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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 of B1-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, at P–T conditions 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 conducting t 2g orbital and a band gap in the e g orbital, resulting in moderate electrical conductivity (~105 S/m) with modest P–T dependence 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 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 Fe1-x O 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.

Abstract Strong waveform complexities, including multipathing of the S diffracted phase and rapid changes in differential ScS‐S times, are observed for multiple deep Fiji earthquakes recorded at the USArray. The complexities occur at the northeastern edge of the Pacific Large Low Shear Velocity Province (LLSVP), about 12 degrees southeast of present‐day Hawaiʻi. Waveform modeling of the multipathing provides good constraints on an ultra‐low velocity zone (ULVZ) with a width of 5 degree located near the inner edge of the LLSVP. Based on the mineralogical‐modeling of the ULVZ as a solid iron‐rich magnesiowüstite‐bearing assemblage with compatible morphology predicted from geodynamical simulations, a ULVZ model with a thickness of 30 km and a shear wave velocity reduction of 18% is preferred. The rapid change in differential ScS‐S travel time is best explained by having both the aforementioned ULVZ and an adjacent high velocity structure near the LLSVP. Furthermore, a low‐velocity plume‐like structure could potentially explain the observed S travel time delay independent of ScS. These seismic features are proposed to be a ULVZ driven toward the edge of the LLSVP while potentially pushed by a subducted slab. This configuration may trigger plume generation due to strong thermal instabilities and is in the same vicinity where mantle flow models place the present‐day Hawaiian plume source. Multiple ScS can potentially be used to verify vertical plume structure in tomographic models but the accuracy of upper mantle structure, which is a key reflection point, needs to be considered.

We describe a method to make even insulation layers for high-pressure laser-heated diamond-anvil cell samples using stepped anvils. The method works for both single-sided and double-sided laser heating using solid or fluid insulation. The stepped anvils are used as matched pairs or paired with a flat culet anvil to make gasket insulation layers and not actually used at high pressures; thus, their longevity is ensured. We compare the radial temperature gradients and Soret diffusion of iron between self-insulating samples and samples produced with stepped anvils and find that less pronounced Soret diffusion occurs in samples with even insulation layers produced by stepped anvils.