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Major and trace element abundances, including highly siderophile elements, and 187Os and 182W isotopic compositions were determined for ca. 89 Ma mafic and ultramafic rocks from the islands of Gorgona (Colombia) and Curaçao (Dutch Caribbean). The volcanic systems of both islands were likely associated with a mantle plume that generated the Caribbean Large Igneous Provence. The major and lithophile trace element characteristics of the rocks examined are consistent with the results of prior studies, and indicate derivation from both a chemically highly-depleted mantle component, and an enriched, or less highly-depleted mantle component. Highly siderophile element abundances for these rocks are generally similar to rocks with comparable MgO globally, indicating that the major source components were not substantially enriched or depleted in these elements. Rhenium-Os isotopic systematics of most rocks of both islands indicate derivation from a mantle source with an initial 187Os/188Os ratio between that of the contemporaneous average depleted mid-ocean ridge mantle and bulk silicate Earth. The composition may reflect either an average lower mantle signature, or global-scale Os isotopic heterogeneity in the upper mantle. Some of the basalts, as well as two of the komatiites, are characterized by calculated initial 187Os/188Os ratios 10–15 % higher than the chondritic reference. These more radiogenic Os isotopic compositions do not correlate with major or trace element systematics, and indicate a mantle source component that was most likely produced by either sulfide metasomatism or ancient Re/Os fractionation. Tungsten-182 isotopic compositions measured for rocks from both islands are characterized by variable μ182W values ranging from modern bulk silicate Earth-like to strongly negative values. The μ182W values do not correlate with major/trace element abundances or initial 187Os/188Os compositions. As with some modern ocean island basalt systems, however, the lowest μ182W value (−53) measured, for a Gorgona olivine gabbro, corresponds with the highest 3He/4He previously measured from the suite (15.8 R/R A). Given the lack of correlation with other chemical/isotopic compositions, the mantle component characterized by negative μ182W and possibly high 3He/4He is most parsimoniously explained to have formed as a result of isotopic equilibration between the mantle and core at the core-mantle boundary. [ABSTRACT FROM AUTHOR]

Iron hydride in Earth's interior can be formed by the reaction between hydrous minerals (water) and iron. Studying iron hydride improves our understanding of hydrogen transportation in Earth's interior. Our high-pressure experiments found that face-centered cubic (fcc) FeH x (x ≤ 1) is stable up to 165 GPa, and our ab initio molecular dynamics simulations predicted that fcc FeH x transforms to a superionic state under lower mantle conditions. In the superionic state, H-ions in fcc FeH become highly diffusive-like fluids with a high diffusion coefficient of ∼3.7 × 10−4 cm2 s−1, which is comparable to that in the liquid Fe-H phase. The densities and melting temperatures of fcc FeH x were systematically calculated. Similar to superionic ice, the extra entropy of diffusive H-ions increases the melting temperature of fcc FeH. The wide stability field of fcc FeH enables hydrogen transport into the outer core to create a potential hydrogen reservoir in Earth's interior, leaving oxygen-rich patches (ORP) above the core mantle boundary (CMB). [ABSTRACT FROM AUTHOR]

Proton self diffusion coefficients for bridgmanite at lower mantle conditions are calculated from ab initio molecular dynamics simulations. We find that the proton self diffusion coefficient, D self is nearly constant ∼ 10−8 m2 s−1 along the lower mantle geotherm but increases by nearly one order of magnitude from ∼10−10 m2 s−1 to ∼ 10−9 m2 s−1 along a cold slab geotherm to about 1800 km depth. These rates imply that the proton diffusion length scale is less than 10 km in lower mantle peridotite in the 150-200 million years timescale for slab material to sink through the lower mantle. Cold wet slabs probably lose less than one percent of their total water content to the ambient mantle on their journey through the lower mantle, indicating that recycled water is far from homogeneously distributed since slab delivery is highly heterogeneous. We estimate that 0.1 to 0.3 ocean masses (<100 ppm wt%) of recycled water may be currently stored in slab remnant materials within the lower mantle. This water is likely not entrained by plumes but is instead captured by background mantle flow before returning to the mid-ocean ridges. By contrast, deep-rooted mantle plumes may entrain materials containing primordial-like water from the lowermost mantle or the core, and preserve these anomalies in fairly small-scale heterogeneities. Over the age of the Earth, the proton diffusion length scale is a few tens of km, which places constraints on the size of possible primordial water reservoirs isolated from convective mixing, and indicates little flux of water across the core-mantle boundary. • Ab initio calculations show that protons diffuse ∼10−8 m2s−1 in bridgmanite. • Slabs lose little water in the lower mantle and thus water is stored heterogeneously. • Less than 0.3 ocean masses of recycled water are stored in lower mantle slabs. • Stiff cold zones surround subducted water and limit hydrous interaction with the core. • Traces of primordial water from the lowermost mantle may be carried upwards by plumes. [ABSTRACT FROM AUTHOR]

In recent decades, ultra-low velocity zones (ULVZs) – thin patches of strongly reduced seismic velocity atop the core-mantle boundary (CMB) – have been inferred from observations of a variety of seismic phases. One such phase is Sdiff and its out-of-plane postcursory energy, Sdiff+. In this study, we present a high quality dataset of Sdiff and Sdiff+ which shows evidence of a ULVZ in the central Pacific, roughly between Hawaii and Marquesas. The observed Sdiff+ have an unusually short dominant period (5–12 seconds) compared to previous Sdiff+ observations, which is indicative of a ULVZ that is approximately 10–15 km thick. We analyse this high frequency dataset using the 2D Wavefront Tracker (2DWT) Bayesian inversion methodology to generate a probabilistic ensemble of ULVZ models. As a result of the uniazimuthal coverage of data, there is a strong southwest-northeast trade-off, but there is a slight preference for the ULVZ to be located just outside the Pacific large low velocity province (LLVP) boundary, roughly centred 20° north of Marquesas and 20° southeast of Hawaii. The 2DWT inversion of Sdiff+ travel times suggest that the ULVZ can be approximated as a cylindrical structure; either as a larger and weaker anomaly (radius 280 km, dVs -20%) close to the LLVP, or a smaller and stronger anomaly (radius 180 km, dVs -30%) further away from the LLVP. Unlike previous broad-scale ULVZs modelled with Sdiff+, this ULVZ is thinner, might lie outside of an LLVP, and lacks a potential relationship to a mantle plume. [ABSTRACT FROM AUTHOR]

Earth's geodynamo has operated for over 3.5 billion years. The magnetic field is currently powered by thermocompositional convection in the outer core, which involves the release of light elements and latent heat as the inner core solidifies. However, since the inner core nucleated no more than 1.5 billion years ago, the early dynamo could not rely on these buoyancy sources. Given recent estimates of the thermal conductivity of the outer core, an alternative mechanism may be required to sustain the geodynamo prior to nucleation of the inner core. One possibility is a silicate dynamo operating in a long-lived basal magma ocean. Here, we investigate the structural, thermal, buoyancy, and magnetic evolution of an Earth-like terrestrial planet. Using modern equations of state and melting curves, we include a time-dependent parameterization of the compositional evolution of an iron-rich basal magma ocean. We combine an internal structure integration of the planet with energy budgets in a coupled core, basal magma ocean, and mantle system. We determine the thermocompositional convective stability of the core and the basal magma ocean, and assess their respective dynamo activity using entropy budgets and magnetic Reynolds numbers. Our conservative nominal model predicts a transient basal magma ocean dynamo followed by a core dynamo after 1 billion years. The model is sensitive to several parameters, including the initial temperature of the core-mantle boundary, the parameterization of mantle convection, the composition of the basal magma ocean, the radiogenic content of the planet, as well as convective velocity and magnetic scaling laws. We use the nominal model to constrain the range of basal magma ocean electrical conductivity and core thermal conductivity that sustain a dynamo. This highlights the importance of constraining the parameters and transport properties that influence planetary evolution using experiments and simulations conducted at pressure, temperature, and composition conditions found in planetary interior, in order to reduce model degeneracies. [Display omitted] • The structural, thermal, buoyancy, and magnetic evolution of an Earth-like planet is investigated • The planet evolution includes a transient iron-rich basal magma ocean with a time-dependent composition • The nominal model predicts a basal magma ocean dynamo followed by a core dynamo after 1 billion years • An early silicate dynamo is possible with an electrical conductivity larger than 21,500 S/m • An early core dynamo is not possible with a thermal conductivity larger than 15.8 W/m/K [ABSTRACT FROM AUTHOR]

• Both 3- and 4-phonon scattering affects the lattice dynamics of MgO at high P-T. • At CMB conditions 3-phonon corrections dominates over 4-phonon effects. • Accounting for only 3-phonon scattering underestimates thermal conductivity up to 45%. • Comparison with experiments highlights the role of extrinsic contributions. • Iron inclusion and extrinsic corrections are suitably treated as a mass disorder. Thermal conductivity of minerals composing planetary mantles plays a fundamental role in controlling the heat propagation and hence the dynamic history of a planet. Here we present ab-initio calculations of the lattice dynamics and thermal conductivity of MgO as a function of temperature and pressure, accounting for phonon scattering and the renormalization it induces. Our calculations, validated by measurements of phonon energies and linewidths, point to a complex interplay between pressure-induced and temperature-induced effects, which influences the predominance of 3- or 4-phonon contributions, the former leading to a reduction in energies, the latter to an increase. Thus, not only the relative magnitude but also the sign of the anharmonic corrections depends strongly on the planetary geotherms. Our study shows that calculations taking into account only 3-phonon scattering underestimate thermal conductivity by 20–45% under the conditions of the Earth's lower mantle, while including anharmonic renormalization up to fourth order provides results in good agreement with high-pressure, high-temperature experiments. The predominance of 3-phonon interactions at core-mantle boundary (CMB) conditions significantly reduces phonon energies, leading to a thermal conductivity of 50.7 Wm-1K-1, further reduced by extrinsic effects, including isotopic disorder, oxygen vacancies and iron inclusion. In particular, oxygen vacancies of up to 1% decrease the thermal conductivity of MgO at CMB conditions by ∼40%, an effect that adds to that of Fe/Mg replacement. Our results indicate that mass disorder effectively reduces thermal conductivity of lower mantle minerals, contributing to the thermal blanketing that limits the heat flux from the core. [ABSTRACT FROM AUTHOR]

FeH x is a crucial mineral in geophysics, but its melting properties remain poorly understood at high pressures. In this Short Communication, we introduce a simple analytical model to access the solid–liquid boundary of FeH x with minimal time and cost. Inspired by the work-heat equivalence principle, our model allows geophysicists to effectively exploit available ab initio resources to directly deduce the melting temperature of FeH x from its equation of state. The obtained results agree quantitatively with cutting-edge atomistic simulations throughout a wide range of hydrogen content and hydrostatic pressure. Therefore, our analyses would have implications for elucidating the internal dynamics of rocky worlds within and beyond our Solar System. • The melting temperature of FeHx is determined up to 1575 GPa via the modified work-heat equivalence principle. • A quantitative agreement between theory and simulation is achieved. • The core crystallization of super-Earths would occur from the bottom up. • The presence of FeHx near Earth's core-mantle boundary can explain the origin of ultralow-velocity zones outside large low-velocity provinces. [ABSTRACT FROM AUTHOR]

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

Elucidating the structure and composition of Mercury is important for understanding its interior dynamics and evolution. The planet is characterised by unusual chemical characteristics and a weak magnetic field generated in a large metallic core, and its early evolution was also marked by the presence of a magnetic field, widespread volcanism and global contraction. Here we develop a parameterised model of coupled core-mantle thermal and magnetic evolution considering a layered Fe-Si(-S) core structure with chemical and physical properties of the mantle and the core based on previous laboratory studies. We seek successful solutions that are consistent with observations of Mercury's long-lived dynamo, total global contraction, present-day crustal thickness, and present-day interior structure. Successful solutions have a mantle reference viscosity > 10 21 Pa s (corresponding to a present-day bulk mantle viscosity > 2 × 10 20 Pa s), a silicon concentration in the core >13 wt%, a present inner core radius of ∼ 1000 − 1200 km and a thermally stable layer ∼ 500 − 800 km thick below the core-mantle boundary. Our results show that if present, a molten FeS layer atop the core has minimal effect on Mercury's long-term thermal and magnetic evolution. Predictions from our models can be tested with upcoming Bepi-Colombo observations. • We model core-mantle thermal-magnetic evolution of Mercury with an Fe-Si-S core. • We seek models matching Mercury's dynamo activity, contraction and crustal thickness. • Viable models have > 13 wt% core Si, a ∼1100 km inner core and ∼700 km stable layer. • A molten FeS layer atop the core has minimal effect on Mercury's evolution. [ABSTRACT FROM AUTHOR]

Palaeomagnetic and modern geomagnetic measurements indicate that the South Atlantic Anomaly (SAA) has undergone rapid changes over the past few hundred years. Its minimum intensity decreased at an average rate of 26 not yr−1, accompanied by a continuous westward drift and spatial expansion. Recently, a secondary minimum of SAA emerged near southern Africa, leading to speculation that expansion of the SAA could indicate an impending geomagnetic reversal. Here, we focus on the evolution and disappearance of the paleo-West Pacific Anomaly (WPA), as another SAA-like structure, which may have implications for the future of SAA evolution. We regard the WPA as SAA-like due to its feature and its association with a reversal flux patch on the core-mantle boundary. Consequently, we suggest that the observed evolutionary pattern in the WPA can serve as a reference for other negative anomalies, such as the SAA. By analysing models that combine datasets of archaeomagnetic and historical records, such as gufm1 and HistKalmag, it is found that the WPA occurred between 1600 and 1820 CE. Over its duration, the WPA experienced phases of rapid expansion, drift, and division. Eventually, its primary component faded away, giving rise to a new segment that continued to expand. The initial two evolutionary phases of the WPA are similar to the evolution of the SAA over the past century. According to the WPA's evolution, it suggests that the current state of the SAA may correspond to an early stage of splitting. Forecasts based on the evolution of the WPA indicate a rapid expansion of the anomalous region in the short term, followed by a gradual reduction in its primary component and continued expansion of a new local minimum. This study provides valuable insight into the evolution of the SAA and highlights the potential utility of the WPA as an evolutionary reference for such geomagnetic phenomena. [Display omitted] • The evolutionary behavior of the West Pacific Anomaly (WPA) was elucidated. • The WPA shares an evolutionary pattern with the South Atlantic Anomaly (SAA). • The future evolution of the SAA is predicted using the WPA evolution. • The SAA is currently in the early stages of splitting. [ABSTRACT FROM AUTHOR]

[Display omitted] Water in the mantle transition zone and the core-mantle boundary plays a key role in Earth's stratification, volatile cycling, and core formation. If water transportation is actively running between the aforementioned layers, the lower mantle should contain water channels with distinctive seismic and/or electromagnetic signatures. Here, we investigated the electrical conductivity and sound velocity of ε-FeOOH up to 71 GPa and 1800 K and compared them with global tomography data. An abrupt three-order jump of electrical conductivity was observed above 50 GPa, reaching 1.24(12) × 103 S/m at 61 GPa. Meanwhile, the longitudinal sound velocity dropped by 16.8% in response to the high-to-low spin transition of Fe3+. The high-conductivity and low-sound velocity of ε-FeOOH match the features of heterogenous scatterers in the mid-lower mantle. Such unique properties of hydrous ε-FeOOH, or possibly other Fe-enriched phases can be detected as evidence of active water transportation in the mid-lower mantle. [ABSTRACT FROM AUTHOR]

Noble gas isotopes in plume-influenced oceanic-island basalts (OIBs) clearly indicate the existence of distinct reservoirs somewhere in the deep Earth that are characterized by their more primitive noble gas isotopic signatures. As one of the largest differentiated layers on Earth, whether the Earth's liquid outer core can preserve 'primordial' noble gases is fundamental for better understanding the Earth's formation and the mantle evolution. Since Earth's core is believed to have formed via liquid iron segregated from magma ocean during the planetary accretion stage, how much noble gases could have been incorporated into Earth's core depends on the partitioning of noble gases between liquid metal and molten silicates. In this study, we perform thermodynamic integration (TI) calculations based on first-principles molecular dynamics (FPMD) simulation to investigate the partition coefficients of noble gases (NG: He, Ne, Ar, Kr, Xe) between liquid Fe and MgSiO 3 melt. The simulations have been performed by adopting conditions that the pressures are from 10 ∼ 135 GPa and the temperatures from 2300 ∼ 5000 K to fully account for possible core-mantle equilibrium conditions. By analyzing the temperature and pressure dependence of the results, we suggest that the metal-silicate partitioning of noble gases (D NG) merely depends on the temperature, with extremely low D NG < 0.001 at temperatures < 2500 K (20 GPa), but increases to different degrees at high temperatures. The D NG tends to be more affected by temperature as the noble gas atomic mass increases, with the least temperature effect on the partition coefficient of He (D He), which increases from ∼ 10−3 at 2300 K (10 GPa) to ∼ 10−1 (5000 K, 135 GPa) near the core-mantle boundary (CMB), and D Ne = 10−6 ∼ 10−3, D Ar = 10−6 ∼ 10−2, D Kr = 10−7 ∼ 10−1 and D Xe = 10−5 ∼ 100 from 2300 to 5000 K, respectively. Analysis of electronic interactions indicates that noble gases have positive chemical bonding with Fe, Mg, and Si atoms, and the bonding strengths increase with pressure as well as increasing noble gas atomic mass. Based on our calculated partition coefficients, we suggest that Earth's core may have preserved significant amounts of noble gases considering the volatile-rich environment from which the core segregated. [ABSTRACT FROM AUTHOR]

We analyzed the impacts of data span on trend estimates using Earth's long-term polar motion time series, 1846-present, and using methodologies including singular spectrum analysis, and Panteleev's filter to mitigate the time series containing transient signals. Our results show that the fluctuations of the mean rotational pole position, the Markowitz wobble, cannot be fully explained by the oceanic and atmospheric excitations. However, there exists plausible similarity with the variations of amplitudes of the Chandler wobble. To explain the abrupt deviation of the mean pole from the previous state after year 2000, we first compute Earth rotation excitations, using the temporal variations of the second-degree Stokes coefficients, C 21 , S 21 , estimated from GRACE, GRACE Follow-On and Satellite Laser Ranging (SLR), 2002–2021. We then compare their trend estimates with that of the Earth's polar motion, and conclude that the drift of the pole is consistent with the climate-induced mass redistributions within the Earth system during the past two decades. However, the observed trend is not in exact agreement with the prediction values using contemporary glacial isostatic adjustment (GIA) process forward models. The analysis of the J 2 variations since 1976 from SLR and the corresponding length of day (LOD) changes, reveals a clear trend reversal around the year 2000. However, the observed J 2 variations can only explain ∼ 5 % of the long-term LOD changes. The remaining decadal signal in the LOD, usually accounted for by the angular momentum exchange at the core-mantle boundary, is observed to be anti-correlated with the Earth surface temperature anomaly. The geophysical explanation on these relationships remains elusive, and necessitates future studies. [ABSTRACT FROM AUTHOR]

Pyrite corona nodules from the ∼2.7 Ga Kapai Slate, a thin, sulfidic carbonaceous shale horizon interbedded with basaltic lava flows in the Yilgarn Craton, Western Australia, have concentric compositional and isotopic zoning with distinctive textural differences between cores and mantles. The sieved-textured cores are enriched in highly compatible trace elements, incl. Ni, As, Ag, Te, Sb, Bi and Pb, and depleted in incompatible Mo and Tl, whereas the radiating-textured mantles are strongly depleted in compatible elements and enriched in incompatible elements, relative to the cores. A striking feature of the data is that both the compatible and incompatible elements are linearly correlated, with correlation coefficients as high as 0.99. A marked drop in the concentration of compatible elements and an increase in incompatible elements at the core-mantle boundary is attributed to a sudden change in the rate of growth of the nodules produced by eruption of the voluminous overlying Paringa Basalt. The weight of the basalt produced sudden compaction of the unconsolidated clays below resulting in upward advection of pore fluid, which thinned the boundary layer around the growing nodules, leading to a marked increase in the rate of pyrite growth. Rapid pyrite growth led to a dramatic depletion in highly compatible elements, and to a build-up in incompatible elements, in the boundary layers around the growing nodule mantles, which resulted in extreme depletion of compatible elements, and enrichment in incompatible elements in the nodule mantles, relative to the cores. The corona nodules are also isotopically zoned with cores that have higher δ34S, with small positive Δ33S values, and mantles that have lower δ34S and higher Δ33S. The increase in Δ33S towards the rims is attributed to S 8 being advected to the growing mantles by upward fluid movement during sudden compaction, and the decrease in δ34S to the lighter S isotope, with its higher reactivity and diffusivity, being preferentially incorporated into the fast growing pyrite mantle. The extreme changes in the growth rates of the Kapai Slate corona pyrite nodules provide a new constraint on the partition coefficients of the trace elements between Archean ocean water and sedimentary pyrite. The compatibility of the analysed trace elements decreases in the order Bi > Te > Sb > Ag > Cu > Pb > Ni ≈ As > (Co, Zn, Se, Cd, Mn, W) > Tl > Mo, which is consistent with the order obtained from modern sedimentary pyrites by Large et al. (2014), except for the redox-sensitive elements Mn, Tl and Mo. These differences are attributed to the lower oxygen content of the Archean atmosphere and oceans. [ABSTRACT FROM AUTHOR]

Earth's deep mantle hosts continental-scale enigmatic structures whose origin, characteristics, and fate are uncovered by advances in tomographic modeling, geochemical analysis, measurements of mantle mineral properties under extreme conditions, and novel methods in mantle convection modeling. Probabilistic tomography revealed that these deep mantle structures, dubbed large low shear velocity provinces (LLSVPs), are likely enriched in iron-bearing bridgmanite, making them denser than the surrounding ambient mantle (establishing a stabilizing negative buoyancy). A distinct geochemical signature observed in ocean-island basalts is thought to be sourced from the deep mantle, which indicates that LLSVPs may be candidates for a primordial reservoir enriched in heat-producing elements (promoting a destabilizing positive buoyancy). Using conductivity measurements from upper and lower mantle minerals, we derived a novel parametrized depth-, temperature-, and composition-dependent thermal conductivity model. With thermochemical mantle convection models, we demonstrated how the thermal state and total buoyancy of primordial reservoirs (as an analogue for LLSVPs) are influenced by a dynamically responsive and (spatially, thermally, and compositionally) heterogeneous thermal conductivity field. We examined the influence of moderate to high temperature-dependent conductivity models on the evolution of primordial reservoirs characterized by intrinsic chemical density contrast between 1.5 and 5% and heat-producing element (HPE) enrichment factors between 1 and 20. The lowermost mantle attains mean conductivities of ∼6 W/m/K within reservoirs and ∼9 W/m/K outside for moderate temperature dependence. For a high temperature dependence, mean conductivities are reduced to ∼2 W/m/K and ∼5 W/m/K, respectively. We found that heterogeneous thermal conductivity exacerbates the thermally driven effect of HPE enrichment, and the strength of the thermal contribution to the total conductivity is decisive in determining the long-term stability (> 10 Gyr of simulation time) of primordial reservoirs. For the parameter space we consider, the stability of primordial reservoirs with a particular HPE factor improves with greater density contrast. However, with a particular density contrast, stability decreases with a greater HPE factor. Thus, pile instability occurs for a combination of parameters that promote the domination of thermal buoyancy over chemical buoyancy. For a moderate HPE enrichment (factor of 10) and a highly temperature-dependent conductivity, reservoirs characterized by a density contrast ∼ 4% attain a mean density ∼ 1.4% and achieve long-term stability. For a moderately temperature-dependent conductivity, reservoirs characterized by a density contrast ∼ 3% attain a mean density ∼ 1.0%, consistent with tomographic estimates of LLSVP density and achieving long-term stability. When HPE enrichment is present, we found that a greater thermal buoyancy is more likely when temperature dependence is stronger due to the piles' larger temperature. Thus, a greater intrinsic chemical density contrast is required to maintain stable and long-lived primordial reservoirs. • Instability is more likely when conditions promote thermal over chemical buoyancy • Heterogeneous thermal conductivity intensifies piles' HPE-induced thermal buoyancy • Conductivity's thermal dependence determines the onset of piles' instability • For 10x HPE enrichment, piles with chemical density contrast > 3% remain stable • Low density contrast piles remain stable if lower mantle conductivity is large enough. [ABSTRACT FROM AUTHOR]

Study of the paleosecular variation of the Earth's Magnetic Field is an almost unique way to estimate the critical conditions at the core-mantle boundary. The continuous and quasi-periodic changes in the magnitude and directions (declination and inclination) could be detected from direct instrumental measurements or through the remanent magnetization of volcanic and sedimentary rocks. Burned archaeological artifacts are another reliable source of geomagnetic information. Besides the great efforts in Central and North America to retrieve the fine characteristics of the geomagnetic field during the last centuries, no homogeneous data distribution has still been achieved in Mexico and the surrounding areas. To improve the analysis of the geomagnetic full vector in Mexico, we compiled and updated available data for the last 200 years exclusively based on full geomagnetic vectors obtained from direct historical measurements, magnetic repeat stations, geomagnetic observatories and maritime exploratory campaigns. A relocation error procedure allowed the selection of 658 reliable data, discarding those belonging to the sites located in the most northern part of the analyzed area. The spectral analysis through the second derivatives reveals relatively fast fluctuations in both intensity and directions between 1890 and 1960. Moreover, a marked difference is observed between the Mexican record and global geomagnetic models between 1840 and 1940. • Updated available data for the last 200 years exclusively based on full geomagnetic vectors. • A relocation error procedure allowed the selection of 658 reliable data. • Homogeneously distribution across Mexico and the southern United States. • Marked differences with previously obtained records and main geomagnetic global models. • Extreme variation episodes between 1840 and 1940, some of which were observed in Europe. [ABSTRACT FROM AUTHOR]

We present first principles predictions of the absolute entropy of a silicate liquid (CaSiO 3) over a wide pressure-temperature range encompassing the Earth's mantle (2000–6000 K, 0–270 GPa). The results are derived from molecular dynamics simulations based on density functional theory and the two-phase thermodynamic (2PT) method, which divides the vibrational density of states into solid-like and gas-like parts, and which we describe in detail. The heat capacity derived from the absolute entropy agrees well with experimental measurements at low pressure. We find that the vibrational contribution accounts for more than 75% of the total entropy over the range of our study. We find that the two-body approximation to the excess entropy (relative to the ideal gas), which is computed with knowledge only of the radial distribution function, is excellent over most of the range considered, except at the lowest pressures and temperatures. We also compute the entropy of CaSiO 3 perovskite and use the entropy of liquid and solid phases to determine their absolute free energies and the melting curve. The melting curve agrees well with experiment and independent theoretical determinations based on Clausius-Clapeyron integration and the ZW method, showing a melting temperature of 5600 K at the Earth's core-mantle boundary. We find a nearly universal scaling of the self-diffusion coefficients with the excess entropy D ∗ = 0.9 exp 1.2 S ex , where D* is the self-diffusion coefficient suitably non-dimensionalized using macroscopic thermodynamic properties, and S ex is the excess entropy in units of the Boltzmann constant per atom. We use our results to estimate the temperature distribution in an isentropic, molten silicate Earth, finding 4.4 kJ/kg/K to be the lowest entropy that is completely molten, and producing a temperature at the core-mantle boundary of 6000 K. [ABSTRACT FROM AUTHOR]

The pyrite-type FeO 2 and FeO 2 H were synthesized at the pressure-temperature conditions relevant to Earth's deep lower mantle. Through the water-iron reaction, the pyrite-phase is a good candidate to explain the chemical heterogeneities and seismological anomalies at the bottom of the mantle. The solid solution of pyrite-type FeO 2 and FeO 2 H, namely the FeO 2 H x (0 ​≤ ​ x ​≤ ​1), is particularly interesting and introduces puzzling chemical states for both the O and H atoms in the deep mantle. While the role of H in the FeO 2 –FeO 2 H system has been primarily investigated, discrepancies remain. In this work, we summarize recent progress on the pyrite-phase, including FeO 2 , FeO 2 H, and FeO 2 H x , which is critical for understanding the water cycling, redox equilibria, and compositional heterogenicities in the deep lower mantle. Image 1 • Previous findings on the pyrite-type FeO 2 –FeO 2 H system are summarized and reviewed. • The mobility of H in the FeO 2 –FeO 2 H system was characterized by x-ray diffraction in diamond anvil cell. • The co-existence of pyrite-type phase with high-low concentrations of H was observed in a quickly decompressed sample. [ABSTRACT FROM AUTHOR]

The isotopic compositions of iron in major mantle minerals may record chemical exchange between deep-Earth reservoirs as a result of early differentiation and ongoing plate tectonics processes. Bridgmanite (Bdg), the most abundant mineral in the Earth's lower mantle, can incorporate not only Al but also Fe with different oxidation states and spin states, which in turn can influence the distribution of Fe isotopes between Bdg and ferropericlase (Fp) and between the lower mantle and the core. In this study, we combined first-principles calculations with high-pressure nuclear resonant inelastic X-ray scattering measurements to evaluate the effects of Fe site occupancy, valence, and spin states at lower-mantle conditions on the reduced Fe partition function ratio (β -factor) of Bdg. Our results show that the spin transition of octahedral-site (B-site) Fe3+ in Bdg under mid-lower-mantle conditions generates a +0.09‰ increase in its β -factor, which is the most significant effect compared to Fe site occupancy and valence. Fe2+-bearing Bdg varieties have smaller β -factors relative to Fe3+-bearing varieties, especially those containing B-site Fe3+. Our models suggest that Fe isotopic fractionation between Bdg and Fp is only significant in the lowermost mantle due to the occurrence of low-spin Fe2+ in Fp. Assuming early segregation of an iron core from a deep magma ocean, we find that neither core formation nor magma ocean crystallization would have resulted in resolvable Fe isotope fractionation. In contrast, Fe isotopic fractionation between low-spin Fe3+-bearing Bdg/Fe2+-bearing Fp and metallic iron at the core-mantle boundary may have enriched the lowermost mantle in heavy Fe isotopes by up to +0.20‰. [ABSTRACT FROM AUTHOR]

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

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

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

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

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

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

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

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

The Fe isotope composition of planetary bodies may provide constraints on their accretion modes and/or differentiation processes, but to do so, the Fe isotope systematics of key planetary reservoirs needs to be determined. To investigate this for the Moon, we measured the Fe isotope compositions for a suite of 33 bulk lunar mare basalts and highland rocks. Combined with published data, a compendium of 73 different lunar bulk rocks reveals a statistically significant Fe isotope difference between low-Ti and high-Ti mare basalts, yielding average δ57Fe = 0.127 ± 0.012‰ (2SE; n = 27) and δ57Fe = 0.274 ± 0.020‰ (2 SE; n = 25), respectively, relative to the IRMM-14 isotopic reference material. As lunar basalts are thought to reflect the Fe isotope composition of their respective mantle sources, the estimated relative proportion of the low-Ti and high-Ti source mantle suggests that the lunar upper mantle δ57Fe value should be close to 0.142 ± 0.026‰. Whilst the composition of highland rocks (ferroan anorthosites and Mg-suite rocks) should provide a more global view of the Moon, the calculation of the mean δ57Fe value of 15 available highland rock analyses yields δ57Fe = 0.078 ± 0.124‰. Such a value is not defined precisely enough to be of critical use for comparative planetology. Ferroan anorthosites and Mg-suite rocks also give unresolvable means. It appears that Fe isotope heterogeneity among the lunar highland rocks is caused by non-representatively too small sample aliquots of coarse-grained rocks. It can also be the result of mixed lithologies for some. When the (kinetic) effect of olivine tending towards low δ57Fe and feldspar with predominantly high δ57Fe is cancelled, a more precise δ57Fe value of 0.094 ± 0.035‰ is calculated. It is indistinguishable from the mean δ57Fe of impact melts and is also similar to the upper lunar mantle estimate obtained from mare basalts. Collectively, this newly determined Fe isotope composition of the bulk Moon is indistinguishable from that of the Earth, and heavier than those reported for other planetary bodies. This planetary isotope relationship is only observed for silicon given the currently available mass-dependent stable isotope database. Because both iron and silicon reside in the Earth's metallic core in significant quantities, this may point to the involvement of metallic cores of the Earth and Moon in the interplanetary Fe and Si isotope fractionation. Rather than via high-pressure metal–silicate fractionation at the core–mantle boundary, this would more likely be achieved by partial vaporization of the liquid outer metallic core in the aftermath of a Moon-forming giant impact. [ABSTRACT FROM AUTHOR]

The origin of micro-diamonds is controversial and although the application to determine the grade and value of macro-diamonds in kimberlite/lamproite bodies continues to receive widespread usage there are several outstanding factors generally not considered, the most important of which is genesis. The issue is addressed in this study in the context that two classes of small diamonds (generally <0.5 mm and rarely <1 mm) are recognized. Micro-diamonds sensu-stricto (MDS) are typically sharp-edged octahedra, free of mineral inclusions and surface etching or corrosion, increase exponentially with decreasing size and are in overwhelming larger concentrations, by orders of magnitude, relative to macro-diamonds (>0.5 mm). The second class of small diamonds (SD <0.5 mm), used in industrial applications, may have modified solution-growth morphologies (e.g. dodecahedra, tetrahexahedra and related forms), and include loosely bonded polycrystalline diamonds (framesite), boart, fibrous cubes and broken fragments. There are large differences in volume to surface-area ratios between MDS and SD, demonstrating unequivocally that pristine and solution-modified forms could not have co-existed in equilibrium under the same P-T-t- f O 2 conditions in the mantle. From detailed studies of N and C in diamond, and experimental results on the redox-partitioning of N in the presence of metallic Fe, it is concluded that MDS are plume-related from the D″ core-mantle boundary, and are melt-derived in lower mantle proto-kimberlite. The lower mantle is expectedly saturated in metallic Fe, and is highly depleted in N which is siderophile under very low f O 2 conditions, a setting in which excessively large (∼100 to 3000 ct), but rare Type II mega-diamonds (but also MDS) are inferred to have originated. These diamonds (Type II, Ib, IaA) are distinct from the majority of N-rich Type Ia upper mantle macro-diamonds that grew slowly by metasomatic processes and annealed over long periods. Two crystal growth laws are possibly applicable to the size-distribution of diamonds encountered in kimberlites/lamproites. Gibrat's Law of proportionate, short-term crystal growth in open systems by advection is applicable to magmatic MDS, whereas macro-diamonds bear some relation to McCabe's Law of long-term, relatively constant crystal growth, by diffusion metasomatism. The range from small to large diamonds (SFD size-frequency-distribution) is lognormal but is composed of two segments: the smaller size (<0.5 mm) fraction has an overall linear distribution, whereas macro-diamonds (>0.5 mm) are quadratic. The two distributions meet or overlap in a marked discontinuity, implying but not proving distinct origins. The power law governing SFD lognormal distributions is fundamental and is widespread across an enormous number of disciplines (from biology to economics), and may be universal (e.g. it is applicable to planetary scale meteorite impact craters, and to the SFD of cosmic-diamonds from supernovae explosions). Industry applications in resource predictions are from mixtures of diamonds (MDS and SD), and extrapolation to larger stones is valid because the fundamental law is independent of origins. [ABSTRACT FROM AUTHOR]

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

The origin of variations in 186Os/188Os ratios amongst mantle-derived basaltic and komatiitic lavas remains controversial, with opposing models arguing for deep core-mantle versus shallow mantle sources. Crustal contamination has generally not been favored due to the low Os contents of such sources, meaning that variations in 186Os/188Os would require involvement of extremely high proportions of crustal material. Here we re-examine crustal contamination as an effective means for generating significant 186Os/188Os variations in Earth materials. Using chromitites and peridotites from the Stillwater, Muskox and Rum layered intrusions, we show that radiogenic 186Os/188Os ratios are correlated with 187Os/188Os ratios and can only be explained by shallow-level mixing processes and crustal contamination. The samples have δ 186 Os ([{(186Os/188Os sample[t] /186Os/188Os PM(t)) − 1} × 1000], where the modern primitive mantle [PM] 186Os/188Os is 0.1198388) values ranging between 0.04 to 0.15 for the ∼2.7 Ga Stillwater Igneous Complex, −0.05 to 0.17 for the ∼1.27 Ga Muskox Intrusion, and 0.02 to 0.13 for the ∼0.06 Ga Rum Layered Suite. The highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) contents of the chromitites and peridotites can be modeled through high sulfide-melt partitioning (typically >8000) and emphasize the role of S-saturation and HSE scavenging. Considering the high sulfide-melt partitioning and accounting for high silicate melt to sulfide melt ratios (R -factors), it is possible to explain the variations in 186Os–187Os in layered intrusions using calculated Os isotope crustal evolution growth models. These calculations indicate that <4% of ancient high Pt/Os crustal contributions can explain the composition of the chromitites and peridotites that were examined. Our observations are consistent with published models for chromitite genesis that invoke either crustal melt-primitive melt mixing, or cumulate assimilation. A crustal origin for radiogenic 186Os is a possible cause for 186Os/188Os ratio variations observed in some komatiites. It is more difficult to explain radiogenic 186Os/188Os measured in Hawaiian lavas by crustal contamination processes. Instead, ancient high Pt/Os oceanic crust, shallow mantle sources such as metasomatic sulfide, or metal-rich large low-shear wave velocity provinces at the core-mantle boundary, all remain valid explanations. • Significant measured and initial 186Os/188Os ratio variations in chromitites from layered intrusions. • HSE contents of chromitites and peridotites modeled through sulfide-melt partitioning typically >8000. • Less than 4% of ancient high Pt/Os crust can explain layered intrusion chromitite and peridotite 186Os–187Os variations. • Crustal origin for radiogenic 186Os is possible in intraplate lavas, but Hawaiian lavas may require alternate sources. [ABSTRACT FROM AUTHOR]

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

The lowermost mantle beneath the central Atlantic is bounded by the western edge of African Large Low Shear Wave Province (LLSVP) and a high-velocity region beneath the eastern Americas, which is attributed to the accumulated subducted slab material at the core-mantle boundary (CMB). Data recorded at broadband seismic arrays in western Europe and Morocco allow us to investigate the detailed structure beneath the Central Atlantic, which provides an important constraint on how remnant slab material and LLSVP interact and reshape the D″ layer. Here, we exploit lower mantle triplicated S waveform data from deep earthquakes beneath South America. Our modelling results show that the D″ discontinuity is constantly observed from north to south beneath the Central Atlantic but at varied depths and with varied shear velocities. In the lowermost mantle beneath east of Caribbean, our results indicate a sharp 2.1% shear velocity increase at 340 km above the CMB, representing a possible bridgmanite dominant normal mantle D″ layer. Whereas, the models of northern and southern portions of Central Atlantic indicate a gradual velocity increase of 4.2% across 200 km starting from 360 km above the CMB downwards. We also find a distinct low velocity zone on top of the velocity jump, which can be interpreted as an upwelling produced by the dynamic response of ancient slab material reaching the CMB. • D″ beneath Central Atlantic shows lateral variation in physical properties. • The gradual change in velocity at the D″ discontinuity may be attributed to the slab debris. • Low velocity layer on top of D″ discontinuity might relate to the upwelling at the edge of subducted slab. [ABSTRACT FROM AUTHOR]

The terrestrial planets accreted from a diverse suite of solar system materials ranging from strongly O-deficient materials similar to enstatite chondrites via ordinary chondrite materials to fully oxidised carbonaceous chondrite and cometary materials. Heliocentric zoning with increasingly oxidised planetesimals outwards through the protoplanetary disc is broadly reflected in core fraction and FeOmantle concentration, ranging from 68 wt% core and 0.5 wt% FeOmantle for Mercury to 18 wt% core and 24 wt% FeOmantle for Vesta. Mercury, Venus and Earth grew mostly from materials which were isotopically similar to enstatite chondrites, although Earth and Venus also received more oxidised material. The elevated (Mg + Fe)/Si ratio, compared to chondrites, in the bulk silicate fraction of the terrestrial planets, except for Mercury, may be related to a combination of nebular fractionation associated with forsterite condensation, concentration of olivine-rich chondrules near the mid-plane of the accretion disc and multi-cycle impact erosion of protocrusts. For the extremely reduced Mercury the silicate magma ocean (MO) and a core with 15 wt% Si might have equilibrated with a melt layer of FeS at the core-mantle boundary (CMB). Recent data from the MESSENGER mission combined with experimentally derived phase relations, support estimates of about 0.5 wt% FeO and 10 wt% S in the MO and the current mantle. Core segregation at very high temperatures for the largest planets, Venus and Earth, led to cores with high Si content, even at relatively high oxygen fugacities and FeOmantle contents, because increasing temperature shifts the equilibrium: SiO 2 MO + 2Fe core = 2FeO MO + Si core towards the products (right side). The hot protocores of Venus and Earth might have started with about 5–7 wt% Si and 2–3 wt% O. Mars and Vesta segregated S-rich cores at high oxygen fugacity and low pressure. Strong partitioning of Fe and Mg to melt and solids, respectively, caused neutrally buoyant bridgmanite (bm) to crystallise from the MO at 1700–1860 km depth (72–80 GPa), resulting in a separate basal magma ocean (BMO) within Earth, and probably also in Venus. Slow cooling of a thermally insulated BMO and core, accompanied by protracted crystallisation of bm and ferropericlase (fp), would facilitate core-BMO chemical exchange by reversing the equilibrium SiO 2 MO + 2Fecore = 2FeOMO + Sicore towards the reactants. Transfer of silica crystals and a liquid SiO 2 component from the core to the BMO, and liquid FeO and Fe 2 O 3 components from the BMO to the core, would increase the Si/Mg, Mg/Fe and bm/fp ratio of the resulting cumulates. Because the solidus temperature of peridotite is <200–300 K above the present temperature of the outermost core, and the melting interval of late-stage BMO melt enriched in Al, Fe, Ca and Na would be lower than that of peridotite, the BMO might have persisted through the Hadean and possibly also the Archean. Low solid state diffusion rates, especially in bm, would have restricted the core-mantle interaction upon BMO solidification, but limited core-mantle interaction could possibly occur via partially molten ultra-low velocity zones. An outermost stagnant low-density and low-velocity core layer (E′-layer), with reduced Si and elevated O contents relative to the convecting core, appears to trace the core-BMO exchange. The E′-layer is compositionally gradational towards the convecting core at 445 km below the CMB. High thermal conductivity and minimal convective entrainment in the low-viscosity core fluid might have developed and stabilised such a gradational layer since the Hadean or early Archean. The primordial bm-dominated cumulates with high Mg/Fe ratios and viscosities may have become convectively aggregated into large refractory domains, remaining neutrally buoyant in the middle to upper parts of the lower mantle and resistant to convective destruction. Late-stage dense BMO cumulates with elevated Fe/Mg ratios relative to the bulk mantle composition might represent a suitable material for 100–200 km thick thermochemical piles at the bottom of the large low S-wave velocity provinces (LLSVPs) under Africa and the Pacific. A degree-2 convection pattern, possibly initiated and stabilised during Earth's early rapid rotation, involving antipodally ascending columns in equatorial positions and an intermediary descending longitudinal belt, might have swept the late-stage, dense bridgmanitic cumulates with high Fe/Mg-ratios, viscosity and bulk modulus towards the root zones of the upwelling columns. Unlabelled Image • Core and mantle compositional estimates for the terrestrial planets are provided • Melting and silicate-metal partitioning led to magma oceans (MO) and cores • Core-mantle separation in Earth and Venus occurred at high temperature and pressure • Mineral-melt density relations led to a separate basal MO (BMO) in Earth and Venus • Cooling of Earth and Venus cores led to core-BMO chemical exchange [ABSTRACT FROM AUTHOR]

Seismic tomography reveals 2 extensive regions in the lowermost mantle, beneath Africa and the Pacific, that exhibit lower-than-average seismic wave speeds. These regions have been named the Large Low Shear Velocity Provinces (LLSVPs), and they have spatial scales on the order of 1000s and 100s of kilometers in width and height, respectively. Discovering their cause remains an important challenge in the deep Earth community because recognizing what they are has important, first-order implications toward understanding the nature of global mantle convection and therefore, heat and chemical transport and evolution through time. At about an order-of-magnitude smaller scale are Ultra Low Velocity Zones (ULVZs) that reside on the core-mantle boundary. ULVZs are typically up to 100s of kilometers laterally and only 10s of kilometers vertically. We don't know what LLSVPs and ULVZs are, and the primary question is whether they are thermal or compositional features, and/or both. In any case, there is every reason to suppose that they are linked by a dynamical relationship, and by better understanding one, we can discover more about the other. Here, we review the observations associated with LLSVPs and the various conceptual mantle models that the community is debating regarding their cause. ULVZs, as they may relate to the larger LLSVPs, are also reviewed, and dynamical linkages between the two are discussed. Better understanding both LLSVPs and ULVZs promises to provide critical insight into global scale mantle convection and therefore provide a foundation for understanding numerous other processes in the Earth's interior. • A review of observations regarding Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle • A review of conceptual hypotheses regarding the cause of Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle • An overview of Ultra Low Velocity Zone (ULVZ) observations and ULVZs may be dynamically connected to LLSVPs. [ABSTRACT FROM AUTHOR]

Highlights • The calculated heat productions from the lunar crust to the core support the magma ocean theory. • The heat flux on the lunar core-mantle boundary indicates that the core convection may have ended. • The calculated temperature in the lunar core accords with those derived from the seismic detections. Abstract We present a study on lunar interior temperature and heat production by 1D geothermal inversion. With the layered structure and thermal state of the lunar interior reported by lunar seismic and magnetic sounding research, a forward modeling of deep temperature is given based on the theory of thermal conduction. Then, the particle swarm optimization (PSO) method is applied to implement the inversion of lunar interior heat production. A six-layer model is solved using a global heat flux of 12 mW/m2 and a near-surface temperature of 250 K. The inversion results show that the crust has an average heat production of approximately 210 nW/m3, the mantle has a depleted heat production varying from 3.7 to 8.1 nW/m3, and the heat production in the lunar core varies from 30 to 36 nW/m3. The distribution of heat production indicates that the present radioisotopes are mainly concentrated in the lunar crust. Different from previous estimates, the heat release in the lunar core may be considerable, similar to the average heat production in the Moon, implying that the residual heat from lunar accretion or radioactive decay is probably substantial; in contrast, perhaps the core was involved to a limit in the material exchange with the mantle. In addition, the obtained heat flux at the core-mantle boundary meets the value required for the core adiabatic process, indicating that the core convection may have stopped. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Absolute "places of birth" of 155 Phanerozoic carbonatites are reconstructed. • 69% of Phanerozoic carbonatite massifs could be linked with mantle superplumes. • 31% of carbonatites originated close to subduction graveyards in lower mantle. Abstract Geodynamic origin of carbonatites is debated for several decades. One of hypotheses links their origin to large-volume mantle plumes rising from the core-mantle boundary (CMB). Some evidence exists for temporal and spatial relationships between the occurrences of carbonatites and large igneous provinces (LIPs), and both carbonatites and LIPs can be related to mantle plumes. A good example is the carbonatites of the Maymecha-Kotuy Province in the Polar Siberia, which were formed at the same time as the Siberian superplume event at ca. 250 Ma. In this study we use a recently published absolute plate kinematic modelling to reconstruct the position of 155 Phanerozoic carbonatites at the time of their emplacement. We demonstrate that 69% of carbonatites may be projected onto the central or peripheral parts of the large low shear-wave velocity provinces (LLSVPs) in the lowermost mantle. This correlation provides a strong evidence for the link between the carbonatite genesis and the locations of deep-mantle plumes. A large group of carbonatites (31%) has no obvious links to LLSVPs and, on the contrary, they plot above the "faster-than-average S-wave" zones in the deep mantle, currently located beneath North and Central America and China. We propose that their origin may be associated with remnants of subducted slabs in the mantle. [ABSTRACT FROM AUTHOR]

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

Highlights • (Mg,Fe)O rheology at CMB conditions is studied through 2.5D dislocation dynamics. • Fe-rich (Mg,Fe)O viscosity is 10 orders of magnitude below that of Mg-rich (Mg,Fe)O. • The influence of weak Fe-rich (Mg,Fe)O on the dynamics of ULVZs is explored. Abstract The viscosity of the lower mantle results from the rheological behavior of its two main constituent minerals, aluminous (Mg,Fe)SiO 3 bridgmanite and (Mg,Fe)O ferropericlase. Understanding the transport properties of lower mantle aggregates is of primary importance in geophysics and it is a challenging task, due to the extreme time-varying conditions to which such aggregates are subjected. In particular, viscosity is a crucial transport property that can vary over several orders of magnitude. It thus has a first-order control on the structure and dynamics of the mantle. Here we focus on the creep behavior of (Mg,Fe)O at the bottom of the lower mantle, where the presence of thermo-chemical anomalies such as ultralow-velocity zones (ULVZ) may significantly alter the viscosity contrast characterizing this region. Two different iron concentrations of (Mg 1–x Fe x)O are considered: one mirroring the average composition of ferropericlase throughout most of the lower mantle (x = 0.20) and another representing a candidate magnesiowüstite component of ULVZs near the base of the mantle (x = 0.84). The investigated pressure-temperature conditions span from 120 GPa and 2800 K, corresponding to the average geotherm at this depth, to core-mantle boundary conditions of 135 GPa and 3800 K. In this study, dislocation creep of (Mg,Fe)O is investigated by dislocation dynamics (DD) simulations, a modeling tool which considers the collective motion and interactions of dislocations. To model their behavior, a 2.5 dimensional dislocation dynamics approach is employed. Within this method, both glide and climb mechanisms can be taken into account, and the interplay of these features results in a steady-state condition. This allows the retrieval of the creep strain rates at different temperatures, pressures, applied stresses and iron concentrations across the (Mg,Fe)O solid solution, providing information on the viscosity for these materials. A particularly low viscosity is obtained for magnesiowüstite with respect to ferropericlase, the difference being around 10 orders of magnitude. Thus, the final section of this work is devoted to the assessment of the dynamic implications of such a weak phase within ULVZs, in terms of the viscosity contrast with respect to the surrounding lowermost mantle. [ABSTRACT FROM AUTHOR]

• MTAM1: first non-zonal time-averaged geomagnetic field model for the Miocene era. • Reversed flux patch under the South Atlantic region in all versions of the models. • Similar features to the last 5 million years, suggesting long-term mangle control. Reconstructions of long-term time-averaged geomagnetic field structures are important to understand geomagnetic field evolution and to identify the longevity and scale of non-dipolar field morphology. This study presents MTAM1 , a non-zonal time-averaged field model for the Miocene era (5.3–23 million years ago), or indeed any time period prior to 5 million years ago. The time averaged field model for the Miocene is an inverse model based on a directional data compilation comprising 38 different localities, each with a minimum of 10 sites, called PSVM (Engbers et al., 2022a). The data were separated into normal (PSVM N) and normalised reversed (PSVM R) datasets, yielding two corresponding models MTAM1 N and MTAM1 R. Allowing for the opposite sign, no substantial differences were found between these two models, suggesting symmetry between the morphology of the normal and reversed fields and no evidence for non-reversing features in the geomagnetic field. Under this assumption of symmetry, the normal and reversed data can be modelled together, enhancing the data distribution and thus the robustness of the complete time-averaged field model for the Miocene. The broad structure of the models resembles previous time-averaged field models for the past 5 Myr but there are some clear differences, particularly under the South Atlantic, where all Miocene models include a reversed flux patch (RFP). To investigate whether this difference is well defined, or could result from differences in modelling methodology or data distribution, the data of the last 5 Myr were inverted with our normal model for the Miocene as a prior constraint. We find no evidence that the Miocene model is inconsistent with the field structures required by data from the past 5 Myr, suggesting an overall stability in the averaged geomagnetic field morphology for the past 23 Myr. This is consistent with long-term mantle control on geomagnetic field morphology leading to consistent deviations from the geocentric axial dipole on a multi-million-year timescale on the Core-Mantle boundary (CMB). [ABSTRACT FROM AUTHOR]

The short-lived radiogenic 182Hf/182W (t½ = 9 Ma) and 146Sm/142Nd (t½ = 103 Ma) systems are useful tools to investigate differentiation processes in Earth's early history. Mantle reservoirs that have been preserved over long geological timescales could carry signatures formed early on in Earth's history and later could act as sources to modern mantle plumes. Negative μ182W anomalies in combination with high 3He/4He ratios have been detected globally as differently sloping negative trends in plume-derived ocean island basalts (OIB) from individual hotspots. Large low shear velocity provinces (LLSVP) and ultra-low velocity zones (ULVZ) are seismically anomalous regions in the lower mantle that have been suggested as sources of primordial mantle components near the core-mantle boundary (CMB) and accessed by deep-rooted mantle plumes. In this work, seven samples from the islands Hiva Oa and Fatu Hiva of the Marquesas Archipelago, showing a range in 3He/4He ratios, have been investigated motivated by its location above the Pacific LLSVP and a recently discovered "mega-ULVZ". Despite elevated 3He/4He ratios of up to 14.4 R/R A , new high precision measurements using thermal ionization mass spectrometry revealed non-anomalous μ182W (−0.9 ± 4.9 ppm) and μ142Nd (1.3 ± 2.4 ppm, 95% confidence interval (c.i.)). These mark the Marquesas Archipelago as the first hotspot to show a decoupling of 3He/4He ratios and μ182W in OIB. A mixing model between an elevated 3He/4He-negative-μ182W and an EM2 or a HIMU endmember suggest that the addition of recycled material is not responsible for this decoupling. Instead, like many previously studied OIB, the Marquesas plume is most likely dominated by an LLSVP component proposed to contain high 3He/4He ratios and μ182W of 0. The unique observed absence of negative μ182W anomalies, on the other hand, implies a lack of involvement of the ULVZ source in the plume's melting region, despite the hotspot being located above a recently discovered mega-ULVZ. This may imply that the expression of ancient mantle domains in hotspot geochemistry is temporally variable, thus, delaying the arrival or involvement of ULVZ material in the origin of the Marquesas OIB. • Marquesas OIB exhibit statistically significant variability in μ142Nd but isotope compositions are unresolved from modern BSE values. • Unlike all other OIB globally, the Marquesas samples show no resolvable negative μ182W anomalies despite elevated 3He/4He ratios. • The global association of negative μ182W with seismically anomalous mantle domains (ULVZ) cannot be observed in the Marquesas hotspot. • The lack of μ182W anomalies in elevated 3He/4He Marquesan OIB suggests temporal variability of ULVZ source contribution in mantle plumes. [ABSTRACT FROM AUTHOR]

The outer core of the Earth, filled with electrically conducting fluid, undergoes thermochemical convection due to super-adiabatic temperature gradients. Near the core-mantle boundary, fluid flow may be restricted due to sub-adiabatic temperature gradients or accumulated light elements forming a layer of stable stratification. The present study investigates the behavior of thermal convection with various buoyancy profiles, using non-uniform radial distribution of heat sources, mimicking the combined presence of convective and stable zones. Role of such modified convection in the evolution and resulting morphology of saturated magnetic fields is the main focus of this study. Apart from the reduction in the threshold for onset, the length scale of the convective instabilities is enhanced with stable stratification, while the frequency is reduced. Despite the confinement of convection to unstable regions, rapid rotation favors penetrative radial convective flows. In presence of a stably stratified layer, the dynamo action is suppressed due to the radial confinement of buoyancy, Coriolis, and Lorentz forces. The suppression of vortex stretching, indicated by the relative asymmetry in axial helicity provides further understanding of the mechanism behind the magnetic field structure. As the stratification becomes stronger, the dynamo action leads to magnetic fields with enhanced axial dipole field strength, although the strength of the dynamo is reduced. The confinement of the toroidal component of the magnetic field to localized concentrated patches in regions of stable stratification near the equatorial plane also inhibits the growth of magnetic fields. Nevertheless, enhanced buoyancy forcing may overcome the suppression of dynamo action and lead to strongly convecting dipolar dominated Earth-like dynamos even with moderate stratification. • Role of a stably stratified layer below the Earth's core-mantle boundary in dynamo action is investigated. • Buoyancy forcing with non-uniform heat sources results in both convective and stable zones. • Threshold and patterns of convective instabilities at onset occur in response to enhanced heat flux due to stable layers. • Topological features of the convective motion characterize the success and failure of dynamo. • Stable layers promote axial dipolarity as well as equatorial anti-symmetry of generated magnetic fields. [ABSTRACT FROM AUTHOR]

The thermal conductivity of the lowermost mantle controls the energy transfer from the core to the mantle, and provides insight into the thermal history, evolution, and magnetic field of the Earth. Bridgmanite (Brg) and post-perovskite (PPv) are the most abundant minerals in the lowermost mantle. However, their thermal conductivities, as well as the impact of Fe impurities, are highly controversial. In this study, the thermal conductivities of Fe-free and Fe-bearing Brg and PPv at high pressure and temperature were predicted, through a combination of non-equilibrium molecular dynamics and machine learning potential (MLP) trained with data from first-principles calculations. The thermal conductivities of Fe-free Brg and PPv are in agreement with those calculated by the Green-Kubo method based on the MLP. We found that the presence of 12.5 mol% Fe in the lowermost mantle decreases the thermal conductivities of Brg and PPv by ∼10% and ∼14%, respectively. Furthermore, the phase transition from Brg to PPv increases the thermal conductivity of pyrolite by ∼22%. Incorporating the distribution of minerals, temperature, and iron content obtained through the inversion based on mineral elasticity and seismic tomography models, a global heat flow with substantial lateral variation at the core-mantle boundary was reported. The total heat flux from the core was found to be 7.1 ± 0.5 TW, implying a geologically young inner core of 0.75 ± 0.35 Ga. • Machine learning potential for Fe-free and Fe-bearing bridgmanite and post-perovskite. • Fe decreases the lattice thermal conductivity of bridgmanite and post-perovskite. • Substantial lateral variation in heat flow at the core-mantle boundary. • The total heat flux from the core is estimated to be 7.1 ± 0.5 TW. • The age of the inner core inferred from the total heat flux is 0.75 ± 0.35 Ga. [ABSTRACT FROM AUTHOR]

The lowermost mantle has been studied with a variety of seismic waves which show that different structures on many length-scales exist there but it is possible that several seismic phases could be generated by one structure. Specifically, we use a randomly distributed scattering layer that is able to generate precursors to the core phase PKPdf as well as waves that resemble reflections off the D″ layer (PdP wave) and here we present a parameter study to understand how scattering parameters in this layer affect the two tested seismic waves and whether it is possible to discriminate between a reflector or a scattering layer as cause for apparent D″-reflected waves. We compute synthetic data for several source-receiver combinations and vary the parameters of the heterogeneity model above the core-mantle boundary: correlation length, velocity (and density) perturbation, taper to soften the transition, thickness, location of the scattering layer and the randomness of the models. We measure the effects on the amplitudes and arrival times of the seismic waves, and in addition for the apparent D″ reflection, we also record the number of reflected waves, their polarity and slowness. We find that the precursor and reflection amplitudes are influenced by all scattering parameters and that trade-offs exist. One conclusion therefore is that it is difficult to match local scattering parameters to fit observations with a unique model since several combinations of scattering parameters result in similar amplitudes and travel times. Interestingly, amplitudes for different correlation lengths are affected differently for larger and smaller epicentral distances, therefore testing the amplitudes of precursors at different distance ranges could potentially help to distinguish between correlation length and perturbation effects. We were not able to further narrow down the scattering parameters of the layer when using a combination of PKP precursors and PdP-like reflections due to the non-uniqueness. Analyzing the effects on PdP arrivals that arrive between P and PcP, we find influences on all seismic observables albeit without clear trends. But based on our modeling it seems prudent to assume that the waves are caused by a scattering layer if 1) the observations show strong scatter in arrival time, slowness and amplitude and 2) if more than one arrival is present between P and PcP. • A scattering layer at the CMB generates PKP precursors and D″ reflections. • Varying scattering parameters affects both waves and seismic observables. • PdP-like arrivals without clear trend but more random than with a discontinuity. • Unique matching of observed seismic waves difficult with scattering due to trade-offs. [ABSTRACT FROM AUTHOR]

Abstract The lowermost mantle is driven to Earth's surface by mantle plumes, providing a volcanic record of its structure and composition. Plumes comprise a head and tail, which melt to form large igneous provinces (LIPs) and ocean island basalts (OIBs), respectively. Recent analyses have shown that LIPs and OIBs exhibit tungsten (W) isotope heterogeneity that was created in the first ∼60 million years of our solar system's evolution. Moreover, the isotopic signature found in LIPs differs to that found in OIBs, revealing that the melt products of plume heads must be dominated by a different ancient mantle reservoir to that of plume tails. However, existing geodynamical studies suggest that plume heads and tails sample the same deep-mantle source region and, therefore, cannot account for any systematic differences in composition. Here, we present a suite of numerical simulations of thermo-chemical plumes and an isotopic model for W sources in the mantle. Our results demonstrate that the W isotope systematics of LIPs and OIBs can, under certain conditions, arise as a dynamical consequence of plumes forming in a heterogeneous, thermo-chemical boundary layer. We find that ultra low-velocity zones (ULVZs), which sit on the core–mantle boundary (CMB), likely contribute to the chemical diversity observed in OIBs but not LIPs, while any dense components residing inside large low shear-wave velocity provinces (LLSVPs) may contribute to both. This study places geochemical observations from Earth's surface in a geodynamically consistent framework and illuminates their relationship with seismically imaged features of the deep mantle. Highlights • Tungsten isotopes reveal vertical structure in mantle plumes. • Plume heads and tails each sample a distinct ancient mantle reservoir. • ULVZs contribute material to OIBs. • LLSVPs may contribute to both OIBs and LIPs. • Chemical heterogeneity in the mantle may be restricted to the base of LLSVPs. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Geophysical and mineral physics data indicate presence of the wet transition zone. • The mantle transition zone contains continental crustal components. • Fluids or volatile-rich magmas may be generated at the top of the lower mantle. • Hydrous δ-H solid solution reduces Al 2 O 3 contents in the lower mantle bridgmanite. • The iron-water reaction creates pyrite-FeOOH at the core-mantle boundary. Abstract The roles of water in the mantle transition zone, lower mantle, and the core-mantle boundary are investigated. The evidence for a wet mantle transition zone has been suggested based on hydrous mineral inclusions in diamond. Seismic wave velocity and electrical conductivity profiles together with mineral physics data are consistent with existence of stagnant slabs in a wet mantle transition zone. The transition zone may contain continental crustal components in these stagnant slabs. Dense hydrous magmas may exist at the base of the upper mantle. Fluids or volatile-rich magmas may also exist at the top of the lower mantle due to the large contrast in water contents between the mineral assemblages in the mantle transition zone and the lower mantle, and the crossing of the convective descent of the cold hydrated materials. Dense magmas are not likely to be formed at the top of the lower mantle and hydrous magmas generated in this region move upwards and metasomatize the overlying mantle transition zone. Water can be transported deeper into the lower mantle by gravitational collapse of the stagnant slabs, which supply water into the lower mantle, including the core-mantle boundary. Hydrous δ-H solid solution may be the most important hydrous phase in lower mantle, and existence of this phase reduces the aluminum content in coexisting bridgmanite and post-perovskite, and thus modifies the physical properties of the lower mantle. Hydrous δ-H solid solution can accumulate at the base of the lower mantle. The iron-water reaction at the core-mantle boundary can also create pyrite-type FeOOH which can be a potential candidate material for the ultralow velocity zone (ULVZ). [ABSTRACT FROM AUTHOR]

Ascertaining the cause of variations in the frequency of geomagnetic polarity reversals through the Phanerozoic has remained a primary research question straddling paleomagnetism and geodynamics for decades. Numerical models suggest the primary control on geomagnetic reversal rates on 10 to 100 Ma timescales is the changing heat flux across the core-mantle boundary and that this is itself expected to be strongly influenced by variations in the flux of lithosphere subducted into the mantle. A positive relationship between the time-dependent global subduction flux and magnetic reversal rate is expected, with a time delay to transmit the thermal imprint into the lowermost mantle. We perform the first test of this hypothesis using subduction flux estimates and geomagnetic reversal rate data back to the early Paleozoic. Subduction area flux estimates are derived from global, full-plate tectonic models, and are evaluated against independent subduction flux proxies based on the global age distribution of detrital zircons and strontium isotopes. A continuous Phanerozoic reversal rate model is built from pre-existing compilations back to ~320 Ma plus a new reversal rate model in the data-sparse mid-to-early Paleozoic. Cross-correlation of the time-dependent subduction flux and geomagnetic reversal rate series reveals a significant correlation with a time delay of ~120 Ma (with reversals trailing the subduction flux). This time delay represents a value intermediate between the seismologically constrained time expected for a subducted slab to transit from the surface to the core-mantle boundary (~150–300 Ma), and the much shorter lag time predicted by some numerical models of mantle flow (~30–60 Ma). While the reason for this large discrepancy remains unclear, it is encouraging that our novel estimate of lag time represents a compromise between them. Although important uncertainties in our proposed relationship remain, these results cast new light on the dynamic connections between the surface and deep Earth, and will help to constrain new models linking mantle convection, the thermal evolution of the lowermost mantle and the geodynamo. [ABSTRACT FROM AUTHOR]

Identification of the primary compositions of mantle-derived melts is crucial for understanding mantle compositions and physical conditions of mantle melting. However, these melts rarely reach the Earth's surface unmodified because of contamination, crystal fractionation and degassing, processes that occur almost ubiquitously after melt generation. Here we report snapshots of the melts preserved in sheared peridotite xenoliths from the Udachnaya-East kimberlite pipe, in the central part of the Siberian craton. These xenoliths are among the deepest mantle samples and were delivered by kimberlite magma from 180–230 km depth interval, i.e. from the base of the cratonic lithosphere. The olivine grains of the sheared peridotites contain secondary inclusions of the crystallized melt with bulk molar (Na + K)/Ca ~ 3.4. Various Na-K-Ca-, Na-Ca-, Na-Mg-, Ca-Mg- and Ca-carbonates, Na-Mg-carbonates with additional anions, alkali sulphates and halides are predominant among the daughter minerals in secondary melt inclusions, whereas silicates, oxides, sulphides and phosphates are subordinate. These inclusions can be considered as Cl–S-bearing alkali-carbonate melts. The presence of aragonite, a high-pressure polymorph of CaCO 3 , among the daughter minerals suggests a mantle origin for these melt inclusions. The secondary melt inclusions in olivine from the sheared peridotite xenoliths and the melt inclusions in phenocrystic olivines from the host kimberlites demonstrate similarities, in daughter minerals assemblages and trace-element compositions. Moreover, alkali-rich minerals (carbonates, halides, sulphates and sulphides) identified in the studied melt inclusions are also present in the groundmass of the host kimberlites. These data suggests a genetic link between melt enclosed in olivine from the sheared peridotites and melt parental to the Udachnaya-East kimberlites. We suggest that the melt inclusions in olivine from mantle xenoliths may represent near primary, kimberlite melts. These results are new evidence in support of the alkali‑carbonate composition of kimberlite melts in their source regions, prior to the kimberlite emplacement into the crust, and are in stark contrast to the generally accepted ultramafic silicate nature of parental kimberlite liquids. [ABSTRACT FROM AUTHOR]

The dynamics of Earth’s lowermost mantle exert significant control on the formation of mantle plumes and the core-mantle boundary (CMB) heat flux. However, it is not clear if and how the variation of CMB heat flux and mantle plume activity are related. Here, we perform geodynamic model experiments that show how temporal variations in CMB heat flux and pulses of mantle plumes are related to morphologic changes of the thermochemical piles of large-scale compositional heterogeneities in Earth’s lowermost mantle, represented by the large low shear velocity provinces (LLSVPs). We find good correlation between the morphologic changes of the thermochemical piles and the time variation of CMB heat flux. The morphology of the thermochemical piles is significantly altered during the initiation and ascent of strong mantle plumes, and the changes in pile morphology cause variations in the local and the total CMB heat flux. Our modeling results indicate that plume-induced episodic variations of CMB heat flux link geomagnetic superchrons to pulses of surface volcanism, although the relative timing of these two phenomena remains problematic. We also find that the density distribution in thermochemical piles is heterogeneous, and that the piles are denser on average than the surrounding mantle when both thermal and chemical effects are included. [ABSTRACT FROM AUTHOR]

Pallasite meteorites, which consist primarily of olivine and metal, may be remnants of disrupted core-mantle boundaries of differentiated asteroids or planetesimals. The early thermal histories of pallasites are potentially recorded by minor- and trace-element zonation in olivine. However, constraining this history requires knowledge of element behavior under the conditions of pallasite formation, which is lacking for many of the main elements of interest (e.g., Co, Cr, Mn). In this study, we experimentally determined metal/olivine partition coefficients for Fe, Ni, Co, Cr, and Mn in a pallasite analogue at subsolidus temperatures. Metal/olivine partition coefficients ( K M ) increase in the order K Mn   <   K Cr   <  1  <   K Fe   <   K Co   <   K Ni , with five orders of magnitude separating K Mn from K Ni . Transition metals also become more siderophile with increasing experimental temperature (900–1550 °C). The experiments incidentally produced diffusion profiles in olivine for these elements; our results suggest they diffuse through olivine at similar rates. Core compositions of pallasite olivines are consistent with high-temperature equilibration with FeNi-metal. Olivine zonation toward crystal rims varies significantly for the investigated transition metals. We suggest rim zonation results from partial re-equilibration during late stage crystallization of minor phases (e.g., chromite, phosphates). This re-equilibration occurred over short timescales relative to overall pallasite cooling, likely tied to initial cooling rates on the order of 100–300 °C/Myr. [ABSTRACT FROM AUTHOR]