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High-resolution full waveform seismic tomography of the Earth's mantle beneath the south and central Atlantic Ocean brings into focus a series of asthenospheric low shear velocity channels, or "fingers" on both sides of the southern and central mid-Atlantic ridge (MAR), elongated in the direction of absolute plate motion with a spacing of ~1,800 to 2,000 km, and associated with bands of shallower residual seafloor depth anomalies that suggest channeled flow over thousands of kilometers. Each of the three most clearly resolved fingers on the African side of the MAR corresponds to a separate group of whole mantle plumes rooted in distinct patches at the core-mantle boundary, feeding hotspots, and volcanic lines with distinct isotopic signatures. Plumes of a given group appear to merge at the top of the lower mantle before separating again, suggesting interaction of deep mantle flow with a more vigorous mesoscale circulation in the upper mantle. The corresponding hotspots are generally offset from the location of the deep mantle plume roots. The distinct isotopic signatures of these hotspot groups are also detected in the mid-ocean ridge basalts at the location where the fingers meet the ridge. Meanwhile, at least some of the variability within each plume group could originate in the upper mantle and extended transition zone where plumes in a given group appear to merge and pond. This study also adds to mounting evidence that the African large low shear velocity province is not a uniform, unbroken pile of dense material rising high above the core-mantle boundary, but rather a collection of mantle plumes rooted in patches of distinct composition. [ABSTRACT FROM AUTHOR]

Recent advancements in seismic data analysis have enhanced our grasp of the seismic heterogeneities near the core–mantle boundary (CMB). Through seismic tomography, persistent lower-mantle structures like the large low shear velocity provinces (LLSVPs) beneath the Pacific and South Africa have been identified. However, variations in the finer-scale features across different models raise questions about their origins. This study utilizes differential travel-time measurements from the USArray, operational across the contiguous United States from 2007 to 2014, to examine the impact of upper-mantle heterogeneities on tomographic models. By averaging the P-wave travel times and calibrating them with diffracted P-waves at the same stations, we mitigate the effects of shallow heterogeneities. The findings confirm that this method accurately maps the seismic anomalies beneath the USArray, consistent with other North American studies. Calibrated Pdiff travel-time data indicate significant anomalies in the mid-Pacific and Bering Sea and lesser anomalies in the northern Pacific, aligning with the global tomographic images. Moreover, the study highlights sharp travel-time variations over short distances, such as those across the northern boundary of the mid-Pacific anomaly, suggesting a chemically heterogeneous Pacific LLSVP. [ABSTRACT FROM AUTHOR]

The electrical resistivity (ρ) and thermal conductivity (κ) of the Earth's core compositions are essential parameters for constraining the core's thermal state, the inner core age, and the evolutionary history of the geodynamo. However, controversies persist between experimental and computational results regarding the electronic transport properties (ρ and κ) of the Earth's core. Iron is the major element in the core, and its transport properties under high-pressure and high-temperature conditions are crucial for understanding the core's thermal state. We measured the ρ values of solid iron using the four-wire van der Pauw method at 300 K and pressures of 3 to 26 GPa within a multi-anvil press. For comparison, we calculated the ρ and κ values of hexagonal close-packed (hcp) iron at 300–4100 K and 22–136 GPa using the first-principles molecular dynamics (FPMD) method. Our calculations generally align with prior studies, indicating that the electrical resistivity of solid hcp iron at Earth's core-mantle boundary (CMB) conditions is ~76–83 μΩ∙cm. The resistivity of hcp iron changes slightly as it melts from solid to liquid at pressures from 98 to 134 GPa. The effects of temperature and pressure on the Lorenz numbers of solid hcp iron were investigated according to our calculation results and previous studies. Under the CMB's pressure conditions, the κ of hcp iron initially decreases with increasing temperature and subsequently increases. The electron-electron scattering plays a dominant role at low temperatures and causes the decrease in κ. At high temperatures, the increase of electronic specific heat significantly increases the Lorentz number and κ. Overall, we estimate the κ of solid hcp iron at the CMB's condition to be 114 ± 6 W/m/K, slightly lower than the room temperature value of 129 ± 9 W/m/K at the same pressure. Our model shows that a 0–525 km thickness of a thermally stratified layer may exist beneath the Earth's CMB, depending on the core's heat flow and thermal conductivity. [ABSTRACT FROM AUTHOR]

Davemaoite (CaSiO3 perovskite) is considered the third most abundant phase in the pyrolytic lower mantle and the second most abundant phase in the subducted mid-ocean ridge basalt (MORB). During the partial melting of the pyrolytic upper mantle, incompatible titanium (Ti) becomes enriched in the basaltic magma, forming Ti-rich MORB. Davemaoite is considered an important Ti-bearing mineral in subducted slabs by forming a Ca(Si,Ti)O3 solid solution. However, the crystal structure and compressibility of Ca(Si,Ti)O3 perovskite solid solution at relevant pressure and temperature conditions had not been systematically investigated. In this study, we investigated the structure and equations of state of Ca(Si0.83Ti0.17)O3 and Ca(Si0.75Ti0.25)O3 perovskites at room temperature up to 82 and 64 GPa, respectively, by synchrotron X-ray diffraction (XRD). We found that both Ca(Si0.83Ti0.17)O3 and Ca(Si0.75Ti0.25)O3 perovskites have a tetragonal structure up to the maximum pressures investigated. Based on the observed data and compared to pure CaSiO3 davemaoite, both Ca(Si0.83Ti0.17)O3 and Ca(Si0.75Ti0.25)O3 perovskites are expected to be less dense up to the core-mantle boundary (CMB), and specifically ~1–2% less dense than CaSiO3 davemaoite in the pressure range of the transition zone (15–25 GPa). Our results suggest that the presence of Ti-bearing davemaoite phases may result in a reduction in the average density of the subducting slabs, which in turn promotes their stagnation in the lower mantle. The presence of low-density Ti-bearing davemaoite phases and subduction of MORB in the lower mantle may also explain the seismic heterogeneity in the lower mantle, such as large low shear velocity provinces (LLSVPs). [ABSTRACT FROM AUTHOR]

Within the last decade, thin ultra-low velocity zone (ULVZ) layering, sitting directly on top of the core–mantle boundary (CMB), has begun to be investigated using the flip-reverse-stack (FRS) method. In this method, pre- and post-cursor arrivals that are symmetrical in time about the ScS arrival, but with opposite polarities, are stacked. This same methodology has also been applied to high velocity layering, with indications that ultra-high velocity zones (UHVZs) may also exist. Thus far, studies using the FRS technique have relied on 1-D synthetic predictions to infer material properties of ULVZs. 1-D ULVZ models predominantly show a SdS precursor that reflects off the top of the ULVZ and an ScscS reverberation within the ULVZ that arrives as a post-cursor. 1-D UHVZ models are more complex and have a different number of arrivals depending on a variety of factors including UHVZ thickness, velocity contrast, and lateral extent. 1-D modelling approaches assume that lower mantle heterogeneity is constant and continuous everywhere across the lower mantle. However, lower mantle features display lateral heterogeneity and are either finite in extent or display local thickness variations. We examine the interaction of the ScS wavefield with ULVZs and UHVZs in 2.5-D geometries of finite extent. We show that multiple additional arrivals exist that are not present in 1-D predictions. In particular, multipath ScS arrivals as well as additional post-cursor arrivals are generated. Subsequent processing by the FRS method generates complicated FRS traces with multiple peaks. Furthermore, post-cursor arrivals can be generated even when the ScS ray path does not directly strike the heterogeneity from above. Analysing these predictions for 2.5-D models using 1-D modelling techniques demonstrates that a cautious approach must be adopted in utilization and interpretation of FRS traces to determine if the ScS wavefield is interacting with a ULVZ or UHVZ through a direct strike on the top of the feature. In particular, traveltime delays or advances of the ScS arrival should be documented and symmetrical opposite polarity arrivals should be demonstrated to exist around ScS. The latter can be quantified by calculation of a time domain multiplication trace. Because multiple post-cursor arrivals are generated by finite length heterogeneities, interpretation should be confined to single layer models rather than to interpret the additional peaks as internal layering. Furthermore, strong trade-offs exist between S -wave velocity perturbation and thickness making estimations of ULVZ or UHVZ elastic parameters highly uncertain. We test our analysis methods using data from an event occurring in the Fiji-Tonga region recorded in North America. The ScS bounce points for this event sample the CMB region to the southeast of Hawaii, in a region where ULVZs have been identified in several recent studies. We see additional evidence for a ULVZ in this region centred at 14°N and 153°W with a lateral scale of at least 250 km × 360 km. Assuming a constant S -wave velocity decrease of −10 or −20 per cent with respect to the PREM model implies a ULVZ thickness of up to 16 or 9 km, respectively. [ABSTRACT FROM AUTHOR]

Earth's magnetic field exhibits a dominant dipole morphology. Notwithstanding, significant deviations from the dipole are evident today, particularly the South Atlantic Anomaly (SAA), characterized by anomalously low-field intensity and high directional variability, diminishing the field's shielding effect. To assess the persistence of SAA-like features over multimillion-year scales, we combine paleomagnetic data from Trindade Island (20°30'S, 29°22'W) with an evaluation of paleosecular variation (PSV) over the past 10 Myr. We employ synthetic models to explore how the position and intensity of magnetic flux patches at the core-mantle boundary can influence the long-term field behavior. Here we present results that reveal anomalous field signatures in the South Atlantic and the Atlantic-Pacific hemispheric asymmetries are enduring features, likely linked to a bottom-up control of PSV by the inner core's heterogeneities but with contributions from mantle anomalies in the long-time range. This study provides evidence of a long-term anomalous behavior of the geomagnetic field in the South Atlantic from paleodirectional data of Trindade Island. Furthermore, field models suggest the persistence of anomalous features for the past 10 Myr. [ABSTRACT FROM AUTHOR]

Seismological observations indicate the presence of chemical heterogeneities at the lowermost mantle, just above the core–mantle boundary (CMB), sparking debate over their origins. A plausible explanation for the enigmatic seismic wave velocities observed in ultra-low-velocity zones (ULVZs) is the process of iron enrichment from the core to the silicate mantle. However, traditional models based on diffusion of atoms and penetration of molten iron fail to account for the significant iron enrichment observed in ULVZs. Here, we show that the chemical reaction between silicate bridgmanite and iron under hydrous conditions leads to profound iron enrichment within silicate, a process not seen in anhydrous conditions. Our findings suggest that the interaction between the core and mantle facilitates deep iron enrichment over a few kilometres at the bottom of the mantle when water is present. We propose that the seismic signatures observed in ULVZs indicate whole mantle convection, accompanied by deep water cycles from the crust to the core through Earth's history. The deep-water cycle in the mantle enhances chemical reactions between mantle minerals and the metallic core, contributing to seismic anomalies observed at the core-mantle boundary, according to high-pressure/temperature multi-anvil experiments. [ABSTRACT FROM AUTHOR]

Ultra-low velocity zones (ULVZs) have been identified as regions of extremely low velocity anomalies in the Earth's lowermost mantle using seismic observations from reflected, refracted and diffracted arrivals along the mantle side of the core–mantle boundary. Estimation of ULVZ geometrical (i.e. shape and size) and elastic (i.e. velocity and density) parameters with uncertainties is crucial in understanding the role of ULVZs in the ongoing dynamic processes within the Earth's mantle; however, these parameters are still poorly known due to uncertainties and tradeoffs of the seismically resolved ULVZ geometries and elastic parameters. Computation of synthetic waveforms for 2-D and 3-D ULVZs shapes is currently computationally feasible, but past studies utilize higher dimensional waveform modelling of mostly only low-frequency diffracted waves. Most studies focusing on high-frequency core-reflected waveforms (e.g. ScP) still use 1-D modelling approaches to determine ULVZ properties. This approach might lead to wrong results if the imaged structures have inherently 3-D geometries. This study investigates high-frequency synthetic ScP waveforms for various 2.5-D ULVZ geometries showing that additional seismic arrivals are generated even when the ScP geometrical ray path does not directly strike the location of the ULVZ. The largest amplitude additional phases in the 2.5-D models are post-cursor arrivals that are generated at the edges of the finite-length ULVZs. These newly identified ScP post-cursors can arrive within the ScsP post-cursor time window traditionally analysed in 1-D ULVZ studies. These post-cursors might then be misidentified or constructively/destructively interfere with the ScsP post-cursor, leading to incorrect estimation of ULVZ parameters. In this study we investigate the bias introduced by the 2.5-D morphologies on the 1-D estimated ULVZ elastic properties in a Bayesian waveform inversion scheme. We further expand the Bayesian method by including the data noise covariance matrix in the inversion and compare it to an autoregressive noise model that was utilized in previous studies. From the application to the observed ScP data, we find that the new approach converges faster, particularly for the inversion of data from multiple events, and the new algorithm retrieves ULVZ parameters with more realistic uncertainties. The inversion of 2.5-D synthetic ScP waveforms suggests that the retrieved ULVZ parameters can be misleading with unrealistically high confidence if we do not consider the data noise covariance matrix in the inversion. Our new approach can also retrieve the shape of a multidimensional Gaussian ULVZ if its length is 12° or longer in the great circle arc direction. However, 2.5-D synthetic waveforms show additional waveform complexity which can constructively interfere with the ScP wavefield. Hence, in many cases the estimation of ULVZ properties using 1-D forward modelling can provide incorrect ULVZ parameters. Hence previous ULVZ modelling efforts using 1-D parameter estimation methods may be incorrect. [ABSTRACT FROM AUTHOR]

Due to their high density, the ilmenite-bearing cumulates (IBC) (with or without KREEP) formed during the late-stage lunar magma ocean solidification are thought to sink into the underlying lunar mantle and trigger lunar mantle overturn. Geophysical evidence implied that IBC may descend deep inside the Moon and remain as a partially molten layer at the core-mantle boundary (CMB). However, partial melting may have occurred on the mixed mantle cumulates during the sinking of IBC/KREEP and the silicate melt may be positively buoyant, thus preventing the IBC/KREEP layer from sinking to the CMB. Here, we perform thermodynamic simulation on the stability of lunar mantle cumulates at different depths mixed with different amounts of IBC/KREEP from an updated LMO model. The modeling results suggest that the sinking of IBC/KREEP will cause at least 5 wt% partial melting in the shallow (~ 120 km) and a much larger degree of partial melting in the deep lunar mantle (~ 420 km). Due to the density contrast with the surrounding mantle, IBC/KREEP-bearing melts could potentially decouple under certain conditions. The modified lunar mantle by sinking of IBC/KREEP can better explain the formation of different kinds of lunar basaltic magma than the primary lunar mantle formed through differentiation of lunar magma ocean. Sinking of IBC/KREEP back into the lunar mantle may introduce plagioclase, clinopyroxene, garnet, and incompatible radioactive elements into the deep lunar mantle, which will further affect the thermal and chemical evolution of the lunar interior. [ABSTRACT FROM AUTHOR]

Radiogenic heat production is fundamental to the energy budget of planets. Roughly half of the heat that Earth loses through its surface today comes from the three long-lived, heat-producing elements (potassium, thorium, and uranium). These three elements have long been believed to be highly lithophile and thus concentrate in the mantle of rocky planets. However, our study shows that they all become siderophile under the pressure and temperature conditions relevant to the core formation of large rocky planets dubbed super-Earths. Mantle convection in super-Earths is then primarily driven by heating from the core rather than by a mix of internal heating and cooling from above as in Earth. Partitioning these sources of radiogenic heat into the core remarkably increases the core-mantle boundary (CMB) temperature and the total heat flow across the CMB in super-Earths. Consequently, super-Earths are likely to host long-lived volcanism and strong magnetic dynamos. [ABSTRACT FROM AUTHOR]

Seismological studies have exposed numerous ultralow velocity zones (ULVZs) exhibiting extraordinary physical attributes at Earth's core-mantle boundary, yet their compositions and origins remain controversial. Water-iron reaction can generate unique phases under lowermost-mantle conditions and likely plays a crucial role in forming ULVZs. Through first-principles molecular dynamic simulations with machine learning techniques, we determine that iron hydride, the product of water-iron reaction, is stable as a superionic phase at the core-mantle boundary. This superionic iron hydride has much slower velocities and a higher density than the ambient mantle under lowermost-mantle conditions. Accumulation of iron hydride, created through either a chemical reaction between subducted water and iron or solidification of core material entrained in the lower mantle by convection, can explain the seismic observations of ULVZs particularly those associated with subduction. This work suggests that water may have a substantial role in creating seismic heterogeneities at the core-mantle boundary. [ABSTRACT FROM AUTHOR]

Conventional Earth evolution models are unable to simultaneously reproduce two fundamental observations: the mantle's secular temperature record and a long-lived geodynamo before inner core nucleation. Today, plate tectonics efficiently cools the mantle, but if assumed to operate throughout Earth's history, past mantle temperature and plate motion become unrealistically high. Through coupled core-mantle modeling that self-consistently predicts multiple mantle convection regimes, we show that over most of the Precambrian, Earth likely operated in a distinct "sluggish-lid" tectonic mode, characterized by partial decoupling between the lithosphere and mantle. This dominant early regime is due to a hotter Earth and the presence of the asthenosphere. This mode regulates the core-mantle boundary heat flow, which powers the geodynamo before inner core nucleation. Both sluggish-lid tectonics and a long-lived dynamo demonstrate the inextricably connected paths of the core-mantle system. Moreover, our simulations simultaneously satisfy diverse geological observations and are consistent with emerging interpretations of such records. [ABSTRACT FROM AUTHOR]

Paleomagnetic analyses have suggested that the lunar magnetic field underwent a significant change from 4.25 to 3.19 Ga, indicating the rapid transition of the lunar dynamo mechanism. We used the van der Pauw (vdP) method to measure the electrical resistivity of Fe‐S‐P alloys under conditions relevant to the lunar core and estimated the thermal conductivity of the Fe‐S‐P lunar core. These values were incorporated into thermal and dynamo models to investigate the evolution of the lunar core. Our model indicates that the inner core began to grow as early as 4.35 Ga, the solidification regime switched at 3.50 Ga, and the thermal dynamo ceased between 3.78 and 3.51 Ga. The cessation of the dynamo could be due to a low buoyancy flux and insufficient entropy dissipation. Thermal and compositional dynamos cannot sustain the ancient strength of the Moon's magnetic field, and require other energy sources. Plain Language Summary: The transport properties of planetary core materials dominate heat transfer through thermal convection, which is closely linked to the geodynamo of the planetary core. The van der Pauw (vdP) method was used to measure the electrical resistivity of Fe‐S‐P alloy, and we assessed the effects of pressure, temperature, and light elements (sulfur and phosphorus) on the electrical resistivity of these alloys. We estimated the thermal conductivity of the Fe‐S‐P lunar core and modeled the growth of the lunar core. We found that the inner core nucleated at 4.35 Ga in the center and then solidified at the core‐mantle boundary (CMB) at 3.50 Ga. The adiabatic heat conduction at the lunar CMB indicated that a thermally driven dynamo persisted until 3.78–3.51 Ga in the Moon's history. We discuss the cessation of the lunar core dynamo in relation to the buoyancy flux, entropy, Reynolds number and Elsasser number and explain the variation in the magnetic field in conjunction with the previous dynamo mechanism. Key Points: The electrical resistivity and thermal conductivity of Fe‐S‐P alloys were determined at 3 and 5 GPaA low thermal conductivity Fe‐S‐P core nucleated earlier and the bottom‐up growth regime switched to a top‐down regime at 3.50 GaMultiple dynamo mechanisms worked together to generate a high magnetic field, low buoyancy flux and entropy dissipation causing the dynamo to cease [ABSTRACT FROM AUTHOR]

We document the arrival of seismic energy in the core shadow zone up to large distances beyond 150° more than 100 s prior to the core phases. Numerical simulations of the energy transport in an established heterogeneity model show that scattering throughout the entire mantle explains these observations. Diffraction at the core‐mantle boundary is unlikely in our 1–2 Hz frequency band and is not required indicating misleading terminology with reference to Pdiff for the scattered P∗P‐energy. Records of the largest deep earthquakes at low‐noise stations are key to the observation of the faint precursory signal which changes appearance with increasing distance from a coda‐like decay over a constant amplitude level around 130° to an emergent wave train. According to our simulations, different depth layers in the mantle dominate different time‐distance windows of the scattered wave train, providing the opportunity to improve the depth resolution of mantle heterogeneity models. Plain Language Summary: Earthquakes producing different types of waves that travel through the Earth help understand the structure of the Earth. We show that there is seismic energy arriving at stations in the shadow of the Earth's core more than 100 s before the waves usually considered the first arrivals. We used computer simulations to explain how this energy can travel so far in such a short time. Our results show that when there are heterogeneous structures distributed throughout the Earth's mantle the seismic energy can change direction at these structures due to a phenomenon called scattering. This allows the seismic energy to travel in the fast mantle material around the slow Earth's core. Previously, seismologists thought that part of this energy travels along the boundary between core and mantle by a process called diffraction. Our study provides a more elegant explanation for the observed energy and offers new possibilities for the investigation of Earth's structure. Key Points: We present observations of high‐frequency Pdiff coda more than 100 s before the core phases even at large distances beyond 150°Seismic energy simulations in published models of Earth's mantle heterogeneity show that scattered energy explains Pdiff and its codaDifferent‐depth mantle layers contribute to different time‐distance windows of the Pdiff coda benefiting imaging the mantle heterogeneity [ABSTRACT FROM AUTHOR]

Lattice thermal conductivity ( k lat ) of mantle minerals is a key factor in determining the Earth's energy budget and influences its dynamic processes. Here, we trained a neuroevolution machine learning potential for CaSiO3 perovskite (CaPv), the third most abundant mineral of the lower mantle, to investigate the k lat of pyrolitic aggregates at the core–mantle boundary (CMB). We show that the k lat of two types of pyrolitic aggregates has increased by 7% and 5% upon the addition of CaPv, demonstrating its significance in shaping the thermal structure of Earth's interior. Considering other mantle minerals and iron content, as well as the global distribution of temperature, we evaluated the heat flow across the CMB to be 7.98 ± 0.4 TW. The estimated heat flow is inconsistent with the value derived from the Fe alloy, which might suggest the presence of a thermally or chemically stratified layer atop the outer core. [ABSTRACT FROM AUTHOR]

There are various hotspots in the Atlantic Ocean, which are underlain by mantle plumes that likely cross the mantle and originate at the core‐mantle boundary. We use teleseismic core‐diffracted shear waves to look for an Ultra‐Low Velocity Zone (ULVZ) at the potential base of central Atlantic mantle plumes. Our data set shows delayed postcursory phases after the core‐diffracted shear waves. The observed patterns are consistent in frequency dependence, delay time, and scatter pattern with those caused by mega‐ULVZs previously modeled elsewhere. Synthetic modeling of a cylindrical structure on the core‐mantle boundary below St. Helena provides a good fit to the data. The preferred model is 600 km across and 20 km high, centered at approximately 15° South, 15° West, and with a 30% S‐wave velocity reduction. Significant uncertainties and trade‐offs do remain to these parameters, but a large ULVZ is needed to explain the data. The location is west of St. Helena and south of Ascension. Helium and neon isotopic systematics observed in samples from this region could point to a less‐outgassed mantle component mixed in with the dominant signature of recycled material. These observations could be explained by a contribution from the Large Low Shear Velocity Province (LLSVP). Tungsten isotopic measurements would be needed to understand whether a contribution from the mega‐ULVZ is also required at St. Helena or Ascension. Plain Language Summary: Nearly 3,000 km beneath the Atlantic to the West of the island of St. Helena, on the boundary between Earth's metal core and rocky mantle, we have discovered a new area where seismic waves diffracting along that boundary travel significantly slower than expected. This area is called an ultra‐low velocity zone. In this study, we use seismic waves which propagate along the core‐mantle boundary. The waves that interact with the ultra‐low velocity zone are scattered and become severely delayed. Using the observations, we have constrained the ultra‐low velocity zone to a broadly cylindrical structure, 600 km across, 20 km high and centered at 15° South, 15° West. The material inside is reduced by 30% in seismic shear wave velocity compared to outside. We confirmed this model by computing and comparing synthetic waveforms for a range of different ultra‐low velocity zone models. This ULVZ location is right beside or just inside a much larger region of low velocity, dubbed the African Large Low Shear‐Velocity Province (LLSVP). The observed ultra‐low velocity zone could be the base of the upwelling or mantle plume rising through the mantle and causing the hotspots of St. Helena and/or Ascension at the surface. Key Points: Observation of significant Sdiff postcursors sampling the CMB beneath the AtlanticModeling of postcursors reveals a previously unknown mega‐ULVZ situated on the CMB to the West of St. HelenaFurther measurements of St. Helena and Ascension samples are needed to identify a potential ULVZ‐associated geochemical signature [ABSTRACT FROM AUTHOR]

Abundant carbon was identified on Mercury by MESSENGER, which is interpreted as the remnant of a primordial graphite flotation crust, suggesting that the magma ocean and core were saturated in carbon. We re-evaluate carbon speciation in Mercury's interior in light of the high pressure-temperature experiments, thermodynamic models and the most recent geophysical models of the internal structure of the planet. Although a sulfur-free melt would have been in the stability field of graphite, sulfur dissolution in the melt under the unique reduced conditions depressed the sulfur-rich liquidus to temperatures spanning the graphite-diamond transition. Here we show it is possible, though statistically unlikely, that diamond was stable in the magma ocean. However, the formation of a solid inner core caused diamond to crystallize from the cooling molten core and formation of a diamond layer becoming thicker with time. A diamond layer that becomes thicker with time is generated from carbon exsolution at the core-mantle boundary of Mercury, owing to cooling of its metallic core and potentially the silicate magma ocean. [ABSTRACT FROM AUTHOR]

Convective flow in the deep mantle controls Earth's dynamic evolution, influences plate tectonics, and has shaped Earth's current surface features. Present and past convection‐induced deformation manifests itself in seismic anisotropy, which is particularly strong in the mantle's uppermost and lowermost portions. While the general patterns of seismic anisotropy have been mapped for the upper mantle, anisotropy in the lowermost mantle (called D′′) is at an earlier stage of exploration. Here we review recent progress in methods to measure and interpret D′′ anisotropy. Our understanding of the limitations of existing methods and the development of new measurement strategies have been aided enormously by the availability of high‐performance computing resources. We give an overview of how measurements of seismic anisotropy can help constrain the mineralogy and fabric of the deep mantle. Specifically, new and creative strategies that combine multiple types of observations provide much tighter constraints on the geometry of anisotropy than have previously been possible. We also discuss how deep mantle seismic anisotropy provides insights into lowermost mantle dynamics. We summarize what we have learned so far from measurements of D′′ anisotropy, how inferences of lowermost mantle flow from measurements of seismic anisotropy relate to geodynamic models of mantle flow, and what challenges we face going forward. Finally, we discuss some of the important unsolved problems related to the dynamics of the lowermost mantle that can be elucidated in the future by combining observations of seismic anisotropy with geodynamic predictions of lowermost mantle flow. Plain Language Summary: Earthquakes cause waves that travel through Earth's interior and are recorded by distant seismometers. These seismic waves behave differently depending on the material that they pass through, revealing Earth's material properties. At the very bottom of the mantle, seismic waves sometimes travel at different speeds depending on their direction. This material property is called seismic anisotropy and is caused by material deformation and flow. Global patterns of mantle flow, which is directly connected to surface processes such as the movement of tectonic plates or hotspot volcanism, can therefore be inferred from seismic anisotropy. Recent years have seen advances in anisotropy imaging in the lowermost mantle as well as in numerical calculations of flow in Earth's lowermost mantle. We review the methods that are used to infer lowermost mantle anisotropy. We further give an overview of previous results and interpretations, which include seismic anisotropy caused by upwelling plumes and ancient slab remnants. Future improvements in the fields of seismology, geodynamics, and mineral physics are needed to improve our understanding of global deep mantle flow. Key Points: Measurement techniques for deep mantle anisotropy have been improved substantially in the last decadeThese improvements enable inferences of deep mantle flow with increasing confidenceKnowledge of mantle dynamics elucidates the drivers of flow, relationships among structures, and Earth's dynamic evolution [ABSTRACT FROM AUTHOR]

Seismic traveltime anomalies of waves that traverse the uppermost 100–200 km of the outer core have been interpreted as evidence of reduced seismic velocities (relative to radial reference models) just below the core–mantle boundary (CMB). These studies typically investigate differential traveltimes of SmKS waves, which propagate as P waves through the shallowest outer core and reflect from the underside of the CMB m times. The use of SmKS and S(m-1)KS differential traveltimes for core imaging are often assumed to suppress contributions from earthquake location errors and unknown and unmodelled seismic velocity heterogeneity in the mantle. The goal of this study is to understand the extent to which differential SmKS traveltimes are, in fact, affected by anomalous mantle structure, potentially including both velocity heterogeneity and anisotropy. Velocity variations affect not only a wave's traveltime, but also the path of a wave, which can be observed in deviations of the wave's incoming direction. Since radial velocity variations in the outer core will only minimally affect the wave path, in contrast to other potential effects, measuring the incoming direction of SmKS waves provides an additional diagnostic as to the origin of traveltime anomalies. Here we use arrays of seismometers to measure traveltime and direction anomalies of SmKS waves that sample the uppermost outer core. We form subarrays of EarthScope's regional Transportable Array stations, thus measuring local variations in traveltime and direction. We observe systematic lateral variations in both traveltime and incoming wave direction, which cannot be explained by changes to the radial seismic velocity profile of the outer core. Moreover, we find a correlation between incoming wave direction and traveltime anomaly, suggesting that observed traveltime anomalies may be caused, at least in part, by changes to the wave path and not solely by perturbations in outer core velocity. Modelling of 1-D ray and 3-D wave propagation in global 3-D tomographic models of mantle velocity anomalies match the trend of the observed traveltime anomalies. Overall, we demonstrate that observed SmKS traveltime anomalies may have a significant contribution from 3-D mantle structure, and not solely from outer core structure. [ABSTRACT FROM AUTHOR]

The Earth's magnetic field is generated by a dynamo in the outer core and is crucial for shielding our planet from harmful radiation. Despite the established importance of the core–mantle boundary (CMB) heat flux as driver for the dynamo, open questions remain about how heat flux heterogeneities affect the magnetic field. Here, we explore the distribution of the CMB heat flux on Earth and its changes over time using compressible global 3-D mantle convection models in the geodynamic modelling software ASPECT. We discuss the use of the consistent boundary flux method as a tool to more accurately compute boundary heat fluxes in finite element simulations and the workflow to provide the computed heat flux patterns as boundary conditions in geodynamo simulations. Our models use a plate reconstruction throughout the last 1 billion years—encompassing the complete supercontinent cycle—to determine the location and sinking speed of subducted plates. The results show how mantle upwellings and downwellings create localized heat flux anomalies at the CMB that can vary drastically over Earth's history and depend on the properties and evolution of the lowermost mantle as well as the surface subduction zone configuration. The distribution of hot and cold structures at the CMB changes throughout the supercontinent cycle in terms of location, shape and number, indicating that these structures fluctuate and might have looked very differently in Earth's past. We estimate the resulting amplitude of spatial heat flux variations, expressed by the ratio of peak-to-peak amplitude to average heat flux, q *, to be at least 2. However, depending on the material properties and the adiabatic heat flux out of the core, q * can easily reach values >30. For a given set of material properties, q * generally varies by 30–50 per cent over time. Our results have implications for understanding the Earth's thermal evolution and the stability of its magnetic field over geological timescales. They provide insights into the potential effects of the mantle on the magnetic field and pave the way for further exploring questions about the nucleation of the inner core and the past state of the lowermost mantle. [ABSTRACT FROM AUTHOR]

The thermal properties of major minerals play a key role in understanding the internal dynamic mechanism and thermal evolution of the Earth and rocky planets. In this study, we first investigated the effect of Fe on the thermal conductivity (κ) and the thermal diffusivity (D) of orthopyroxene at 1–3 GPa and 293–873 K by the transient plane source method. The κ and D both decrease with increasing temperature and decreasing pressure. With increasing Fe content, the two parameters both quickly decrease from the beginning and then slack off. We further modeled the thermal evolution of S‐type asteroids, which strongly depends on the composition model and the dimension of the planet. Combining the present data with surface heat flow and heat production, the lunar's geotherm until 1,400 km is constructed. The core‐mantle boundary temperature of the Moon is refined from 1,883 to 1,754 K. Plain Language Summary: The thermal state and the thermal evolution of rocky planets are strongly influenced by the thermal properties of the major constituent minerals. Orthopyroxene is one of such minerals for rocky planets (e.g., S‐type asteroids and moon). The Fe content can potentially affect the thermal properties of orthopyroxene. However, there are no relevant studies up to now. In this study, we systematically measured the thermal conductivity and the thermal diffusivity of pyroxene with variable Fe content at high temperature and high pressure. Our research shows that the thermal conductivity and the thermal diffusivity of orthopyroxene decrease with increasing Fe content. Adopting the results of this study, we simulate the thermal evolution of S‐type asteroids with different compositions and dimensions and construct the lunar's geotherm until the core‐mantle boundary. Key Points: Both the thermal conductivity and the thermal diffusivity of orthopyroxene decrease with temperature and increase with pressureThe thermal conductivity and the thermal diffusivity of orthopyroxene quickly decrease with iron contentThe thermal evolutions of S‐type asteroids are first simulated and the thermal structure of the lunar interior until the CMB is constrained [ABSTRACT FROM AUTHOR]

FeO is an important component in both mantle silicates and core iron alloys. Understanding its melting behavior and physical properties is crucial for exploring the chemistry and physics of our planet. Here we report the melting curve of FeO up to 186 GPa from laser‐heating experiments in a diamond‐anvil cell coupled with synchrotron X‐ray diffraction (XRD) techniques. In‐situ observations of both temperature plateau and changes in XRD patterns were used as primary melting criteria. The ex‐situ examination of a recovered sample shows consistent melting temperatures of FeO with in‐situ determinations. Our melting curve of FeO agrees with existing low‐pressure data within uncertainties and is much lower than earlier experimental results above 100 GPa including those extrapolated by Lindemann's law. Our results indicate that FeO‐rich materials could be present as melts coexisting with surrounding solids in the lowermost mantle, providing plausible explanations to the seismically observed ultra‐low velocity zones. Plain Language Summary: As an endmember of mantle silicates, the melting behavior of FeO is critical to understand the early magma ocean crystallization as well as present mantle signatures. However, the question on the high‐pressure melting curve of FeO remains because of the use of different melting criteria in earlier experiments. In this study, we conducted melting experiments on FeO up to 186 GPa in a laser‐heated diamond‐anvil cell using both in‐situ synchrotron X‐ray diffraction techniques and supplementary ex‐situ analyses of a recovered sample. Our results show that the melting temperature of FeO is 3,440 ± 250 K at 135 GPa, the Earth's core‐mantle boundary (CMB) pressure. The MgO‐FeO‐SiO2 melting phase diagram shows that fractional crystallization of mantle silicate melts would end up with a cotectic ternary minimum containing up to 95 mol% FeO, which would have a similar or slightly lower melting temperature than FeO. Considering the temperature at present‐day CMB, we anticipate that FeO‐rich materials, possible products of late‐stage magma ocean crystallization, would still exist as melts with surrounding solids at the bottom of the mantle. Due to its liquid nature and high density, the existence of FeO‐rich melts above the CMB could provide important insights for explaining the seismologically observed ultra‐low velocity zones. Key Points: The melting curve of FeO was determined up to 186 GPa based on in‐situ observations with supplementary ex‐situ examinationsOur determined melting temperature of FeO is much lower than earlier experimental results above 100 GPaAt the late magma ocean crystallization, the residual FeO‐rich materials could exist as melts in the lowermost mantle and explain ultra‐low velocity zones [ABSTRACT FROM AUTHOR]

Hadean zircons provide a potential record of Earth's earliest subduction 4.3 billion years ago. It remains enigmatic how subduction could be initiated so soon after the presumably Moon‐forming giant impact (MGI). Earlier studies found an increase in Earth's core‐mantle boundary (CMB) temperature due to the accumulation of the impactor's core, and our recent work shows Earth's lower mantle remains largely solid, with some of the impactor's mantle potentially surviving as the large low‐shear velocity provinces (LLSVPs). Here, we show that a hot post‐impact CMB drives the initiation of strong mantle plumes that can induce subduction initiation ∼200 Myr after the MGI. 2D and 3D thermomechanical computations show that a high CMB temperature is the primary factor triggering early subduction, with enrichment of heat‐producing elements in LLSVPs as another potential factor. The models link the earliest subduction to the MGI with implications for understanding the diverse tectonic regimes of rocky planets. Plain Language Summary: Plate tectonics remains unique to Earth, but when and how it started is debated. Earth's oldest minerals indicate a clement surface by 4.3 Ga, resembling the modern Earth with its granitic crust and oceans. Granite is most easily explained as originating from subduction. However, the mechanisms for subduction initiation, especially so soon after the Moon‐forming giant impact, remain elusive. Earlier studies indicate that the core‐mantle boundary (CMB) temperature is increased due to accumulation of the impactor's core during the impact. Our recent work further shows that the lower half of Earth's mantle remains mostly solid after this impact and that parts of the impactor's mantle might have survived as the two seismically‐observed large low‐shear velocity provinces (LLSVPs). In this study, we perform whole‐mantle convection models to illustrate that strong mantle plumes can arise, weaken the lithosphere, and eventually initiate subduction ∼200 Myr after the giant impact. Our systematic computations reveal that the hot CMB temperature after the impact is the primary factor determining whether there is early subduction initiation, with enrichment of heat‐producing elements in LLSVPs as another potential contributor. Our model ties the Moon's formation to incipient subduction, providing insights for understanding the diverse tectonic regimes of rocky planets. Key Points: The mantle thermochemical structure left by the Moon‐forming impact triggers strong plumes that may have initiated the first subductionThe strong mantle plumes may rise from a heated core or from large low‐shear velocity provinces enriched in heat‐producing elementsEarly giant impact events may have a profound influence on the diverse tectonic regimes of rocky planets [ABSTRACT FROM AUTHOR]

A strong negative anomaly of seismic wave velocities at the core-mantle boundary (the Perm Anomaly) beneath the East European platform is attributed to the remnant of a deep mantle upwelling. The interaction between the upwelling and the East European lithosphere in the geological past and its resulting surface manifestations are still poorly understood. Using mantle plume modelling and global plate motion reconstructions, we show here that the East European lithosphere is likely to have been situated over the weakening Perm Anomaly upwelling for about 150–200 million years. As the East European platform moved above the Perm Anomaly in post-Jurassic times, the vertical tectonic movements recorded in sedimentary hydrocarbon-rich basins show either hiatus/uplift or insignificant subsidence. Analytical modelling of heat conduction through the lithosphere demonstrates that the basins have been slowly heated for a long time by the Perm Anomaly upwelling, creating suitable conditions for hydrocarbon maturation. This suggests a profound relationship between mantle plume dynamics, basin evolution, and hydrocarbon generation. Since the Jurassic, East European basins have likely been situated over a weakening mantle upwelling, which heated the basins and created suitable conditions for hydrocarbon maturation, according to geodata combined with modelling. [ABSTRACT FROM AUTHOR]

The heat flux across the core–mantle boundary (CMB) is a fundamental variable for Earth evolution and internal dynamics. Seismic tomography provides access to seismic heterogeneities in the lower mantle, which can be related to present-day thermal heterogeneities. Alternatively, mantle convection models can be used to either infer past CMB heat flux or to produce statistically realistic CMB heat flux patterns in self-consistent models. Mantle dynamics modifies the inertia tensor of the Earth, which implies a rotation of the Earth with respect to its spin axis, a phenomenon called true polar wander (TPW). This rotation must be taken into account to link the dynamics of the mantle to the dynamics of the core. In this study, we explore the impact of TPW on the CMB heat flux over long timescales (∼1 Gyr) using two recently published mantle convection models: one model driven by a plate reconstruction and a second that self-consistently produces a plate-like behaviour. We compute the geoid in both models to correct for TPW. In the plate-driven model, we compute a total geoid and a geoid in which lateral variations of viscosity and density are suppressed above 350 km depth. An alternative to TPW correction is used for the plate-driven model by simply repositioning the model in the original paleomagnetic reference frame of the plate reconstruction. The average TPW rates range between 0.4 and 1.8° Myr -1 , but peaks up to 10° Myr -1 are observed. We find that in the plate-driven mantle convection model used in this study, the maximum inertia axis produced by the model does not show a long-term consistency with the position of the magnetic dipole inferred from paleomagnetism. TPW plays an important role in redistributing the CMB heat flux, notably at short timescales (≤10 Myr). Those rapid variations modify the latitudinal distribution of the CMB heat flux, which is known to affect the stability of the magnetic dipole in geodynamo simulations. A principal component analysis (PCA) is computed to obtain the dominant CMB heat flux pattern in the different cases. These heat flux patterns are representative of the mantle convection cases studied here and can be used as boundary conditions for geodynamo models. [ABSTRACT FROM AUTHOR]

We investigate the geodynamic and melting history of Mars using 3D spherical shell models of mantle convection, constrained by the recent InSight mission results. The Martian mantle must have produced sufficient melt to emplace the Tharsis rise by the end of the Noachian–requiring on the order of 1–3 × 109 km3 of melt after accounting for limited (∼10%) melt extraction. Thereafter, melting declined; however, abundant evidence for limited geologically recent volcanism necessitates some present‐day melt even in the cool mantle inferred from InSight data. We test models with two mantle activation energies and a range of crustal Heat Producing Element (HPE) enrichment factors and initial core‐mantle boundary temperatures. We also test the effect of including a hemispheric (spherical harmonic degree‐1) step in lithospheric thickness to model the Martian dichotomy. We find that a higher activation energy (350 kJ mol−1) rheology produces present‐day geotherms consistent with InSight results, and crustal HPE enrichment factors of 5–10‐times produce localized melting near or up to present‐day. The 10‐times crustal HPE enrichment is consistent with both InSight and geochemical results and also produces present‐day geoid power spectra consistent with Mars. However, calculations that match the present‐day geoid power spectra require more than 60% melt extraction to produce the Tharsis swell. The addition of a degree‐1 hemispheric dichotomy, as an equatorial step in lithospheric thickness, does not significantly improve upon melt production or the geoid. Plain Language Summary: Mars' mantle needed to produce an extremely high volume of melt by ∼3.7 billion years ago in order to build the immense volcanic plateau of Tharsis. There is also abundant evidence for small volumes of geologically recent volcanism, yet the InSight mission results are consistent with a relatively cool present‐day mantle. We use 3D numerical models of the Martian mantle to determine what properties can produce a melting history and present interior temperatures consistent with InSight results and Mars' volcanic history. We test sets of models with two different sensitivities of the mantle viscosity to changes in temperature (i.e., activation energy), and a range of enrichment factors in Heat Producing Elements in the crust. We also test the effect of including a simplified version of the Martian hemispheric dichotomy. Our models with the higher activation energy and 10‐times crustal enrichment in HPEs (consistent with Mars' crustal composition) produce melt near the present‐day as well as temperature profiles consistent with InSight. However, it is very difficult to produce sufficient melt to form Tharsis in such a cool mantle, without assuming most of the melt produced in the mantle reaches the surface. Addition the simplified dichotomy does not significantly improve our results. Key Points: Our results favor a higher activation energy mantle rheology and 10‐times crustal heat producing element enrichment factorEven with the cool mantle consistent with InSight results, we obtain models having melt up to present‐dayProducing sufficient melt to form Tharsis requires a degree of melt extraction 6‐times as efficient as Earth's mid‐ocean ridges [ABSTRACT FROM AUTHOR]

Abstract Seismic energy arriving before the compressional (P) wave passing through the core (PKP), called PKP precursors, have been detected for decades, but the origin of those arrivals is ambiguous. The largest amplitude arrivals are linked to scattering at small‐scale lowermost mantle structure, but because these arrivals traverse both source and receiver sides of the mantle, it is unknown which side of the path the energy is scattered from. To address this ambiguity, we apply a new seismic array method to analyze PKP waveforms from 58 earthquakes recorded in North America that allows localization of the origin of the PKP precursors at the core‐mantle boundary (CMB). We compare these measurements with high frequency 2.5‐D synthetic predictions showing that the PKP precursors are most likely associated with ultra‐low velocity zone structures beneath the western Pacific and North America. The most feasible scenario to generate ULVZs in both locations is through melting of mid‐ocean ridge basalt in subducted oceanic crust.

Recent advancements in seismic data analysis have enhanced our grasp of the seismic heterogeneities near the core–mantle boundary (CMB). Through seismic tomography, persistent lower-mantle structures like the large low shear velocity provinces (LLSVPs) beneath the Pacific and South Africa have been identified. However, variations in the finer-scale features across different models raise questions about their origins. This study utilizes differential travel-time measurements from the USArray, operational across the contiguous United States from 2007 to 2014, to examine the impact of upper-mantle heterogeneities on tomographic models. By averaging the P-wave travel times and calibrating them with diffracted P-waves at the same stations, we mitigate the effects of shallow heterogeneities. The findings confirm that this method accurately maps the seismic anomalies beneath the USArray, consistent with other North American studies. Calibrated Pdiff travel-time data indicate significant anomalies in the mid-Pacific and Bering Sea and lesser anomalies in the northern Pacific, aligning with the global tomographic images. Moreover, the study highlights sharp travel-time variations over short distances, such as those across the northern boundary of the mid-Pacific anomaly, suggesting a chemically heterogeneous Pacific LLSVP.

Systematic studies of numerical dynamo simulations reveal that the transition from dipole-dominated non-reversing fields to models that exhibit reversals occurs when inertial effects become strong enough. However, the inertial force is expected to play a secondary role in the force balance in Earth's outer core. Here we show that reversals in numerical dynamo models with heterogeneous outer boundary heat flux inferred from lower mantle seismic anomalies appear when the amplitude of heat flux heterogeneity is increased. The reversals are triggered at regions of large heat flux in which strong small-scale inertial forces are produced, while elsewhere inertial forces are substantially smaller. When the amplitude of heat flux heterogeneity is further increased so that in some regions sub-adiabatic conditions are reached, regional skin effects suppress small-scale magnetic fields and the tendency to reverse decreases. Our results reconcile the need for inertia for reversals with the theoretical expectation that the inertial force remains secondary in the force balance. Moreover, our results highlight a non-trivial non-monotonic behavior of the geodynamo in response to changes in the amplitude of the core-mantle boundary heat flux heterogeneity. [ABSTRACT FROM AUTHOR]

The CsCl‐type (B2) phase of FeSi (B2‐FeSi) has been proposed as a candidate phase in the ultralow‐velocity zones (ULVZs) at the base of the lower mantle and in the Earth's inner core. However, the elastic properties of B2‐FeSi under relevant conditions remain unclear. Here we determine the density, elastic constants, and velocities of B2‐FeSi at high pressures (90–390 GPa) and temperatures (3,000–6,000 K) relevant to the Earth's lower most mantle and the inner core, using first‐principles molecular dynamics simulations. At the base of the lower mantle, B2‐FeSi shows significantly lower velocities and a higher density than those of the ambient mantle. Mechanical mixing models suggest the presence of ∼27–39 vol% B2‐FeSi in the silicate mantle is consistent with the reduced velocities and the elevated density of ULVZs observed seismically. On the other hand, the hcp‐Fe and B2‐FeSi mixture exhibits higher bulk sound velocity compared to the PREM under inner core conditions. Adding superionic H in the interstitial sites of B2‐FeSi lowers its density but has little effect on the bulk sound velocity of B2‐FeSi, precluding H‐bearing B2‐FeSi as a major component in the Earth's inner core. Plain Language Summary: Under Earth's deep lower mantle and inner core pressures and temperatures, iron silicide (FeSi) takes a CsCl‐type crystalline structure (B2‐FeSi). It has been proposed that B2‐FeSi is a potential component in the ultralow‐velocity zones (ULVZs) at the base of the lower mantle and in the inner core. Here, we apply first‐principles molecular dynamics simulations to determine the density and elastic wave velocities of B2‐FeSi under deep Earth conditions and compare its properties to seismic observations. At core‐mantle boundary conditions, a mixture of B2‐FeSi with the ambient mantle exhibits elastic behaviors consistent with the observations of ULVZs, suggesting its possible presence. However, at inner core conditions, B2‐FeSi and H‐bearing B2‐FeSi have substantially higher bulk sound velocity than that of the seismic model, invalidating its presence in the inner core as a major component. Key Points: B2‐FeSi shows markedly lower velocities than those of the PREM at the base of lower mantle but higher velocities at inner core conditionsThe presence of H in B2‐FeSi reduces both the P‐ and S‐wave velocities of B2‐FeSi, but has little effect on the bulk sound velocityThermoelastic properties of B2‐FeSi are more consistent with its presence in the ultra‐low velocity zones than in the Earth's inner core [ABSTRACT FROM AUTHOR]

Ultra‐low velocity zones (ULVZs) are anomalous structures, generally associated with decreased seismic velocity and sometimes an increase in density, that have been detected in some locations atop the Earth's core‐mantle boundary (CMB). A wide range of ULVZ characteristics have been reported by previous studies, leading to many questions regarding their origins. The lowermost mantle beneath Antarctica and surrounding areas is not located near currently active regions of mantle upwelling or downwelling, making it a unique environment in which to study the sources of ULVZs; however, seismic sampling of this portion of the CMB has been sparse. Here, we examine core‐reflected PcP waveforms recorded by seismic stations across Antarctica using a double‐array stacking technique to further elucidate ULVZ structure beneath the southern hemisphere. Our results show widespread, variable ULVZs, some of which can be robustly modeled with 1‐D synthetics; however, others are more complex, which may reflect 2‐D or 3‐D ULVZ structure and/or ULVZs with internal velocity variability. Our findings are consistent with the concept that ULVZs can be largely explained by variable accumulations of subducted oceanic crust along the CMB. Partial melting of subducted crust and other, hydrous subducted materials may also contribute to ULVZ variability. Plain Language Summary: Earth's core‐mantle boundary (CMB), the interface between the solid silicate mantle and the molten iron‐rich outer core, is associated with a range of anomalous structures, including ultra‐low velocity zones (ULVZs). While generally associated with reduced seismic wave velocities and sometimes increased density, prior studies have reported a wide range of ULVZ characteristics, leading to many questions regarding their origins. The lowermost mantle beneath the southern hemisphere provides a unique environment to study ULVZs because it is located away from regions of large‐scale mantle upwelling and downwelling. Our study uses core reflected P‐waves (PcP) recorded by seismic stations in Antarctica to investigate this portion of the CMB for ULVZ presence. We find widespread evidence for variable ULVZ structure. Some of the imaged ULVZs can be modeled with a single layer, but others are more complex. We suggest that the ULVZs beneath the southern hemisphere are predominantly associated with subducted oceanic crust that has variable accumulations along the CMB. In some regions, hydrated subducted materials may also experience partial melting, which may contribute to the complicated ULVZ structures imaged in some locations. Key Points: Core‐reflected P‐waves are used to investigate ultra‐low velocity zones in the lowermost mantle beneath the southern hemisphereResults show widespread evidence for variable ultra‐low velocity zones, some of which may indicate layered or gradational structureVariable accumulations of subducted oceanic crust with localized partial melting can explain these anomalous structures [ABSTRACT FROM AUTHOR]

We present new observations of core‐diffracted shear waves which contain anomalous waveforms sampling the lowermost mantle beneath the southern Pacific region. Data in two distinct geometries, one from New Zealand to North America and the other from the Fiji and Solomon Islands to South America, show evidence of postcursor phases. The postcursor delays and move‐outs imply that they are caused by an ultra‐low velocity zone (ULVZ). Beamforming analyses of the observed diffracted postcursors show a strong backazimuthal deviation, suggesting this new ULVZ is likely to have a cylindrical shape similar to broad ULVZs sampled by shear diffracted waves elsewhere. Full‐waveform modeling suggests that the postcursors seen in North America might be due to the previously modeled ULVZ located to the west of the Galápagos, while those seen in South America are due to a previously unknown ULVZ beneath the Southern Pacific. We cannot fit observations in both geometries by a single ULVZ. For the new location, we propose one cylindrical ULVZ model with a radius of 400 km and a shear wave velocity decrease of 20% centered at geographical coordinates (−33.6, 130) close to the Pitcairn hotspot. Despite some uncertainty in the west‐east direction, this new ULVZ observation likely provides another example to support the hypothesis that ULVZs exist at the base of mantle plumes where primordial signatures are observed in the ocean island basalts. Plain Language Summary: We analyze shear waves that diffract along the core‐mantle boundary, called Sdiff, beneath the southern part of the Pacific Ocean. Sdiff waves are very sensitive to the seismic velocity structure of the lowermost mantle just above the boundary. For diffracted phases from several earthquakes and across a range of seismic stations, we observe anomalous waveform signals, which we refer to as Sdiff postcursors, and which arrive up to tens of seconds after the main Sdiff phase. We use the seismic data from many seismic stations to find from what direction the postcursor energy is coming from. We infer the cause of the anomalous postcursors are ultra‐low velocity zones (ULVZs) on the core‐mantle boundary, which are the strongest heterogeneities observed in the lower mantle. Additionally, we compute synthetic data for a range of potential ULVZ models. We find that some of the data can be explained by a previously modeled ULVZ near the Galápagos, but that other data point to a previously unknown ULVZ, beneath the southern Pacific region, although its exact location is somewhat uncertain, particularly in the west‐east direction. ULVZs, like this one, might represent the bases of upwelling mantle plumes that feed the hotspots at the surface. This particular one is in a location roughly beneath several southern Pacific hotspots, where geochemists find primordial isotope anomalies. Key Points: Detection of anomalous Sdiff postcursors sampling the core‐mantle boundary beneath the Southern PacificBeamforming and forward waveform modeling analyses find a new ultra‐low velocity zone (ULVZ) with an uncertain location beneath the southern PacificThis provides a potential new seismic example to support the correlation between surface ocean island basalts and ULVZs [ABSTRACT FROM AUTHOR]

Accurate thermoelastic properties and thermal conductivity are crucial for understanding the thermal evolution of the Martian core. A fitting method based on ab initio calculated pressure‐volume‐temperature data was proposed for the formulation of the equation of state with high accuracy, by which the pressure and temperature dependent thermoelastic properties can be directly calculated by definitions. Ab initio results showed that Fe0.75S0.25 liquid under Martian core conditions was thoroughly in a magnetic state without existing spin crossover. The Fe0.75S0.25 liquid in magnetic calculations had a low thermal conductivity (21–23 W/m/K) when compared with non‐magnetic calculations at the same state. Based on Insight's estimated Martian core properties (Stähler et al., 2021, https://doi.org/10.1126/science.abi7730) and ab initio calculated properties of the Fe0.75S0.25 liquid, the scenario for the thermal evolution of the Martian core is the iron‐snow model crystallization regime. The parameter uncertainty effect on the cessation time of the dynamo and zone of iron snow was systematically analyzed. Plain Language Summary: The knowledge of the density and thermoelastic properties of the Martian core is very limited. The thermoelastic properties of Fe0.75S0.25 liquid under Martian core conditions were calculated by ab initio molecular dynamics, and the thermal evolution of the Martian core was modeled by the thermoelastic properties and the reported Insight data (Stähler et al., 2021, https://doi.org/10.1126/science.abi7730). The Fe0.75S0.25 liquid under the Martian core condition is magnetic and its thermal conductivity is 21–23 W/m/K. If the temperature at the core‐mantle boundary is higher than the core's melting temperature, the Martian core is entirely liquid with simple cooling. If the temperature is lower than the melting temperature, there exists iron snow at the top of the Martian core at present. The parameter uncertainty on the effect of the iron snow evolution of the Martian core was explored. Key Points: The thermoelastic properties were calculated by the polynomial fitted equation of stateThe Fe0.75S0.25 liquid under the Martian core condition was thoroughly in the magnetic stateThe Fe0.75S0.25 liquid in magnetic calculation had a low thermal conductivity (21–23 W/m/K) [ABSTRACT FROM AUTHOR]

Mars experienced a dynamo process that generated a global magnetic field ~4.3 (or earlier) to 3.6 billion years ago (Ga). The cessation of this dynamo strongly affected Mars' history and is expected to be linked to thermochemical evolution of Mars' iron-rich liquid core, which is strongly influenced by its thermal conductivity. Here, we directly measured thermal conductivities of solid iron-sulfur alloys to pressures relevant to the Martian core and temperatures to 1023 Kelvin. Our results show that a Martian core with 16 weight % sulfur has a thermal conductivity of ~19 to 32 Watt meter-1 Kelvin-1 from its top to the center, much higher than previously inferred from electrical resistivity measurements. Our modeled thermal conductivity profile throughout the Martian deep-mantle and core indicates a ~4-to 6-fold discontinuity across the core-mantle boundary. The core's efficient cooling resulting from the depth-dependent, high conductivity diminishes thermal convection and forms thermal stratification, substantially contributing to cessation of Martian dynamo. [ABSTRACT FROM AUTHOR]

Three-dimensional models of Earth's seismic structure can be used to identify temperature-dependent phenomena, including mineralogical phase and spin transformations, that are obscured in 1-D spherical averages. Full-waveform tomography maps seismic wave-speeds inside the Earth in three dimensions, at a higher resolution than classical methods. By providing absolute wave speeds (rather than perturbations) and simultaneously constraining bulk and shear wave speeds over the same frequency range, it becomes feasible to distinguish variations in temperature from changes in composition or spin state. We present a quantitative joint interpretation of bulk and shear wave speeds in the lower mantle, using a recently published full-waveform tomography model. At all depths the diversity of wave speeds cannot be explained by an isochemical mantle. Between 1000 and 2500 km depth, hypothetical mantle models containing an electronic spin crossover in ferropericlase provide a significantly better fit to the wave-speed distributions, as well as more realistic temperatures and silica contents, than models without a spin crossover. Below 2500 km, wave speed distributions are explained by an enrichment in silica towards the core-mantle boundary. This silica enrichment may represent the fractionated remains of an ancient basal magma ocean. This study reveals that in the Earth's mid-mantle, ferropericlase (the second most abundant mineral) undergoes a major electronic reconfiguration. At the base of the mantle, an enrichment in silica may represent a crystallised ancient magma ocean. [ABSTRACT FROM AUTHOR]

Many seismic tomography studies have indicated that the African Large Low Velocity Province (LLVP) extends from the lower mantle beneath southern Africa into the upper mantle beneath eastern Africa; however, it has been questioned whether the LLVP structure may also extend to the north or northwest beneath south‐central Africa. Debates regarding the upper mantle structure beneath the Damara Belt contribute to this uncertainty. Some studies suggest the Damara Belt is underlain by thermally perturbed upper mantle; however, other studies indicate the region is not associated with anomalous structure. Here, we use a comprehensive P‐wave travel‐time data set and an adaptive model parameterization to develop a new tomographic model for the Damara Belt and surrounding regions. Our results show that seismically slow structure beneath the Damara Belt is relegated to depths greater than ∼1,200 km, indicating that the LLVP is not significantly affecting this region. However, further to the northeast, the LLVP structure obliquely rises and crosses the mantle transition zone near the Irumide Belt, where it then extends into the upper mantle. The seismic structure beneath the Damara Belt and neighboring areas in our model correlates well with tectonic observations at the surface, including variations in heat flow, the distribution of geothermal features, the locations of rifts, and estimates of dynamic topography. Plain Language Summary: The African Large Low Velocity Province (LLVP) is an anomalous feature in the Earth's mantle, thought to be associated with unique temperature and compositional characteristics. Many prior studies have shown that the LLVP originates near the core‐mantle boundary beneath southern Africa but then ascends to the northeast, reaching the upper part of the mantle beneath eastern Africa. However, it is unclear whether the LLVP also extends to the north or northwest beneath the Damara Belt in south‐central Africa. Using the travel‐times of earthquake signals recorded by stations across the continent, we have created a new model of the seismic velocity structure beneath south‐central Africa, which can be interpreted in terms of thermal and compositional variations. Slow velocities associated with the LLVP are constrained to depths greater than ∼1,200 km beneath the Damara Belt, which indicates that the LLVP has little affect on this region. However, the slow LLVP structure rises to the northeast, moving into the upper mantle beneath the Irumide Belt and the East African Rift System. The trend of the LLVP structure in our model well matches various features observed at the surface. Key Points: New P‐wave tomography model highlights the structure of the Large Low Velocity Province beneath south‐central AfricaAnomalously slow seismic velocities beneath the Damara Belt are constrained to depths below ∼1,200 kmSlow velocities ascend to the northeast, crossing into the upper mantle beneath the Irumide Belt and the East African Rift System [ABSTRACT FROM AUTHOR]

The D″ region of the lower mantle, which lies just above the core–mantle boundary, is distinct from the bulk of the lower mantle in that it exhibits complex seismic heterogeneity and seismic anisotropy. Seismic anisotropy in this region is likely to be largely due to the deformation-induced texture (crystallographic preferred orientation) development of the constituent mineral phases. Thus, seismic anisotropy can provide a marker for deformation processes occurring in this dynamic region of the Earth. Post-perovskite-structured (Mg,Fe)SiO3 is believed to be the dominant mineral phase in many regions of the D". As such, understanding deformation mechanisms and texture development in post-perovskite is important for the interpretation of observed seismic anisotropy. Here, we report on high-pressure diamond anvil cell deformation experiments on NaMgF3 neighborite (perovskite structure) and post-perovskite. During deformation, neighborite develops a 100 texture, as has been previously observed, both in NaMgF3 and MgSiO3 perovskite. Upon transformation to the post-perovskite phase, an initial texture of {130} at high angles to compression is observed, indicating that the {100} planes of perovskite become the ~{130} planes of post-perovskite. Further compression results in the development of a shoulder towards (001) in the inverse pole figure. Plasticity modeling using the elasto-viscoplastic self-consistent code shows this texture evolution to be most consistent with deformation on (001)[100] with some contribution of glide on (100)[010] and (001)<110> in NaMgF3 post-perovskite. The transformation and deformation mechanisms observed in this study in the NaMgF3 system are consistent with the behavior generally observed in other perovskite–post-perovskite systems, including the MgSiO3 system. This shows that NaMgF3 is a good analog for the mantle bridgmanite and MgSiO3 post-perovskite. [ABSTRACT FROM AUTHOR]

Unraveling the short‐term behavior of the Earth's past geomagnetic field at regional scales is crucial for understanding its global behavior and, thus, the dynamics of the deep Earth. In this context, obtaining accurate full‐vector geomagnetic field records from regions where archeomagnetic data are absent becomes essential. Here, we present the first full‐vector archeomagnetic data from Central Asia, derived from the analysis of nine archeological kilns sampled in South Uzbekistan, dating back to the period between 200 BCE and 1429 CE. To obtain these new data, we conducted thermal and alternating field demagnetization procedures, along with Thellier‐Thellier paleointensity experiments, including partial thermoremanent magnetization checks, thermoremanent magnetization anisotropy and cooling rate corrections. The comparison between the new data, previous selected data from Central Asia, and available global models reveals important differences between approximately 400 BCE and 400 CE, especially concerning the geomagnetic field intensity element. In order to investigate this in detail, we have developed a regional update of the SHAWQ global models family by incorporating, for the first time, high‐quality data from Central Asia. The results suggest that this deviation is linked to non‐dipolar sources of the geomagnetic field in Central Asia reaching a maximum contribution around the first century BCE. According to the updated global paleoreconstruction, this non‐dipole feature, manifested at the Earth's surface as low intensities, is associated with the presence of a reversed flux patch at the core‐mantle boundary beneath this region. Plain Language Summary: The geomagnetic field is a global feature that changes over time and space. These changes are known over the last few centuries through direct observations collected by satellites and geomagnetic observatories. However, indirect measurements, based on the paleomagnetic study of archeological and geological materials, are needed to disentangle geomagnetic field behavior over longer time scales. In this context, archeomagnetism is the discipline that studies the direction and strength of the past geomagnetic field by investigating the magnetic properties of well dated archeological baked clay material. In order to properly describe the behavior of the Earth's magnetic field it is crucial to have a good temporal and spatial coverage of archeomagnetic data. In this work, we present the first full‐vector archeomagnetic data for Central Asia, a large region that remained unexplored from an archeomagnetic point of view. The new data were obtained from 9 kilns excavated in South Uzbekistan, with ages ranging from 200 BCE to 1429 CE. We then developed a regional update of the SHAWQ global models family by incorporating, for the first time, high‐quality archeomagnetic data from Central Asia. The results indicate a relevant non‐dipole feature presented over this region around the first century BCE. Key Points: First nine full‐vector archeomagnetic data for Central Asia for the last 2k yearsResults indicate a large non‐dipole contribution around the first century BCE in Central AsiaCentral Asia intensity data reveal a reversed flux patch at the core‐mantle boundary [ABSTRACT FROM AUTHOR]

The thermal properties of the Earth's primordial magma are the key factors that constrained crystallization and other thermo‐chemical processes in Earth's primitive magma ocean and therefore controlled the Earth's long‐term evolution. Thermal conductivity of the primordial magma is conventionally assumed to be a constant of about 4 W m−1 K−1 under the high pressure‐temperature conditions of the primitive magma ocean. Here we measured the lattice thermal conductivity of a variety of basaltic and silicate glasses at high pressures and a wide range of temperatures. Our results suggest that the primordial magma, if it is indeed represented by basaltic melts, had a thermal conductivity of ∼1.0–1.9 W m−1 K−1, much lower than previously thought. Such low thermal conduction reduced heat loss and thus prolonged the cooling time of the early magma ocean, promoting convection in the solidifying mantle and preventing a global overturn. Moreover, if the seismic ultralow velocity zones presently observed in the lowermost mantle are made of basaltic melts, originating either from remnants of the primitive magma ocean or pieces of subducted crust, the material in these zones must have an ultralow thermal conductivity, which would reduce cooling and thus influence the thermo‐chemical evolution of the present day core‐mantle boundary. Plain Language Summary: It is believed that a global magma ocean existed in the early Earth's interior. The thermo‐chemical evolution of the primitive magma ocean holds the key to understand the Earth's subsequent evolution and even its present day structure. Thus, knowledge of the fundamental physical properties of the primitive magma ocean, in particular, the thermal properties, would significantly advance our comprehension of our planet's history. Here, we precisely determined the thermal conductivity of a series of basaltic and silicate glasses at the high pressure‐temperature conditions assumed to be characteristic of the primitive magma ocean. Our results suggest that the primitive magma ocean, if it indeed consisted of basaltic melts, had a thermal conductivity much lower than previously assumed, which substantially slowed down cooling and allowed convection of the magma ocean before it entirely solidified. Furthermore, if the ultralow seismic velocity zones observed at present are remnants of the primitive magma ocean, their exceptionally low thermal conductivity should hinder heat transfer and thus strongly affect the thermo‐chemical state of the core‐mantle boundary. Key Points: We measured lattice thermal conductivity of basaltic and silicate glasses at high pressures and wide range of temperaturesIf primordial magma can be modeled as basaltic melts, its thermal conductivity in the mantle is much lower than previously thoughtSuch poorly conductive primitive magma ocean has a longer lifetime, facilitating solid‐state convection in the solidified mantle [ABSTRACT FROM AUTHOR]

The bottom of the lower mantle is a key region for material circulation and energy exchange within the Earth, with extremely high heterogeneity and complex dynamics processes. Although tomography models have revealed the large-scale velocity structure at the bottom of the lower mantle, the nature of the small-scale lateral heterogeneity structure remains controversial due to resolution limitations. The scattering observations of PKP precursors have been widely used to constrain the small-scale structures at the bottom of the lower mantle due to their special sampling paths and arrival time characteristics. This study cross-validates the presence of scatterers at the bottom of the lower mantle in the northern Bay of Bengal and the northern Gulf of Mexico through migration and array analysis of PKP precursors sampled from seismic arrays in the Sichuan–Yunnan and adjacent areas in China. The forward modelling of the envelope of PKP precursors using the Monte Carlo seismic phonon method reveals that their P -wave velocity perturbations are 0.3 and 0.55 per cent in each area, respectively. Based on the distribution range of the small-scale scatterers, we infer that the northern Bay of Bengal scatterer lies within 200 km above the core–mantle boundary, whereas the thickness of the scattering layer in the northern Gulf of Mexico is approximately 250 km. We propose that the small-scale lateral heterogeneities observed in both regions originate from subducted slabs and may have been transitioned into post-perovskite. [ABSTRACT FROM AUTHOR]

The fractional crystallization of the lunar magma ocean (LMO) results in a gravitationally unstable layering, with dense Fe‐ and Ti‐oxides overlying lighter mafic cumulates. Due to their high density, these are prone to overturn via Rayleigh‐Taylor instability. However, this instability competes with the downward growth of the cold and stiff stagnant lid, which tends to trap the cumulates preventing their overturn. The fate of high‐Ti cumulates (HTC) plays a major role in several aspects of the Moon's history, including mare volcanism, early magnetism, and the presence of a partially molten layer at the core‐mantle boundary (CMB). To assess the extent of the overturn of HTC, we use 2D simulations of thermo‐chemical mantle convection in the presence of a solidifying LMO. The long lifetime of the magma ocean caused by the insulating effect of the plagioclase crust delays the growth of the stagnant lid and promotes the onset of solid‐state convection during magma ocean solidification. Both phenomena favor a high degree of mobilization of the HTC. Independent of the rheology of the mafic and HTC, the overturn is always characterized by small‐scale instabilities and is completed within ∼300–600 Myr. High‐Ti material accumulates at the CMB where it undergoes partial melting until present‐day, in agreement with the existence of a deep, partially molten layer inferred from geodetic data. Part of the overturned cumulates is entrained by mantle flow and can participate in secondary melting until ∼1 Ga, in agreement with the age of high‐Ti mare basalts. Plain Language Summary: The Moon likely underwent a hot accretion resulting in a deep magma ocean. An important consequence of the solidification of the lunar magma ocean is the formation of a shallow layer, rich in Titanium (Ti) and denser than the underlying mantle. This configuration is gravitationally unstable, with the Ti‐rich layer that is prone to sink to the bottom of the mantle—an event referred to as the lunar mantle overturn. Yet, cooling of the lunar crust and shallow mantle can be efficient enough to stiffen this layer, preventing its mobilization and overturn. Understanding to what extent the Ti‐rich layer sunk through the mantle is important because its fate influences many aspects of the lunar evolution, including volcanism, magnetism, and the present‐day thermal state of the Moon. Here, we use numerical simulations to show that, if the Ti‐rich layer started sinking while the magma ocean was still extant, the overturn would have been much more efficient than if it started only after the magma ocean solidified completely, as assumed in previous studies. Indeed, the hot magma ocean delays the cooling of the crust and shallow mantle and facilitates the mobilization of Ti‐rich material as soon as it crystallizes. Key Points: Solid‐state mantle convection during lunar magma ocean solidification enhances the overturn of high‐Ti cumulatesThe overturn always occurs via small‐scale downwellings and lasts at most a few hundred MyrOverturned material accumulates above the core‐mantle boundary where it heats up and partially melts without undergoing any large‐scale destabilization [ABSTRACT FROM AUTHOR]

Abstract There are various hotspots in the Atlantic Ocean, which are underlain by mantle plumes that likely cross the mantle and originate at the core‐mantle boundary. We use teleseismic core‐diffracted shear waves to look for an Ultra‐Low Velocity Zone (ULVZ) at the potential base of central Atlantic mantle plumes. Our data set shows delayed postcursory phases after the core‐diffracted shear waves. The observed patterns are consistent in frequency dependence, delay time, and scatter pattern with those caused by mega‐ULVZs previously modeled elsewhere. Synthetic modeling of a cylindrical structure on the core‐mantle boundary below St. Helena provides a good fit to the data. The preferred model is 600 km across and 20 km high, centered at approximately 15° South, 15° West, and with a 30% S‐wave velocity reduction. Significant uncertainties and trade‐offs do remain to these parameters, but a large ULVZ is needed to explain the data. The location is west of St. Helena and south of Ascension. Helium and neon isotopic systematics observed in samples from this region could point to a less‐outgassed mantle component mixed in with the dominant signature of recycled material. These observations could be explained by a contribution from the Large Low Shear Velocity Province (LLSVP). Tungsten isotopic measurements would be needed to understand whether a contribution from the mega‐ULVZ is also required at St. Helena or Ascension.

The melting temperatures of Fe–Ni alloys and their densities in the liquid state at relevant pressure–temperature (P–T) conditions present in the core are of great importance for understanding the composition and thermal structure of the Earth's core. We measured shock temperatures of the Fe–11Ni–18Cr (wt. %) alloy up to ∼304 GPa using a special target configuration, a quasi-spectral pyrometer, and velocimeter diagnostics in a two-stage light-gas gun. The present results show that Fe–11Ni–18Cr starts to melt at the pressure of 210 (8) GPa and 4700 (300) K and completes at the pressure of 280 (10) GPa and 5250 (350) K under shock loading, which is ∼1000 K lower than some previous shock temperature measurements. The melting temperatures of the Fe–11Ni–18Cr alloy are 4100 (250) K and 5500 (450) K at the pressures present at the core-mantle boundary (∼136 GPa) and inner-core boundary (∼330 GPa), respectively, which are slightly lower than that of pure iron. Combined with the previous results of the pressure–density measurements at Hugoniot states, our results indicate that the presence of limited amounts of Ni and Cr into Fe has a minor effect on its melting curve and density in the liquid state, suggesting that sufficient light elements are required in the outer core to satisfy both the core density deficit and the reduced melting temperature. [ABSTRACT FROM AUTHOR]

Recent seismological studies challenge the traditional view that the interface between the core and mantle is a straightforward discontinuity. As seismology is pushed to its observational limits, a complex - potentially compositionally layered - region between the core and mantle is emerging. [ABSTRACT FROM AUTHOR]

The core‐mantle boundary (CMB) marks the most dramatic changes in physical properties within the Earth, and plays a critical role in the understanding of the Earth's dynamics. PmKP waves are seismic phases that reflect (m − 1) times under the CMB and are useful for studying the complex CMB structure. We present an automated workflow for detecting PmKP phases using multi‐station records from global seismic stations. We employ a novel sampling method to extract PmKP waveforms into a 2‐D matrix. Two deep neural networks are then utilized for initial phase detections and subsequent slowness validations. Numerous PmKPab (3 ≤ m ≤ 7) and their CMB diffracted signals were identified for deep earthquakes (magnitude >6) occurred from 2000 to 2020, including diffracted P7KPab waves with diffraction lengths of nearly 20°. Our approach significantly improves the efficiency of PmKP phase identification and holds the capability to detect other weak core phases, such as PKiKP. Plain Language Summary: Seismic waves provide invaluable information about the Earth's core when sampling the core‐mantle boundary (CMB) complex structures. However, adequate sampling of this region remains a challenge due to the uneven distribution of earthquakes and seismic stations. PmKP is a seismic phase that reflects multiple times (m − 1) within the CMB, and even though it has been considered elusive, it offers an opportunity to overcome the sampling limitation caused by the earthquake‐station distribution and enhance the CMB seismic coverage. Here, we propose a new automated approach to detect unprecedent high reverberations of PmKP. Our method utilizes a combination of two deep neural networks to analyze seismic records from stations distributed globally. By applying this approach to seismic records between 2000 and 2020, we successfully detected up to six underside reflections of PmKP phases and their CMB diffracted waves. Our method enhances the efficiency of identifying PmKP phases and has the potential to be applied to other core seismic phases. Key Points: We propose a workflow based on neural networks to detect rarely observed core seismic phases and their diffractionsOur method successfully detects PmKP up to six underside reflections on the core‐mantle boundary (CMB), increasing the seismic sampling of the CMBThe new workflow can be used for future automated identification of other rare or weak seismic phases [ABSTRACT FROM AUTHOR]

Surface hotspot motions are approximately a factor of two faster in the Pacific than the Indo‐Atlantic, and the Indo‐Atlantic large low shear velocity province (LLSVP) appears to be significantly taller than the Pacific LLSVP. Hypothesizing that surface hotspot motions are correlated with the motion of plume sources on the upper surface of chemically distinct, intrinsically dense LLSVPs, we use 3D spherical mantle convection models to compute the velocity of plume sources and compare with observed surface hotspot motions. No contrast in the mean speed of Pacific and Indo‐Atlantic hotspots is predicted if the LLSVPs are treated as purely thermal anomalies and plume sources move laterally across the core‐mantle boundary. However, when LLSVP topography is included in the model, the predicted hotspot speeds are, on average, faster in the Pacific than the Indo‐Atlantic, even when modest topography is assigned to both LLSVPs (e.g., 100–300 km). The difference in mean hotspot speed increases to a factor of two for larger and laterally variable LLSVP topography estimated from seismic tomographic model S40RTS (up to 1,100–1,500 km for the Indo‐Atlantic region vs. 700–1,400 km for the Pacific region) and our results also broadly reproduce the convergence of Pacific hotspots toward the center of the Pacific LLSVP. These largescale features of global hotspot motions are only reproduced when ambient mantle material flows over large, relatively stable topographical features, suggesting that LLSVPs are chemically distinct and intrinsically dense relative to ambient mantle material. Plain Language Summary: Seismic observations reveal two continent‐sized anomalies at the base of the Earth's mantle, referred to as Large Low‐Shear‐Velocity Provinces (LLSVPs), through which seismic waves travel slowly. Slow seismic velocity is generally interpreted as increased temperature. However, buoyant plumes of hot material rise from the LLSVPs and produce hotspot volcanoes on the Earth's surface, and the geochemistry of these lavas suggest that LLSVPs are primordial, which suggests the LLSVPs cannot be purely thermal anomalies. To explain why the LLSVPs have remained at the base of the mantle for billions of years, they must have higher intrinsic density that the ambient mantle in addition to being hot, but this is still debated. To contribute to understanding whether LLSVPs are intrinsically dense, chemically distinct piles versus purely thermal, we compare the velocity of ambient mantle flow, which may advect plume sources over LLSVPs, with surface hotspot motions. Results show that the largescale differences in surface hotspot motions are reproduced only when LLSVPs behave like topography that deflects ambient mantle flow. Because LLSVPs would only behave like topography that deflects ambient mantle flow if they are denser than ambient mantle material, our results provide additional evidence in support of LLSVPs as dense thermochemical piles. Key Points: Pacific hotspot motions are on average two times faster than Indo‐Atlantic hotspots in commonly used reference framesThe contrast in mean hotspot speed is reproduced in the models only if Large Low‐Shear‐Velocity Provinces (LLSVPs) behave like topography that plume sources are advected overLLSVPs would behave like topography that plume sources are advected over only if they are denser than ambient mantle material [ABSTRACT FROM AUTHOR]

It is a well-accepted hypothesis that deep-mantle primary plumes originate from a buoyant source layer at the core–mantle boundary (CMB), where Rayleigh–Taylor instabilities (RTIs) play a key role in the plume initiation process. Previous studies have characterized their growth rates mainly in terms of the density, viscosity and layer-thickness ratios between the denser overburden and the source layer. The RTIs, however, develop in the presence of global flows in the overlying mantle, which can act as an additional factor in the plume mechanics. Combining 2-D computational fluid dynamic (CFD) model simulations and a linear stability analysis, this paper explores the influence of a horizontal global mantle flow in the instability dynamics. Both the CFD simulation results and analytical solutions reveal that the global flow is a dampening factor in reducing the instability growth rate. At a threshold value of the normalized global flow velocity, short- as well as long-wavelength instabilities are completely suppressed, allowing the entire system to advect in the horizontal direction. Using a series of real-scale numerical simulations, this paper also investigates the growth rate  as a function of the density contrast, expressed in Atwood number |${A}_T = ({{{\rho }_1 - {\rho }_2}})/({{{\rho }_1 + {\rho }_2}})$|⁠ , and the viscosity ratio |$\ {\mu }^* = \ {\mu }_1/{\mu }_2$|⁠ , where |${\rho }_1,\ {\mu }_{1\ }$| and |${\rho }_{2,}\ {\mu }_{2\ }$| are densities and viscosities of the overburden mantle and source layer, respectively. It is found that increase in either |${A}_T$| or |${\mu }^*$| promotes the growth rate of a plume. In addition, the stability analysis predicts a nonlinearly increasing RTI wavelength with increasing global flow velocity, implying that the resulting plumes widen their spacing preferentially in the flow direction of kinematically active mantle regions. The theory accounts for additional physical parameters: source-layer viscosity and thickness in the analysis of the dominant wavelengths and their corresponding growth rates. The paper finally discusses the problem of unusually large inter-hotspot spacing, providing a new conceptual framework for the origin of sporadically distributed hotspots of deep-mantle sources. [ABSTRACT FROM AUTHOR]

Solidification of the cores of small planets and moons is thought to occur in the "iron snow" regime, in which iron crystals form near the core‐mantle boundary and fall until re‐melting at greater depth. The resulting buoyancy flux may sustain convection and dynamo action. This regime of crystallization is poorly understood. Here we present the first laboratory experiments designed to model iron snow. We find that solidification happens in a cyclic pattern, with intense solidification bursts separated by crystal‐free periods. This is explained by the necessity of reaching a finite amount of supercooling to re‐initiate crystallization once the crystals formed earlier have migrated away. When scaled to planetary cores, our results suggest that crystallization and the associated buoyancy flux would be strongly heterogeneous in time and space, which eventually impacts the time variability and geometry of the magnetic field. Plain Language Summary: In small planets or moons with iron core, solidification proceeds from the top down, producing solid iron crystals at the top of the core. These crystals then fall until they melt at deeper depth, where the temperature is larger. By analogy with snow in the atmosphere, this regime is called iron snow. It creates motions in the liquid core and provides energy for generating a magnetic field. However, the key aspects of this regime remain largely unknown. Using analog laboratory experiments, we have found that solidification occurs in a cyclic pattern, with periods of intense crystal formation followed by quiet periods with no crystals. This happens because crystallization needs a certain amount of cooling below the solidification temperature to be triggered, during which all crystals have risen and melted. Applied to planetary cores, it means that the iron snow would be heterogeneous in space and time, with intermittent and localized crystals falling. This would affect the shape and strength of the planet's magnetic field. Key Points: We have carried out an experimental study of the dynamics of iron snowOur experiments present crystallization cycles, with intense solidification bursts separated by quiet periodsThis cyclic pattern is controlled by thermal diffusion and by the amount of supercooling required for crystallization [ABSTRACT FROM AUTHOR]

Geodynamic modeling and seismic studies have highlighted the possibility that a thin layer of low seismic velocities, potentially molten, may sit atop the core‐mantle boundary but has thus far eluded detection. In this study we employ normal modes, an independent data type to body waves, to assess the visibility of a seismically slow layer atop the core‐mantle boundary to normal mode center frequencies. Using forward modeling and a data set of 353 normal mode observations we find that some center frequencies are sensitive to one‐dimensional kilometer‐scale structure at the core‐mantle boundary. Furthermore, a global slow and dense layer 1–3 km thick is better‐fitting than no layer. The well‐fitting parameter space is broad with a wide range of possible seismic parameters, which precludes inferring a possible composition or phase. Our methodology cannot uniquely detect a layer in the Earth but one should be considered possible and accounted for in future studies. Plain Language Summary: Normal modes are long‐period oscillations of the whole Earth as it vibrates after large earthquakes. The frequency that a mode oscillates at depends on the interior structure of the Earth. Research suggests a global and thin layer of anomalous composition and low seismic wave speeds may have formed at the base of Earth's mantle, but would be difficult to observe seismically. We test and quantify the effect of this layer on the frequencies at which normal modes vibrate. We then compare these predictions to a large data set of normal mode frequency measurements to examine whether such a layer is consistent with observed data. We find that not only is a layer of 1–3 km thickness permitted by the modes, but that a layer being present improves the fit to the data. There are a wide range of parameters that adequately fit the data set so we cannot be specific about its properties. Furthermore a layer is likely not a unique way to improve the model. A seismically slow layer at the core‐mantle boundary has implications for processes in the mantle and outer core and the interaction between them. Key Points: Normal mode center frequencies are a sensitive data type to detect seismically anomalous thin layers at the core‐mantle boundaryA slow and dense layer on the order of 1 km thick atop the core‐mantle boundary can improve the fit to normal mode dataThe inclusion of a layer is likely not a unique way to improve the 1D model and layer properties remain uncertain [ABSTRACT FROM AUTHOR]