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SUMMARY Seismic observations reveal significant anisotropy in the D$$^{\prime \prime }$$ region, providing direct constraints on mantle flow and deformation. However, the global anisotropy pattern and its relationship with subduction history, mineral deformation and rheology in the lower mantle remain unclear. We analyse published regional shear-wave splitting and null measurements, along with waveform inversions, which reveal rapid lateral variations in anisotropy near the edges of large low shear velocity provinces (LLSVPs). We combine mineral physics results of temperature- and pressure-dependent elastic tensors, slip systems and phase transition mechanisms to explore potential deformation scenarios. We set up models that begin with dynamic thermochemical convection, tracking the deformation history driven by the subduction, evolving crystal fabrics and cumulative seismic anisotropy. Models show that post-perovskite (pPv) with a (001)-dominant slip system, combined with viscosity changes and texture inheritance during the bridgmanite-post-perovskite (Br-pPv) phase transition and the reverse transition, best reproduces the distinct anisotropy patterns observed in upwelling regions such as plume roots and LLSVP edges. The nominal model is time-dependent, showing strong seismic anisotropy when slabs impinge on the core–mantle boundary that diminishes toward the LLSVP, followed by plume development at the LLSVP edge with significant anisotropy. Within LLSVPs, internal convective upwellings and downwellings can explain the intermittent, spatially clustered anisotropy. We further demonstrate the potential for constraining LLSVP composition through the observed weaker anisotropy within these structures compared to the surrounding mantle, with our results favouring a Br-rich composition. Computations indicate that the bulk of the lower mantle remains nearly isotropic despite significant texture accumulation through dislocation glide, and that seismic anisotropy can extend several hundred kilometers above the core–mantle boundary. 

Oxyhydroxide phases in the (Al,Fe)OOH–MgSiO2(OH) system may form within oceanic lithosphere and transport hydrogen in their crystal structures into the lowermost mantle via cold, subducted slabs. In this work, we present new measurements of the seismic wavespeeds of the dense oxyhydroxide (Al,Fe)-phase H (Al0.84FeMg0.02Si0.06OOH) to 100 GPa constrained by nuclear resonant inelastic X-ray scattering, incorporating previous constraints on the equation of state of this phase. At 300 K and pressure greater than 70 GPa, (Al,Fe)-phase H exhibits high P-wave speeds (lnVp +12%) and low S-wave speeds (lnVs −7%) relative to the preliminary reference Earth model (PREM). Experimentally determined sound velocities are incorporated into a model of a hydrous metabasalt including (Al,Fe)-phase H and compared with the seismic wavespeeds of pyrolitic mantle along appropriate adiabats. Hydrous metabasalt may reproduce an anti-correlation of negative shear wave velocity and positive bulk sound velocity at the upper edges of large, low velocity provinces when compared to pyrolitic mantle but has similar wavespeeds to PREM in this region. Hydrous metabasalt with conceivable concentrations of (Al,Fe)-phase H can be distinguished from PREM in Vs at mid-mantle depths (1100–1700 km) and in V at shallower depths (750–1000 km). Subducted hydrous metabasalt could contribute to scattering of seismic waves across the depth interval of the post-stishovite transition, which may be affected by the formation of (Al,Fe)-phase H. 

Abstract Römerite, a triclinic hydrous sulfate in the P1¯ space group with the chemical formula Fe2+Fe23+(SO4)4·14(H2O), is of potential interest in studies of planetary environments, with particular relevance to Mars and the icy jovian satellites. Past work has indicated the presence of hydrous sulfates on said bodies, and the mixed-valence iron in römerite’s structure makes the mineral a worthwhile end-member composition in thermodynamic models. Such models should be constrained by measurements at the low temperatures relevant to the planetary environments in question. We characterized single crystals of römerite with time-domain Mössbauer spectroscopy, Raman spectroscopy, and X-ray diffraction methods. Through our X-ray diffraction experiment, we refined the unit-cell parameters of the crystal between 100 and 300 K. The resulting temperature-variant lattice parameters and volumes are reported and are fit by physical and empirical models of the thermal expansion coefficient. The physical model considered, a Debye model of thermal expansion, provides estimates of additional thermodynamic parameters: the ratio of the bulk modulus at 0 K and 1 bar to the thermodynamic Grüneisen parameter (K0,0K/γth), the volume at 0 K and 1 bar (V0,0K), and the Debye temperature (θD). 

Abstract Seismic and mineralogical studies have suggested regions at Earth’s core-mantle boundary may be highly enriched in FeO, reported to exhibit metallic behavior at extreme pressure-temperature (P–T) conditions. However, underlying electronic processes in FeO remain poorly understood. Here we explore the electronic structure ofB1-FeO at extreme conditions with large-scale theoretical modeling using state-of-the-art embedded dynamical mean field theory (eDMFT). Fine sampling of the phase diagram reveals that, instead of sharp metallization, compression of FeO at high temperatures induces a gradual orbitally selective insulator-metal transition. Specifically, atP–Tconditions of the lower mantle, FeO exists in an intermediate quantum critical state, characteristic of strongly correlated electronic matter. Transport in this regime, distinct from insulating or metallic behavior, is marked by incoherent diffusion of electrons in the conductingt_2gorbital and a band gap in thee_gorbital, resulting in moderate electrical conductivity (~10⁵S/m) with modestP–Tdependence as observed in experiments. Enrichment of solid FeO can thus provide a unifying explanation for independent observations of low seismic velocities and elevated electrical conductivities in heterogeneities at Earth’s mantle base. 

Abstract Over geologic time‐scales, large volumes of exogenic sulfur ions from Io's plasma torus have been supplied to the surface of Europa and Ganymede, which, combined with recent interpretations of orbiter images, dynamical modeling, and surface‐subsurface exchange, suggests further sulfur transport into the interior of the icy worlds. These observations motivate mixed‐phase spectral modeling for interpreting orbiter spectroscopy data and determination of hydration states of candidate surface materials including hydrous sulfates. In this work, we present a combined experimental and theoretical study of the low temperature and high pressure vibrational spectral signature of the iron‐sulfate monohydrate endmember, szomolnokite (FeSO₄·H₂O). By employing synchrotron Fourier‐transform infrared spectroscopy (FTIR) in the diamond anvil cell up to 23 GPa and down to 20 K, we explore the extreme range of pressure‐temperature domains relevant to icy environments throughout our solar system and beyond. Combined with our density‐functional theory quantum‐mechanics molecular dynamics results, we demonstrate that experimentally observed infrared features in the O‐H stretching region commonly associated withnH₂O (n > 1) hydration states can be attributed to a pure monohydrate without the need for pressure‐induced exsolved ice, other coexisting hydrous iron sulfates, or strong overtone and combination modes. We further discuss the possibility of lateral variations in density and shear properties on icy worlds associated with temperature variations and the high‐pressure phases of kieserite group monohydrated sulfates. 

Abstract The high-pressure melting curve of FeO controls key aspects of Earth’s deep interior and the evolution of rocky planets more broadly. However, existing melting studies on wüstite were conducted across a limited pressure range and exhibit substantial disagreement. Here we use an in-situ dual-technique approach that combines a suite of >1000 x-ray diffraction and synchrotron Mössbauer measurements to report the melting curve for Fe_1-xO wüstite to pressures of Earth’s lowermost mantle. We further observe features in the data suggesting an order-disorder transition in the iron defect structure several hundred kelvin below melting. This solid-solid transition, suggested by decades of ambient pressure research, is detected across the full pressure range of the study (30 to 140 GPa). At 136 GPa, our results constrain a relatively high melting temperature of 4140 ± 110 K, which falls above recent temperature estimates for Earth’s present-day core-mantle boundary and supports the viability of solid FeO-rich structures at the roots of mantle plumes. The coincidence of the defect order-disorder transition with pressure-temperature conditions of Earth’s mantle base raises broad questions about its possible influence on key physical properties of the region, including rheology and conductivity. 

Interacting magmatic sills at about 40-kilometer depth under the island of Hawai‘i feed into multiple different volcanoes. 

Abstract. Shear properties of mantle minerals are vital for interpreting seismic shear wave speeds and therefore inferring the composition and dynamics of a planetary interior. Shear wave speed and elastic tensor components, from which the shear modulus can be computed, are usually measured in the laboratory mimicking the Earth's (or a planet's) internal pressure and temperature conditions. A functional form that relates the shear modulus to pressure (and temperature) is fitted to the measurements and used to interpolate within and extrapolate beyond the range covered by the data. Assuming a functional form provides prior information, and the constraints on the predicted shear modulus and its uncertainties might depend largely on the assumed prior rather than the data. In the present study, we propose a data-driven approach in which we train a neural network to learn the relationship between the pressure, temperature and shear modulus from the experimental data without prescribing a functional form a priori. We present an application to MgO, but the same approach works for any other mineral if there are sufficient data to train a neural network. At low pressures, the shear modulus of MgO is well-constrained by the data. However, our results show that different experimental results are inconsistent even at room temperature, seen as multiple peaks and diverging trends in probability density functions predicted by the network. Furthermore, although an explicit finite-strain equation mostly agrees with the likelihood predicted by the neural network, there are regions where it diverges from the range given by the networks. In those regions, it is the prior assumption of the form of the equation that provides constraints on the shear modulus regardless of how the Earth behaves (or data behave). In situations where realistic uncertainties are not reported, one can become overconfident when interpreting seismic models based on those defined equations of state. In contrast, the trained neural network provides a reasonable approximation to experimental data and quantifies the uncertainty from experimental errors, interpolation uncertainty, data sparsity and inconsistencies from different experiments. 

Abstract Transport of heat from the interior of the Earth drives convection in the mantle, which involves the deformation of solid rocks over billions of years. The lower mantle of the Earth is mostly composed of iron-bearing bridgmanite MgSiO 3 and approximately 25% volume periclase MgO (also with some iron). It is commonly accepted that ferropericlase is weaker than bridgmanite 1 . Considerable progress has been made in recent years to study assemblages representative of the lower mantle under the relevant pressure and temperature conditions 2,3 . However, the natural strain rates are 8 to 10 orders of magnitude lower than in the laboratory, and are still inaccessible to us. Once the deformation mechanisms of rocks and their constituent minerals have been identified, it is possible to overcome this limitation thanks to multiscale numerical modelling, and to determine rheological properties for inaccessible strain rates. In this work we use 2.5-dimensional dislocation dynamics to model the low-stress creep of MgO periclase at lower mantle pressures and temperatures. We show that periclase deforms very slowly under these conditions, in particular, much more slowly than bridgmanite deforming by pure climb creep. This is due to slow diffusion of oxygen in periclase under pressure. In the assemblage, this secondary phase hardly participates in the deformation, so that the rheology of the lower mantle is very well described by that of bridgmanite. Our results show that drastic changes in deformation mechanisms can occur as a function of the strain rate.