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

Abstract

The structural adaptation in MgSiO3 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 MgSiO3 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.