Properties of magmas at depth from SiO2 density and local structure measured using X-ray absorption and X-ray Raman spectroscopy
Abstract
Melt properties at high pressure are crucial to model the evolution of the deep part of the Earth formation and evolution. SiO2 is the main component of silicate melts in the Earth's deep mantle and controls their network structure and physical properties with pressure (e.g. density, viscosity). Knowledge of the short-range atomic and electronic structure in melts brings important constraints about their compressibility and viscosity at depth. We measured the O K-edge and the Si L2,3-edge in silica up to 110 GPa using X-ray Raman scattering spectroscopy, with a striking match to calculated spectra of the quenched high-pressure melt based on structures from molecular dynamics simulations. Our data show two major discontinuities at high pressure that are related to coordination changes. Between 20 and 27 GPa, at the transition zone (660 km), [4]Si species are converted into a mixture of [5]Si and [6]Si species. Between 60 and 70 GPa, in the lower mantle, a further transition marks the decrease of [5]Si species with [6]Si becoming dominant above 70 GPa without crossing-over the 6-fold references stishovite and CaCl2 phases up to at least 110 GPa. We also found two discontinuities in the compressibility and density trends measured on the same SiO2 glass using the X-ray absorption method up to 110 GPa [1]. The changes of coordination and density measured on SiO2 may have direct influence on the properties of silicate melts at depth with changes in viscosity and partitioning of elements at such pressures. Higher coordination than 6 only takes place beyond 140 GPa corroborating Brillouin scattering measurements in agreement with our results with a further increase in density at such pressure. Silicate melts containing network modifiers elements may potentially densify at a lower pressure making magmas neutrally buoyant at the depth of the core-mantle boundary.
[1] S. Petitgirard et al., SiO2 glass density to lower-mantle pressures. Phys. Rev. Lett. 119, 215701 (2017).- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMMR13A..01P
- Keywords:
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- 3919 Equations of state;
- MINERAL PHYSICSDE: 3924 High-pressure behavior;
- MINERAL PHYSICSDE: 3954 X-ray;
- neutron;
- and electron spectroscopy and diffraction;
- MINERAL PHYSICSDE: 3994 Instruments and techniques;
- MINERAL PHYSICS