![]() Based on thermal conductivity measurements of iron–silicon alloys at high pressure and temperature conditions, the authors here propose Earth’s inner core could be older than previously expected. ![]() Thermal conductivity of Earth’s core affects Earth’s thermal structure, evolution and dynamics. Our results provide key constraints on inner core age that could be older than two billion-years. This suggests a lower minimum heat flow, around 3 TW, across the core–mantle boundary than previously expected, and thus less thermal energy needed to operate the geodynamo. An outer core with 15 at% Si would have a conductivity of about 20 W m−1 K−1, lower than pure Fe at similar pressure–temperature conditions. 15 at% Si alloyed in Fe substantially reduces its conductivity by about 2 folds at 132 GPa and 3000 K. Here we directly measured thermal conductivities of solid Fe and Fe–Si alloys up to 144 GPa and 3300 K. Its thermal conductivity critically affects Earth’s thermal structure, evolution, and dynamics, as it controls the magnitude of thermal and compositional sources required to sustain a geodynamo over Earth’s history. ![]() Our results show that less than 8.4% of FeSi by volume in the aggregate are enough to explain both the amplitude of the velocity reduction and the high density anomaly observed in a wide range of ULVZs.Įarth’s core is composed of iron (Fe) alloyed with light elements, e.g., silicon (Si). ![]() FeSi is thus a plausible candidate to explain the origin of ULVZs. We found that the sound velocities of FeSi are significantly lower than those of the average lowermost mantle measured in seismic studies, and also lower than those of other mineral phases in this region, which are also considered as potential sources of ULVZs. From these measurements we derived the sound velocities of the high‐pressure phase of FeSi at the CMB. Here, we test this hypothesis in an experimental study, where the elastic behavior of FeSi is measured while the sample is exposed to very high pressures and temperatures comparable to those in the Earth's lower mantle. One plausible source could be FeSi, which is assumed to form from chemical reactions between material from the core and the mantle. However, the origin of these so‐called ultralow‐velocity zones (ULVZs) remains unclear. Seismic studies have revealed patches on the Earth's core‐mantle boundary (CMB), which have extremely low seismic velocities. Our results on single crystals of δ-(Al,Fe)OOH demonstrate the sensitivity of NRIXS to vibrational anisotropy and provide an in-depth description of the vibrational behavior of Fe3+ cations in a crystal structure that may motivate future applications of NRIXS to study anisotropic vibrational properties of minerals. As a potential application of single-crystal NRIXS at high pressures, we discuss the evaluation of anisotropic thermal stresses in the context of elastic geobarometry for mineral inclusions. We further show how the anisotropy of the Lamb–Mössbauer factor can be translated into anisotropic displacement parameters for 57Fe atoms and relate our findings on vibrational anisotropy to the crystal structure of δ-(Al,Fe)OOH. To describe the anisotropy of central vibrational properties, we define and derive tensors for the partial phonon density of states, the Lamb–Mössbauer factor, the mean kinetic energy per vibrational mode, and the mean force constant of 57Fe atoms. From the recorded single-crystal NRIXS spectra, we calculated projections of the partial phonon density of states along different crystallographic directions. We used nuclear resonant inelastic X-ray scattering (NRIXS) to probe lattice vibrations that involve motions of 57Fe atoms in δ-(Al0.87Fe0.13)OOH single crystals. Vibrational properties can be highly anisotropic, in particular for materials with crystal structures of low symmetry that contain directed structural groups or components. Understanding the vibrational properties of high-pressure phases provides the basis for assessing their thermal properties, which are required to compute phase diagrams and physical properties. The formation of high-pressure oxyhydroxide phases spanned by the components AlOOH–FeOOH–MgSiO2(OH)2 in experiments suggests their capability to retain hydrogen in Earth's lower mantle.
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