The northeastern margin of the Pacific Large Low Shear Velocity Province (LLSVP) is particularly interesting as many seismic modeling studies have suggested the presence of multiple structural anomalies in this lowermost mantle region. There is a strong velocity change from the interior of the Pacific LLSVP toward the margin, inferred to be a high velocity, post-perovskite lens thins toward the edge while an ultra-low velocity zone (ULVZ) layer increases in thickness (Lay et al., 2006). To et al. ( 2011) also proposed a localized, slow ULVZ-like anomaly embedded inside or at the margin of the LLSVP from their modeling of the anomalous S and diffracted S (Sdiff) behaviors observed with a limited number of stations.The deployment of USArray, recording many deep earthquakes from Fiji-Tonga subduction zone, greatly increases the sampling points to study the lateral structural variation along this margin. Through deconvolution

The composition of ultralow velocity zones (ULVZs) remains an open question, despite advances in both seismology and experimental work. We investigate the hypothesis of iron-rich (Mg,Fe)O (magnesiowüstite) as a cause of ULVZ seismic signatures. We report new quasi-hydrostatic X-ray diffraction measurements to constrain the equation of state of (Mg0.06Fe0.94)O with fit parameters V0 = 9.860 ± 0.007 Å3, K0T = 155.3 ± 2.2 GPa, K’0T = 3.79 ± 0.11, as well as synchrotron Mössbauer spectroscopy measurements to characterize the high-pressure magnetic and spin state of magnesiowüstite. We combine these results with information from previous studies to calculate the elastic behavior at core–mantle boundary conditions of magnesiowüstite, as well as coexisting bridgmanite and calcium silicate perovskite. Forward models of aggregate elastic properties are computed, and from these, we construct an inverse model to determine the proportions of magnesiowüstite that best reproduce ULVZ observations within estimated mutual uncertainties. We find that the presence of magnesiowüstite can explain ULVZ observations exhibiting 1:2 VP:VS reduction ratios relative to the Preliminary Reference Earth Model (PREM), as well as certain 1:3 VP:VS reductions within estimated uncertainty bounds. Our work quantifies the viability of compositionally distinct ULVZs containing magnesiowüstite and contributes to developing a framework for a methodical approach to evaluating ULVZ hypotheses.

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.

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Earth's interior consists primarily of an insulating rocky mantle [1, 2] and a metallic iron-dominant core [3, 4]. Recent work has shown that mountainscale structures at the core-mantle boundary may be highly enriched in , reported to exhibit high conductivity and metallic behavior at extreme pressure-temperature (P-T ) conditions [9]. However, the underlying electronic processes in FeO remain poorly understood and controversial. Here we systematically 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) [10]. Fine sampling of the phase diagram at more than 350 volume-temperature conditions 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 [5, 6, 11]. 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 (∼ 10 5 S/m) with modest P-T dependence as observed in experiments [9]. FeO-rich regions in Earth's lowermost mantle could thus influence electromagnetic interactions between the mantle and the core, producing several features observed in Earth's rotation and magnetic field evolution [14].