [ABSTRACT FROM AUTHOR]

Analyses of Lunar Laser Ranging data show that the spin symmetry axis of the Moon is ahead of its expected Cassini state by an angle of ϕp = 0.27 arcsec. This indicates the presence of one or more dissipation mechanisms acting on the lunar rotation. A combination of solid‐body tides and viscous core‐mantle coupling have been proposed in previous studies. Here, we investigate whether viscoelastic deformation within a solid inner core at the center of the Moon can also account for a part of the observed phase lead angle ϕp. We build a rotational dynamic model of the Cassini state of the Moon that comprises an inner core, a fluid core, and a mantle and where solid regions are allowed to deform viscoelastically in response to an applied forcing. We show that the presence of an inner core does not change the global monthly Q of the Moon and, hence, that the contribution from solid‐body tides to ϕp is largely unaffected by an inner core. However, we also show that viscoelastic deformation within the inner core, acting to realign its figure axis with that of the mantle, can contribute significantly to ϕp through inner core‐mantle gravitational coupling. We show that the contribution to ϕp is largest when the inner core viscosity is in the range of 1013 to 1014 Pa s, when the inner core radius is large and when the free inner core nutation frequency approaches a resonance with the precession frequency of 2π/18.6 yr−1. Plain Language Summary: Analyses of the lunar rotation have revealed that there exists one or more mechanisms that act to dissipate a part of the rotational energy. Previous mechanisms that have been suggested include friction at the boundary between the fluid core and the mantle, and a delayed response of a deformable "soft" mantle to tidal forces. Here, we investigate whether the dissipation of rotational energy could also originate from deformation within a soft solid inner core located at the center of the Moon. We show that if the inner core is sufficiently large and if its viscosity is sufficiently low, its contribution to the rotational energy dissipation is not negligible. Key Points: Viscoelastic deformation in the lowermost mantle and inner core of the Moon dissipates a part of the lunar rotation energyWe focus on the role of the inner core and show that maximal lead angles of the Cassini state result for viscosities of 1013–1014 Pa sThe largest dissipation occurs for large inner cores and when the free inner core nutation frequency is close to the precession frequency [ABSTRACT FROM AUTHOR]

The Cassini state equilibrium associated with the precession of the Moon predicts that the mantle, fluid core, and solid inner core precess at different angles. We present estimates of the dissipation from viscous friction associated with the differential precession at the core‐mantle boundary (CMB), Qcmb, and at the inner core boundary (ICB), Qicb, as a function of the evolving lunar orbit. We focus on the latter and show that, provided the inner core was larger than 100 km, Qicb may have been as high as 1010–1011 W for most of the lunar history for a broad range of core density models. This is larger than the power required to maintain the fluid core in an adiabatic state; therefore, the heat released by the differential precession at the ICB can drive a past lunar dynamo by thermal convection. This dynamo can outlive the dynamo from precession at the CMB and may have shut off only relatively recently. Estimates of the magnetic field strength at the lunar surface are of the order of a few μT, compatible with the lunar paleomagnetic intensities recorded after 3 Ga. We further show that it is possible that a transition of the Cassini state associated with the inner core may have occurred as a result of the evolution of the lunar orbit. The heat flux associated with Qicb can be of the order of a few mW m−2, which should slow down inner core growth and be included in thermal evolution models of the lunar core. Plain Language Summary: While the Moon today no longer has a large‐scale magnetic field generated by dynamo action in its small iron core, magnetic studies on lunar rocks collected during the Apollo missions suggest that it did in the past. However, the mechanism responsible for this dynamo is still debated. In this study, we investigate whether the precession motion of the Moon may have been capable to sustain a past lunar dynamo. The mantle, fluid core, and solid inner core of the Moon precess at different angles today. These precession angles were larger in the past when the Moon was orbiting closer to Earth. We calculate the dissipation generated by the viscous friction from the differential precession at the boundaries between the fluid core and the mantle (CMB) and between the fluid and solid cores (ICB). We focus of the latter and show that dissipation at the ICB in the past was high enough to drive thermal convection and sustain a dynamo. A lunar dynamo driven by this mechanism is long lived and may have shut off as late as 1 billion years ago and perhaps even more recently. Key Points: Viscous heating from the differential precession between the Moon's fluid and solid cores was high enough in the past to power a dynamoThe surface magnetic field that it produces is of the order of a few microTeslas, compatible with the lunar field recorded after 3 GaThe associated heat flux at the inner core boundary is an important contribution to the thermal evolution of the lunar core [ABSTRACT FROM AUTHOR]

Deep‐rooted mantle plumes are thought to originate from the margins of the Large Low Shear Velocity Provinces (LLSVPs) at the base of the mantle. Visible in seismic tomography, the LLSVPs are usually interpreted to be intrinsically dense thermochemical piles in numerical models. Although piles deflect lateral mantle flow upward at their edges, the mechanism for localized plume formation is still not well understood. In this study, we develop numerical models that show plumes rising from the margin of a dense thermochemical pile, temporarily increasing its local thickness until material at the pile top cools and the pile starts to collapse back toward the core‐mantle boundary (CMB). This causes dense pile material to spread laterally along the CMB, locally thickening the lower thermal boundary layer on the CMB next to the pile, and initiating a new plume. The resulting plume cycle is reflected in both the thickness and lateral motion of the local pile margin within a few hundred km of the pile edge, while the overall thickness of the pile is not affected. The period of plume generation is mainly controlled by the rate at which slab material is transported to the CMB, and thus depends on the plate velocity and the sinking rate of slabs in the lower mantle. A pile collapse, with plumes forming along the edges of the pile's radially extending corner, may, for example, explain the observed clustering of Large Igneous Provinces (LIPs) in the southeastern corner of the African LLSVP around 95–155 Ma. Plain Language Summary: Deep‐rooted upwellings in the Earth's mantle, so‐called "plumes", are responsible for volcanism such as Hawaii and seem to cluster around two continent‐sized regions at the core‐mantle boundary that show anomalously low seismic velocities. These regions have been suggested to have a different composition than the surrounding mantle, causing them to be denser and stiffer which allows them to survive billions of years at the base of the mantle without being completely eroded. In this study, we use numerical simulations to show that mantle plumes and dense piles at the core‐mantle boundary interact with each other, potentially resulting in a periodic plume initiation. A starting upwelling increases the pile thickness locally by pulling dense pile material upward. It then cools down, causing a density increase that results in gravitational collapse of the dense material toward the core‐mantle boundary. As a result, this material spreads along the boundary and pushes hot ambient mantle in front of it, thereby triggering a new upwelling, and the cycle starts from the beginning. The main control on how often upwellings are generated is the rate at which material from subduction zones at the Earth's surface reaches the core‐mantle boundary, which relates to the velocity of tectonic plates. Key Points: Plumes can be periodically generated at the margins of thermochemical piles by local pile collapseVariations in pile thickness and lateral motion of the pile edge reflect cycles of plume initiationThe period of plume cycles is controlled by plate velocity and the sinking rate of slabs [ABSTRACT FROM AUTHOR]

During the last decade, rapid or extreme geomagnetic field intensity variations associated with rates greater than the maximum currently observed have been inferred from archeomagnetic data in the Near-East and in Western Europe. The most extreme events, termed geomagnetic spikes, are defined as intensity peaks occurring over a short time (a few decades), and are characterized by high variation rates, up to several μT yr–1. Magnetic flux expulsion from the Earth's outer core has been suggested as one possible explanation for these peaks but has not yet been examined in detail. In this study, we develop a 2-D kinematic model for magnetic flux expulsion whose key control parameter is the magnetic Reynolds number R m , the ratio of magnetic diffusion time to advection time. This model enables the tracking of magnetic field lines which are distorted and folded by a fixed flow pattern. Two processes govern the magnetic evolution of the system. The first is the expulsion of magnetic flux from closed streamlines, whereby flux gradually concentrates near the boundaries of the domain, which leads to an increase of the magnetic energy of the system. If the upper boundary separates the conducting fluid from an insulating medium, a second process then takes place, that of diffusion through this interface, which we can quantify by monitoring the evolution of the vertical component of magnetic induction along this boundary. It is the conjunction of these two processes that defines our model of magnetic flux expulsion through the core–mantle boundary. We analyse several configurations with varying flow patterns and magnetic boundary conditions. We first focus on flux expulsion from a single eddy. Since this specific configuration has been widely studied, we use it to benchmark our implementation against analytic solutions and previously published numerical results. We next turn our attention to a configuration which involves two counter-rotating eddies producing an upwelling at the centre of the domain, and comprises an upper boundary with an insulating medium. We find that the characteristic rise time and maximum instantaneous variation rate of the vertical component of the magnetic field that escapes the domain scale like |$\sim R_m^{0.15}$| and |$\sim R_m^{0.45}$|⁠ , respectively. Extrapolation of these scaling laws to the Earth's régime is compared with various purported archeointensity highs reported in the Near-East and in Western Europe. According to our numerical experiments magnetic flux expulsion is unlikely to produce geomagnetic spikes, while intensity peaks of longer duration (one century and more) and smaller variation rates appear to be compatible with this process. [ABSTRACT FROM AUTHOR]

Observations of seismic anisotropy in the lowermost mantle—D″—are abundant. As seismic anisotropy is known to develop as a response to plastic flow in the mantle, constraining lowermost mantle anisotropy allows us to better understand mantle dynamics. Measuring shear-wave splitting in body wave phases which traverse the lowermost mantle is a powerful tool to constrain this anisotropy. Isolating a signal from lowermost mantle anisotropy requires the use of multiple shear-wave phases, such as SKS and SKKS. These phases can also be used to constrain azimuthal anisotropy in D″: the ray paths of SKS and SKKS are nearly coincident in the upper mantle but diverge significantly at the core–mantle boundary. Any significant discrepancy in the shear-wave splitting measured for each phase can be ascribed to anisotropy in D″. We search for statistically significant discrepancies in shear-wave splitting measured for a data set of 420 SKS–SKKS event–station pairs that sample D″ beneath the Eastern Pacific. To ensure robust results, we develop a new multiparameter approach which combines a measure derived from the eigenvalue minimization approach for measuring shear-wave splitting with an existing splitting intensity method. This combined approach allows for easier automation of discrepant shear-wave splitting analysis. Using this approach we identify 30 SKS–SKKS event–station pairs as discrepant. These predominantly sit along a backazimuth range of 260°–290°. From our results we interpret a region of azimuthal anisotropy in D″ beneath the Eastern Pacific, characterized by null SKS splitting, and mean delay time of |$1.15 \, \mathrm{ s}$| in SKKS. These measurements corroborate and expand upon previous observations made using SKS–SKKS and S–ScS phases in this region. Our preferred explanation for this anisotropy is the lattice-preferred orientation of post-perovskite. A plausible mechanism for the deformation causing this anisotropy is the impingement of subducted material from the Farallon slab at the core–mantle boundary. [ABSTRACT FROM AUTHOR]

Seismology provides important constraints on the structure and dynamics of the deep mantle. Computational and methodological advances in the past two decades improved tomographic imaging of the mantle and revealed the fine-scale structure of plumes ascending from the core-mantle boundary region and slabs of oceanic lithosphere sinking into the lower mantle. We discuss the modeling aspects of global tomography including theoretical approximations, data selection, and model fidelity and resolution. Using spectral, principal component, and cluster analyses, we highlight the robust patterns of seismic heterogeneity, which inform us of flow in the mantle, the history of plate motions, and potential compositionally distinct reservoirs. In closing, we emphasize that data mining of vast collections of seismic waveforms and new data from distributed acoustic sensing, autonomous hydrophones, ocean-bottom seismometers, and correlation-based techniques will boost the development of the next generation of global models of density, seismic velocity, and attenuation. ▪ Seismic tomography reveals the 100-km to 1,000-km scale variation of seismic velocity heterogeneity in the mantle. ▪ Tomographic images are the most important geophysical constraints on mantle circulation and evolution. [ABSTRACT FROM AUTHOR]

Seismic studies show two antipodal regions of lower shear velocity at the core–mantle boundary (CMB) called large low-shear-velocity provinces (LLSVPs). They are thought to be thermally and chemically distinct and therefore might have a different density and viscosity than the ambient mantle. Employing a composite rheology, using both diffusion and dislocation creep, we investigate the influence of grain size evolution on the dynamics of thermochemical piles in evolutionary geodynamic models. We consider a primordial layer and a time-dependent basalt production at the surface to dynamically form the present-day chemical heterogeneities, similar to earlier studies, e.g. by. Our results show that, relative to the ambient mantle, grain size is higher inside the piles, but, due to the high temperature at the CMB, the viscosity is not remarkably different from ambient mantle viscosity. We further find that although the average viscosity of the detected piles is buffered by both grain size and temperature, the viscosity is influenced predominantly by grain size. In the ambient mantle, however, depending on the convection regime, viscosity can also be predominantly controlled by temperature. All pile properties, except for temperature, show a self-regulating behaviour: although grain size and viscosity decrease when downwellings or overturns occur, these properties quickly recover and return to values prior to the downwelling. We compute the necessary recovery time and find that it takes approximately 400 Myr for the properties to recover after a resurfacing event. Extrapolating to Earth values, we estimate a much smaller recovery time. We observe that dynamic recrystallisation counteracts grain growth inside the piles when downwellings form. Venus-type resurfacing episodes reduce the grain size in piles and ambient mantle to a few millimetres. More continuous mobile-lid-type downwellings limit the grain size to a centimetre. Consequently, we find that grain-size-dependent viscosity does not increase the resistance of thermochemical piles to downgoing slabs. Mostly, piles deform in grain-size-sensitive diffusion creep, but they are not stiff enough to counteract the force of downwellings. Hence, we conclude that the location of subduction zones could be responsible for the location and stability of the thermochemical piles of the Earth because of dynamic recrystallisation. [ABSTRACT FROM AUTHOR]

After accretion and formation, terrestrial planets go through at least one magma ocean episode. As the magma ocean crystallises, it creates the first layer of solid rocky mantle. Two different scenarios of magma ocean crystallisation involve that the solid mantle either (1) first appears at the core-mantle boundary and grows upwards, or (2) appears at mid-mantle depth and grows in both directions. Regardless of the magma ocean freezing scenario, the composition of the solid mantle and liquid reservoirs continuously change due to fractional crystallisation. This chemical fractionation has important implications for the long-term thermo-chemical evolution of the mantle, as well as its present-day dynamics and composition. In this work we use numerical models to study convection in a solid mantle bounded at either or both boundaries by magma ocean(s), and in particular, the related consequences for large-scale chemical fractionation. We use a parameterisation of fractional crystallisation of the magma ocean(s) and (re-)melting of solid material at the interface between these reservoirs. When these crystallisation/re-melting processes are taken into account, convection in the solid mantle occurs readily and is dominated by large wavelengths. Related material transfer across the mantle magma-ocean boundaries promotes chemical equilibrium, and prevents extreme enrichment of the last-stage magma ocean (as would otherwise occur due to pure fractional crystallisation). The timescale of equilibration depends on the convective vigour of mantle convection and on the efficiency of material transfer between the solid mantle and magma ocean(s). For Earth, this timescale is comparable to that of magma ocean crystallisation suggested in previous studies (Lebrun et al., 2013), which may explain why the Earth's mantle is rather homogeneous in composition, as supported by geophysical constraints. [ABSTRACT FROM AUTHOR]

The degree to which the lowermost mantle influences behaviour of the geodynamo has been debated over the past quarter century. Our analysis of a comprehensive set of 17 Cenozoic palaeomagnetic transitional field records obtained from lavas in the Southern Hemisphere provides robust evidence of stable mantle control since the Pliocene. The records come from a region where—given a significantly weakened axial dipole—the magnetic field today would be largely controlled by the non-axial dipole (NAD) flux patch currently emanating from Earth's outer core beneath western Australia. The palaeomagnetic recording sites from west to east include the south Indian Ocean, eastern Australia, New Zealand and French Polynesia. The analysed records contain from 2 to 26 sequential transitional virtual geomagnetic poles (VGPs). 10 of the 17 records supply at least one VGP within a narrow longitudinal band 10°-wide between 60°S and the equator, centred along 102.4°E. That is, transitional data from 59 per cent of the Cenozoic recordings are found to reside in a region that encompasses a mere 2.8 per cent of the VGP transitional area on Earth's surface. A robust Monte Carlo approach applied to this data set, one that takes into account the number of transitional VGPs contained in each record, finds this result highly improbable (p-value  = 0.0006). The present-day pattern of vertical flux at the core–mantle boundary shows an anomalously strong, thin Southern Hemisphere longitudinal band off the west coast of Australia that strikingly coincides with this unusual palaeomagnetic finding. We conclude with a high degree of confidence that this band of flux has remained virtually unmoved for at least the past 3 Myr. Seemingly independent of the behaviour of the axial dipole, our findings indicate that it has dominated the magnetic field over an area of considerable size during attempts by the geodynamo to reverse polarity. [ABSTRACT FROM AUTHOR]

Comparing compositional models of the terrestrial planets provides insights into physicochemical processes that produced planet-scale similarities and differences. The widely accepted compositional model for Mars assumes Mn and more refractory elements are in CI chondrite proportions in the planet, including Fe, Mg, and Si, which along with O make up >90% of the mass of Mars. However, recent improvements in our understandings on the composition of the solar photosphere and meteorites challenge the use of CI chondrite as an analog of Mars. Here we present an alternative model composition for Mars that avoids such an assumption and is based on data from Martian meteorites and spacecraft observations. Our modeling method was previously applied to predict the Earth's composition. The model establishes the absolute abundances of refractory lithophile elements in the bulk silicate Mars (BSM) at 2.26 times higher than that in CI carbonaceous chondrites. Relative to this chondritic composition, Mars has a systematic depletion in moderately volatile lithophile elements as a function of their condensation temperatures. Given this finding, we constrain the abundances of siderophile and chalcophile elements in the bulk Mars and its core. The Martian volatility trend is consistent with ⩽ 7 wt% S in its core, which is significantly lower than that assumed in most core models (i.e., >10 wt% S). Furthermore, the occurrence of ringwoodite at the Martian core-mantle boundary might have contributed to the partitioning of O and H into the Martian core. [ABSTRACT FROM AUTHOR]

Small‐scale heterogeneities contain important information on the composition of the Earth's interior and are vital in understanding the dynamics and evolution of the Earth. Here we explore a global data set of PKP waves in both array and network data and search for scattered waves arriving as precursors to PKPdf. We classify the observed precursors into incoherent and coherent types and find that scattering regions for these two types overlap at the base of the mantle. In addition, a global map of scattering strength displays strong regional variations with strong scattering mixed with weak scattering. The complex distribution of scattering strength indicates lateral variations of heterogeneities on short length scales in the lowermost mantle. A mixture model of localized strong scatterers embedded in a random medium could explain the observed precursors in this study. Plain Language Summary: The properties of small‐scale seismic scattering heterogeneities are crucial for understanding the composition and evolution of the Earth's interior. However, the nature of these scattering heterogeneities is widely debated. Here we analyze a global data set of seismic observations to investigate this problem. Our results show that there exist strong lateral variations of scattering heterogeneities on short length scales in the lowermost mantle. Furthermore, scattering heterogeneities show different scattering modes that overlap with each other. Therefore, we propose a mixture model of localized strong seismic heterogeneities surrounded by weaker/smaller‐scale heterogeneities to explain the observations in our study. This model indicates that the small‐scale heterogeneities in the Earth's lowermost mantle are more complex than thought before, implying complex chemical mixing processes and dynamic convections in the deepest mantle. Key Points: We find coherent and incoherent precursors generated by small‐scale scattering heterogeneities in the lower mantleScattering strength displays strong regional variations with strong scattering mixed with weak scattering in the lowermost mantleA mixture model of localized strong seismic heterogeneities surrounded by weaker/smaller‐scale heterogeneities is proposed [ABSTRACT FROM AUTHOR]

Previous geodynamic studies have indicated that the presence of a compositionally anomalous and intrinsically dense (CAID) mantle component can impact both core heat flux and surface features, such as plate velocity, number and size. Implementing spherical annulus geometry mantle convection models, we investigate the influence of intrinsically dense material in the lower mantle on core heat flux and the surface velocity field. The dense component is introduced into a system that features an established plate-like surface velocity field, and subsequently we analyse the evolution of the surface velocity as well as the interior thermal structure of the mantle. The distribution and mobility of the CAID material is investigated by varying its buoyancy ratio relative to the ambient mantle (ranging from 0.7 to 1.5), its total volume (3.5–10 per cent of the mantle volume) and its intrinsic viscosity (0.01–100 times the ambient mantle viscosity). We find at least three distinct distributions of the dense material can occur adjacent to the core–mantle boundary (CMB), including multiple piles of varying topography, a core enveloping layer and two diametrically opposed provinces (which can on occasion break into three distinct piles). The latter distribution mimics the morphology of the seismically observed large low shear wave velocity provinces (LLSVPs) and can occur over the entire range of CAID material viscosities. However, diametrically opposed provinces occur primarily in cases with CAID material buoyancy numbers of 0.7–0.85 (corresponding to contrasts in density between ambient and CAID material of 130 and 160 kg m−3, respectively) in our model (with an effective Rayleigh number of order 106). Steep and high topography piles are also obtained for cases featuring buoyancy ratios of 0.85 and viscosities 10–100 times that of the ambient mantle. An increase in relative density, as well as larger volumes of CAID material, lead to the development of a core enveloping layer. Our findings show that when two provinces are present core heat flux can be reduced by up to 50 per cent relative to cases in which CAID material is absent. Surface deformation quantified by Plateness is minimally influenced by variation of the properties of the dense material. Surface velocity is found to be reduced in general but mostly substantially in cases featuring high CAID material viscosities and large volumes (i.e. 10 per cent) or buoyancy ratios. [ABSTRACT FROM AUTHOR]

Plate tectonics describes the horizontal motions of lithospheric plates, the Earth's outer shell, and interactions among them across the Earth's surface. Since the establishment of the theory of plate tectonics about half a century ago, considerable debates have remained regarding the driving forces for plate motion. The early "Bottom up" view, i.e., the converting mantle-driven mechanism, states that mantle plumes originating from the core-mantle boundary act at the base of plates, accelerating continental breakup and driving plate motion. Toward the present, however, the "Top down" idea is more widely accepted, according to which the negative buoyancy of oceanic plates is the dominant driving force for plate motion, and the subducting slabs control surface tectonics and mantle convection. In this regard, plate tectonics is also known as subduction tectonics. "Top down"tectonics has received wide supports from numerous geological and geophysical observations. On the other hand, recent studies indicate that the acceleration/deceleration of individual plates over the million-year timescale may reflect the effects of mantle plumes. It is also suggested that surface uplift and subsidence within stable cratonic areas are correlated with plume-related magmatic activities over the hundred-million-year timescale. On the global scale, the cyclical supercontinent assembly and breakup seem to be coupled with superplume activities during the past two billion years. These correlations over various spatial and temporal scales indicate the close relationship and intensive interactions between plate tectonics and plume tectonics throughout the history of the Earth and the considerable influence of plumes on plate motion. Indeed, we can acquire a comprehensive understanding of the driving forces for plate motion and operation mechanism of the Earth's dynamic system only through joint analyses and integrated studies on plate tectonics and plume tectonics. [ABSTRACT FROM AUTHOR]

The short-lived 182Hf-182W isotope system (t 1/2 = 9 Ma) left evidence in both ancient and modern terrestrial rock record of processes that took place during the earliest stages of Earth's accretionary and differentiation history. We report µ182W values (the deviation of 182W/184W of a sample from that of laboratory standards, in parts per million) and corresponding 3He/4He ratios for rocks from 15 different hotspots. These rocks are characterized by µ182W values that range from ∼0 to as low as −23 ± 4.5. For each volcanic system that includes rocks with negative µ182W values, the values tend to be negatively correlated with 3He/4He. The W-He isotopic characteristics of all samples can be successfully modeled via mixing involving at least three mantle source reservoirs with distinct µ182W-3He/4He characteristics. One reservoir has 3He/4He ≈ 8 R/R A and μ182W ≈ 0, which is indistinguishable from the convecting upper mantle. Based on high 3He/4He, the other two reservoirs are presumed to be relatively un-degassed and likely primordial. One reservoir is characterized by µ182W ≈ 0, while the other is characterized by µ182W ≤ −23. The former reservoir likely formed from a silicate differentiation process more than 60 Myr after the origin of the solar system, but has remained partially or wholly isolated from the rest of the mantle for most of Earth history. The latter reservoir most likely includes a component that formed while 182Hf was extant. Mass balance constraints on the isotopic composition of the core suggest it has a strongly negative µ182W value of ∼−220. Thus, it is a candidate for the origin of the negative µ182W in the plume sources. Mixing models show that the direct addition of outer core metal into a plume rising from the core-mantle boundary would result in collateral geochemical effects, particularly in the abundances of highly siderophile elements, which are not observed in OIB. Instead, the reservoir characterized by negative µ182W most likely formed in the lowermost mantle as a result of core-mantle isotopic equilibration. The envisioned equilibration process would raise the W concentration and lower the µ182W of the resulting silicate reservoir, relative to the rest of the mantle. The small proportion (<0.3 %) of this putative core-mantle equilibrated reservoir required to account for the µ182W signatures observed in OIB is insufficient to result in observable effects on most other elemental and/or isotopic compositions. The presumed primordial reservoirs may be linked to seismically distinct regions in the lower mantle. Seismically imaged mantle plumes appear to preferentially ascend from the vicinity of large low-shear velocity provinces (LLSVPs), which have been interpreted as thermochemical piles. We associate the LLSVPs with the primordial reservoir characterized by high 3He/4He and µ182W = 0. Smaller, ultra-low velocity zones (ULVZs) present at the core-mantle boundary have been interpreted to consist of (partially) molten lower mantle material. The negative µ182W signatures observed in some plume-derived lavas may result from small contributions of ULVZ material that has inherited its negative µ182W signature through core-mantle equilibration. [ABSTRACT FROM AUTHOR]

During the Galileo spacecraft's flyby of Europa, magnetic field measurements detected an inductive signal due to the response of Europa's interior conductors to temporal fluctuations in the Jovian magnetic field. In contrast, no signatures of intrinsic magnetic field originating from the dynamo motion in the metallic core were acquired. These measurements suggest that a global sub-surface ocean containing electrolytes exists beneath the solid ice shell and that the metallic core lacks convection. Europa's interior is expected to be divided into the metallic core, rocky mantle and hydrosphere based on the moment of inertia factor estimated from gravity field measurements. Specifically, the thickness of the outermost water layer is 120 – 170 km, and the radius of the metallic core is 0.12 – 0.43 times the surface radius. No systematic investigation of Europa's internal evolution has been conducted to estimate the current state of the subsurface ocean and to explain the absence of a core dynamo field within such uncertainty for internal structure and material properties (especially ice properties). Herein, I performed a numerical simulation of the long-term thermal evolution of Europa's interior and investigated the temporal changes in the ocean thickness as well as the temperature and heat flow of the metallic core. If the ice reference viscosity is greater than 5 × 1014 Pa s, the sub-surface ocean can persist even in the absence of tidal heating. In the case of a tidal heating of 10 mW/m2 and 20 mW/m2, the ice shell thickness is ≤ 90 km if the ice reference viscosity is ≥ 1 × 1015 and 1 × 1014 Pa s, respectively. Regardless of the ice reference viscosity, if the tidal heating is ≥ 50 mW/m2, the shell thickness will be ≤ 40 km. The thermal history of the metallic core is determined by the hydrosphere thickness and the metallic core density, and is unaffected by variations in the ice shell (ocean) thickness. Preferred conditions for the absence of the core dynamo include CI chondritic abundance for the long-lived radioactive isotopes, lower initial core–mantle boundary (CMB) temperature and thicker hydrosphere. The core may be molten without convection if the composition is near the eutectic in a Fe–FeS alloy, or not molten (without convection) if the composition is near the Fe or FeS endmember. Specifically, if the rocky mantle has a CI chondritic radioisotope abundance, any core composition and hydrosphere thickness allow the absence of the core dynamo if the initial temperature at the CMB is lower than 1,250 K. If the rocky mantle has the ordinary chondritic radioisotope abundance, or a higher initial temperature (∼ 1,500 K) at the CMB, the core density lower than 6,000 kg/m3 is preferred for the absence of the core dynamo. In the case of the core composition near the eutectic one, a hydrosphere thicker than 150 km is required for the lacking core dynamo. The lower pressure of Europa's rocky mantle due to its thinner hydrosphere compared with that of Ganymede may facilitate heat transfer in the mantle, lowering its temperature and making dynamo motion more challenging. [Display omitted] • Evolutional models for Europa's ocean without a core magnetic field are presented. • Found conditions for ice viscosity and tidal heating to sustain the ocean now. • Narrowed interior structure range than only the moment of inertia-based model. [ABSTRACT FROM AUTHOR]

• Acoustic and electrical measurements were performed on ilmenite-rutile aggregates. • Experiments were conducted at the pressure and temperature conditions of the lunar mantle. • Fe-Ti oxides increase the electrical conductivity and the seismic velocities of the lunar mantle. • Field datasets are reproduced with overturned ilmenite-bearing cumulates containing 20% melt. The overturn of titanium-rich mantle cumulates has been invoked to explain the structure and dynamics of the Moon. These dense cumulates are stable at the core-mantle boundary (CMB) and could explain field observations inferred from geophysical studies. We report acoustic and electrical experiments on natural ilmenite-rutile aggregates up to 4.5 GPa and 1920 K. Seismic velocities show a weak pressure and temperature dependence, with Vs ∼4.2 (+/-0.2) km/s and Vp ∼ 8.0 (+/-0.2) km/s at the CMB conditions. Conductivity increases by a factor of 104 from 373 to 1920 K and is >103 S/m above 1573 K. Seismic and electrical models for the lunar mantle based on our results, considering mixtures of Fe-Ti oxides and olivine, indicate that field velocity and conductivity estimates are reproduced satisfactorily with 3–16 vol.% Fe-Ti oxides and 20 vol.% melt. Interactions between a Ti-rich, melt-bearing layer and the adjacent core likely affect the cooling and magnetic history of the Moon. [ABSTRACT FROM AUTHOR]

Earth's magnetic field changes in both space and time: the temporal changes are called geomagnetic and paleomagnetic secular variations. Westward drift has been noted as a feature of secular variation for several centuries, but eastward drift has received less attention. We use three global geomagnetic field models covering the past 100 kyr to extend temporal coverage for tracking the zonal (azimuthal) motion of the radial magnetic field. The models we use are GGF100k (100–0 ka), GGFSS70 (70–15 ka), LSMOD.2 (50–30 ka); the extent of the models enables the inclusion of the extreme secular variations found during excursions, particularly the Laschamp excursion (42–40 ka). GGFSS70 and LSMOD.2 have higher temporal resolution than GGF100k, but their underlying data have poorer spatial coverage. Spatial structure is greatly diminished in all models for spherical harmonic degrees l > 4. We use two types of time-longitude plots, one of the full radial field to expose reverse and intense flux patches at the core-mantle boundary. The second time-longitude plot is processed to enhance zonal motion signatures and allows us to use Radon drift analyses to uncover characteristic time scales of both westward and eastward drift at mid to high latitudes in both the northern and southern hemispheres. Our results differ across the three models, which we attribute to varying degrees of resolution, accuracy, and data distribution. Nevertheless, recurrent episodes of both eastward and westward drift ranging from ± 0.05 o / yr to ± 0.18 o / yr occur in both the northern and southern hemispheres. Westward drift dominates. We also observe 8–20 kyr intervals between occurrences of high-latitude reverse flux patches correlated with strong drift signals. Focusing on the period 50–30 ka, we observe dominant eastward drift preceding the Laschamp excursion and westward drift subsequently. In a period not associated with an excursion, 90–80 ka, we see strong mid to high latitude drift signatures in the northern hemisphere. • Time-longitude plots of zonal drift in the geomagnetic field span 100–0 ka. • Both eastward and westward drift are recurring features. • A transition in drift direction occurs during the Laschamp excursion. [ABSTRACT FROM AUTHOR]

Sanne Cottaar, Carl Martin, Stuart Russell, and Lisanne Jagt explore the surprisingly variable nature – and unsatisfactory terminology – of Earth's core-mantle boundary [ABSTRACT FROM AUTHOR]

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.

Abstract Diffracted waves around Earth’s core could provide important information of the lowermost mantle that other seismic waves may not. We examined PKKPab diffraction waves from 52 earthquakes occurring at the western Pacific region and recorded by USArray to probe the velocity structure along the core–mantle boundary (CMB). These diffracted waves emerge at distances up to 10° past the theoretical cutoff epicentral distance and show comparable amplitudes. We measured the ray parameters of PKKPab diffraction waves by Radon transform analysis that is suitable for large-aperture arrays. These ray parameters show a wide range of values from 4.250 to 4.840 s/deg, suggesting strong lateral heterogeneities in sampling regions at the base of the mantle. We further estimated the P-wave velocity variations by converting these ray parameters and found the CMB regions beneath the northwestern edge of African Anomaly (Ritsma et al. in Science 286:1925–1928, 1999) and southern Sumatra Islands exhibit velocity reductions up to 8.5% relative to PREM. We suggest that these low velocity regions are Ultra-low velocity zones, which may be related to partial melt or iron-enriched solids. Graphical abstract .

A discontinuity in the seismic velocity associated with the lithosphere-asthenosphere interface, known as the Gutenberg discontinuity, is enigmatic in its origin. While partial mantle melts are frequently suggested to explain this discontinuity, it is not well known which factors critically regulate the melt production. Here, we report geochemical evidence showing that the melt fractions in the lithosphere-asthenosphere boundary were enhanced not only by accumulation of compacted carbonated melts related to recycled ancient marine sediments, but also by partial melting of a pyroxene-rich mantle domain related to the recycled oceanic eclogite/pyroxenites. This conclusion is derived from the first set of Mg isotope data for a suite of young petit-spot basalts erupted on the northwest Pacific plate, where a clearly defined Gutenberg discontinuity exists. Our results reveal a specific linkage between the Gutenberg discontinuity beneath the normal oceanic regions and the recycling of ancient subducted crust and carbonate through the deep Earth. The seismic Gutenberg discontinuity has long been associated with the lithosphere-asthenosphere boundary, yet the physical explanation of what is causing the discontinuity remains debated. Here, the authors report geochemical evidence, including Mg isotopes, and suggest that melting of recycled crust is responsible for the Gutenberg discontinuity. [ABSTRACT FROM AUTHOR]