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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. 

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 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 As the most massive geochemical reservoir, the lower mantle affects the Earth's budget of volatile elements, including hydrogen or O. The properties of minerals in the lower mantle are further affected by changes in the electronic configurations of iron cations, that is, by spin transitions. The feedback between spin transitions and potential storage of O in solid hydrous phases in the lower mantle, however, remains unexplored. By combining high‐pressure nuclear resonant inelastic X‐ray scattering and high‐pressure high‐temperature X‐ray diffraction experiments, we constrained the thermal equation of state of δ‐(Al,Fe)OOH, a member of the phase H solid solution. Based on the derived thermal equation of state of δ‐(Al,Fe)OOH and the underlying thermodynamic model, we calculate the excess Gibbs free energy that arises from the spin transition of ferric iron in this compound and evaluate the effect on phase equilibria. The results of our analysis show that the spin transition of ferric iron in phase H may significantly reduce the thermodynamic activity and hence the concentration of O in a coexisting hydrous melt. As a consequence, nominally anhydrous minerals of the lower mantle may become dehydrated in the presence of phase H. Our analysis further suggests that, under certain conditions, the spin transition may expand the thermal stability of ‐bearing phase H and create a geochemical link between the storage of O in phase H and ferric iron in the lower mantle. 

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. 

Global seismic tomography consistently identifies two large low shear velocity provinces (LLSVPs) beneath Africa and the Pacific in the lower mantle. These structures are generally hypothesized to have a thermochemical origin with a higher bulk modulus () than ambient mantle. Regional high-resolution seismic studies have revealed that LLSVPs exhibit diverse edge morphologies, though the factors controlling these variations remain unclear. Here we quantitatively investigate the evolution of LLSVP boundary topographies through high- thermochemical convection models. The calculations show that the boundary morphology of a thermochemical pile is primarily controlled by its density and viscosity. Comparison with observed boundary shapes suggests that the African LLSVP may be less dense and thus less stable than the Pacific LLSVP, potentially reflecting differences in their compositions and evolutions. Additionally, the observed boundary complexity indicates that the viscosity of LLSVPs is likely no more than an order of magnitude higher than that of the surrounding mantle.