Because of dissolution of lighter elements such as sulfur, carbon, hydrogen, and oxygen, Earth's outer core is about 10 percent less dense than molten iron at the relevant pressure and temperature conditions. To determine whether hydrogen can account for a major part of the density deficit and is therefore an important constituent in the molten iron outer core, the hydrogen concentration in molten iron was measured at 7.5 gigapascals. From these measurements, the metal-silicate melt partitioning coefficient of hydrogen was determined as a function of temperature. If the magma ocean of primordial Earth was hydrous, more than 95 mole percent of H ₂ O in this ocean should have reacted with iron to form FeH x , and about 60 percent of the density deficit is reconciled by adding hydrogen to the core.

Silicate melts at the top of the transition zone and the core-mantle boundary have significant influences on the dynamics and properties of Earth's interior. MgSiO ₃ -rich silicate melts were among the primary components of the magma ocean and thus played essential roles in the chemical differentiation of the early Earth. Diverse macroscopic properties of silicate melts in Earth's interior, such as density, viscosity, and crystal-melt partitioning, depend on their electronic and short-range local structures at high pressures and temperatures. Despite essential roles of silicate melts in many geophysical and geodynamic problems, little is known about their nature under the conditions of Earth's interior, including the densification mechanisms and the atomistic origins of the macroscopic properties at high pressures. Here, we have probed local electronic structures of MgSiO ₃ glass (as a precursor to Mg-silicate melts), using high-pressure x-ray Raman spectroscopy up to 39 GPa, in which high-pressure oxygen K-edge features suggest the formation of tricluster oxygens (oxygen coordinated with three Si frameworks; ^[3] O) between 12 and 20 GPa. Our results indicate that the densification in MgSiO ₃ melt is thus likely to be accompanied with the formation of triculster, in addition to a reduction in nonbridging oxygens. The pressure-induced increase in the fraction of oxygen triclusters >20 GPa would result in enhanced density, viscosity, and crystal-melt partitioning, and reduced element diffusivity in the MgSiO ₃ melt toward deeper part of the Earth's lower mantle.

Knowledge of the electronic structure of amorphous and liquid silica at high pressures is essential to understanding their complex properties ranging from silica melt in magma to silica glass in optics, electronics, and material science. Here we present oxygen near $K$-edge spectra of ${\mathrm{SiO}}_2$ glass to $51\mathrm{GPa}$ obtained using x-ray Raman scattering in a diamond-anvil cell. The x-ray Raman spectra below $\sim 10\mathrm{GPa}$ are consistent with those of quartz and coesite, whereas the spectra above $\sim 22\mathrm{GPa}$ are similar to that of stishovite. This pressure-induced spectral change indicates an electronic bonding transition occurring from a fourfold quartzlike to a sixfold stishovitelike configuration in ${\mathrm{SiO}}_2$ glass between $10\mathrm{GPa}$ and $22\mathrm{GPa}$. In contrast to the irreversible densification, the electronic bonding transition is reversible upon decompression. The observed reversible bonding transition and irreversible densification call for a coherent understanding of the transformation mechanism in compressed ${\mathrm{SiO}}_2$ glass.

High‐quality single‐crystals of (Al,Fe)‐bearing bridgmanite, Mg_0.88 Fe³⁺_0.065Fe²⁺_0.035Al_0.14Si_0.90O₃, of hundreds of micrometer size were synthesized at 24 GPa and 1800 °C in a Kawai‐type apparatus from the starting hydrous melt containing ~6.7 wt% water. Analyses of synthesized bridgmanite using petrographic microscopy, scanning electron microscopy, and transmission electron microscopy show that the crystals are chemically homogeneous and inclusion free in micrometer‐ to nanometer‐spatial resolutions. Nanosecondary ion mass spectrometry (NanoSIMS) analyses on selected platelets show ~1,020(±70) ppm wt water (hydrogen). The high water concentration in the structure of bridgmanite was further confirmed using polarized and unpolarized Fourier‐transform infrared spectroscopy (FTIR) analyses with two pronounced OH‐stretching bands at ~3,230 and ~3,460 cm⁻¹. Our results indicate that lower‐mantle bridgmanite can accommodate relatively high amount of water. Therefore, dehydration melting at the topmost lower mantle by downward flow of transition zone materials would require water content exceeding ~0.1 wt%.

Significance: Presence of iron in Earth’s lower mantle may critically affect its physical properties and thermochemical structures. However, its effects on thermal conductivity and dynamics of deep mantle remain unknown. We studied lattice thermal conductivity of lower-mantle ferropericlase to 120 GPa using ultrafast optics. We observed an enhanced iron substitution effect in the low-spin iron-rich ferropericlase, with thermal conductivity that significantly drops across spin transition. Combined with bridgmanite data, we provided a self-consistent radial profile of lower-mantle thermal conductivity, which is dominated by pressure, temperature, and iron effects and shows a twofold increase throughout the lower mantle. If ultralow velocity zones are hot and strongly enriched in iron, their exceptionally low thermal conductivity will delay their cooling, influencing lowermost mantle dynamics.