Structure of Mg2SiO4 glass up to 140 GPa

Abstract:

The physical properties of melts at temperature and pressure conditions of the Earth’s mantle have a fundamental influence on the chemical and thermal evolution of the Earth. However, direct investigations of melt structures at these conditions are experimentally very difficult or even impossible with current capabilities. In order to still be able to obtain an estimate of the structural behavior of melts at high pressures and temperatures, amorphous materials have been widely used as analogue materials.

In particular the investigation of sound wave velocities of amorphous SiO₂ and MgSiO₃ as analogues for silicate melts indicate structural changes at about ~30-40 GPa and ~130-140 GPa [1]. The transition pressures are lower for MgSiO₃ than for SiO₂ indicating that these transitions are affected by the degree of polymerization of the SiO₂ network of the glasses. Nevertheless, these measurements only give a hint about the occurrence of structural transitions but lack information on the actual structural changes accompanied by the sound wave velocity discontinuities.

The pressure of the second structural transition at ~130-140 GPa is of vital importance for geophysics. If it causes silicate melts to become denser than the surrounding solid material, it would result in negatively buoyant melts close to the core-mantle boundary, which could be a major factor affecting the chemical stratification of the Earth’s mantle during an early magma ocean after the moon forming impact.

In order to resolve the structural transition and estimate the effect of a different degree of polymerization further, we studied the structural behavior of Mg₂SiO₄ glass up to 140 GPa using X-ray total scattering and pair distribution function analysis. The measurements were performed at the GSECARS 13-IDD beamline at the APS employing the newly developed multichannel collimator (MCC) setup. The MCC effectively removes unwanted Compton scattering of the diamond anvils and enables easy extraction of X-ray total scattering intensity up to the highest pressures achieved.

We will present data on structural changes and densification mechanisms of Mg₂SiO₄glass at high pressures, and elaborate on the potential of negatively buoyant melts at the core-mantle boundary.

 [1] Murakami et al., 2011. Proc. Natl. Acad. Sci. U.S.A. 108, 17286–9.