Structural Transitions in MgSiO₃ Glasses and Melts at the Core‐Mantle Boundary Observed via Inelastic X‐ray Scattering

Abstract

The structural adaptation in MgSiO 3 melts under compression up to 130 GPa is the key to revealing the origins of the pronounced negative buoyancy of the melts at the core-mantle boundary (CMB). A full understanding of the melt densification requires study of the pressure-induced changes in the bonding configuration around oxygen at the CMB, which has proven to be difficult to measure. Here, the experimental breakthrough in O K-edge inelastic X-ray scattering enables collection of the spectra of MgSiO 3 glasses up to~130 GPa, along with ab initio molecular dynamics simulations, revealing the electronic bonding transitions around heavily compressed oxygen. The spectral results indicate the emergence of denser network structures around oxygen, stemming from contractions in the Mg-O and O-O distances associated with flexible topological and short-range rearrangements around Si. The results unveil the electronic structure and thus the nature of densification in dense partial melts at the CMB.Plain Language Summary A thin ultralow velocity zone at the bottom of mantle is characterized by reduced seismic wave velocities and enhanced density, suggesting the possible presence of molten silicates at a depth of~2,850 km (~130 GPa of pressure). How the melts densify near the core-mantle boundary is unclear. Because oxygen occupies the major volume fraction of silicates, the melt densification is dominated by the reorganization of oxygen during compression. However, the oxygen bonding environments in MgSiO 3 melt-a model mantle melt-near the core-mantle boundary are unknown. Here, the inelastic X-ray scattering spectra of MgSiO 3 glasses and liquids up to~130 GPa revealed the electronic bonding transitions around compressed oxygen that are not observed at low pressures and substantially differ from those in the crystalline MgSiO 3 bridgmanite. The spectral results indicate the pressure-induced emergence of denser network structures around oxygen, stemming from gradual contractions in the Mg-O and O-O distances associated with flexible topological and short-range rearrangements around Si. The densification around oxygen near megabar pressures potentially contributes to the gravitational stabilization of the complex partial melts in the deeper part of the mantle toward ultralow velocity zone.

Reductive Adsorption of Atmospheric Oxidized Mercury on Ice: Insights from Density Functional Calculations

“…DFT has been used in many studies to complement experimental investigations of environmentally important materials: for example, the surface adsorption behavior, − the metal intercalation tendency into clay minerals, , the electronic structure of high-pressure minerals, − and electronic transition spectroscopy (X-ray Raman scattering, X-ray absorption, optical properties, etc. ). ,,− We established detailed calculation conditions by referring to these previous studies with suitable adjustments (see also Section 2 in the Supporting Information). Electronic structures used for structure optimization and for determining E B and E vib were calculated using CASTEP .…”

Structure and disorder in MgSiO3 glasses above megabar pressures via nuclear magnetic resonance: DFT calculations

“…[22, 23] and references therein). Advances in element‐specific nonresonant inelastic X‐ray scattering (IXS) revealed the densification of MgSiO 3 glass, suggesting the formation of tricluster oxygen and related changes in medium‐range structures above 20 GPa 8 and the reduction of O–O distance up to 130 GPa 24,25 . However, overlap among the binary correlation functions in the pair distribution functions (from X‐ray and neutron scattering) of oxide glasses is more prevalent at elevated pressure conditions.…”

“…Nevertheless, pioneering elastic, inelastic X-ray, and neutron scattering studies have shown pressure-induced structural transitions in the glass (e.g., Refs. [8,[18][19][20][21][22][23][24]), suggesting an increase in coordination numbers for Si and Mg at relatively low-pressure conditions up to ∼20 GPa 18,19 ; scattering studies of MgSiO 3 glasses above 100 GPa revealed pressure-induced changes in elastic wave velocities and systematic shift in the first peak of the structure factor above 100 GPa, implying the major structural changes in the amorphous netwokrs (see Refs. [22,23] and references therein).…”

“…As ab initio molecular dynamics (MD) simulations provided structural information for MgSiO 3 melts and glasses under compression (e.g., Refs. [6,24,80,81]), these two ab initio techniques can be combined to reveal the hidden nature of nuclear spins under extreme compression. This study explores the evolution of MgSiO 3 glasses under pressure above 1.5 Mbar based on ab initio calculations of the NMR parameters of 25 Mg, 29 Si, and 17 O to unravel the detailed nature of bonding transitions and nuclear spin interactions under compression.…”

Reductive Adsorption of Atmospheric Oxidized Mercury on Ice: Insights from Density Functional Calculations

“…In the current study, the calculation parameters were referred from previous studies and adjusted in the context of calculation efficiency: the surface adsorption behavior, [1][2][3] the metal intercalation tendency into clay minerals, [4][5] the electronic structure of high-pressure minerals, [6][7][8][9][10][11] and the electronic transition spectroscopy (X-ray Raman scattering, X-ray absorption, and optical properties, etc.). [6][7][10][11] As summarized in the manuscript, the adsorption behaviors of atmospheric Hg on ice were explored via DFT calculations with following conditions: the ultrasoft pseudopotential, the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional with Tkatchenko-Scheffler (TS) dispersion correction, the 400 eV of cut-off energy, and the Γ point Brillouin zone sampling. [12][13][14] When using the non-localized plane-wave basis sets (e.g., WIEN2k [15][16] and CASTEP 12 ), the accuracy is determined primarily from the cutoff energy and the number of irreducible k points (i.e., the density of Monkhorst-Pack grid).…”