X-ray Raman scattering study of MgSiO3 glass at high pressure: implication for triclustered MgSiO3 melt in Earth's mantle.

School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, Korea.
Proceedings of the National Academy of Sciences (Impact Factor: 9.81). 07/2008; 105(23):7925-9. DOI: 10.1073/pnas.0802667105
Source: PubMed

ABSTRACT 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. MgSiO3-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 MgSiO3 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; 3O) between 12 and 20 GPa. Our results indicate that the densification in MgSiO3 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 MgSiO3 melt toward deeper part of the Earth's lower mantle.

1 Bookmark
  • [Show abstract] [Hide abstract]
    ABSTRACT: The detailed atomic structures of shock compressed basaltic glasses are not well understood. Here, we explore the structures of shock compressed silicate glass with a diopside-anorthite eutectic composition (Di{sub 64}An{sub 36}), a common Fe-free model basaltic composition, using oxygen K-edge X-ray Raman scattering and high-resolution {sup 27}Al solid-state NMR spectroscopy and report previously unknown details of shock-induced changes in the atomic configurations. A topologically driven densification of the Di{sub 64}An{sub 36} glass is indicated by the increase in oxygen K-edge energy for the glass upon shock compression. The first experimental evidence of the increase in the fraction of highly coordinated Al in shock compressed glass is found in the {sup 27}Al NMR spectra. This unambiguous evidence of shock-induced changes in Al coordination environments provides atomistic insights into shock compression in basaltic glasses and allows us to microscopically constrain the magnitude of impact events or relevant processes involving natural basalts on Earth and planetary surfaces.
    Geophysical Research Letters 05/2012; 39(2012). · 3.98 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Ab initio molecular dynamics simulations on MgSiO3 melt have been performed over the pressure range 200–500 GPa. The results calculated by local density approximation (LDA) and generalized gradient approximation (GGA) were compared. Our results showed that compared with experimental data, LDA can yield more accurate pressure and thus the thermal equation of state and Hugoniot curve than GGA. The reason is that in MgSiO3 melt GGA overestimates the average length of Mg − 4 2
    Journal of Non-Crystalline Solids 01/2014; 385:169–174. · 1.72 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We perform a detailed visualization-based analysis of atomic-position series data for model basalt melt obtained from first-principles (quantum mechanical) molecular dynamics simulations. To gain insight into the short- and mid-range order of the melt structure, we extract and visualize the details of radial distribution function (RDF) and coordination environment. The first peaks of all partial RDFs lie in the distance range of 1.6–4 Å and the corresponding mean coordination numbers vary from less than 1 to more than 9. The coordination environments involving cations and anions differ substantially from each other, each consisting of a rich set of coordination states. These states vary both spatially and temporally: The per-atom coordination information extracted on the fly is rendered instantaneously as the spheres and polyhedra as well as along the corresponding trajectories using a color-coding scheme. The information is also visualized as clusters formed by atoms that are coordinated at different time intervals during the entire simulation. The Si–O coordination is comprised of almost all tetrahedra (4-fold) whereas the Al–O coordination includes both tetrahedra (4-fold) and pentahedra (5-fold). The animated visualization suggests that the melt structure can be viewed as a dynamic (partial) network of Al/Si–O coordination polyhedra connected via bridging oxygen in an inhomogeneous distribution of mobile magnesium and calcium atoms.
    Computers & Geosciences 08/2013; 57:166–174. · 1.83 Impact Factor


Available from
Jun 4, 2014