A Molecular Link to Thermodynamic Properties of Multi-component Silicate Melts in the Earth’s Interior

Whereas the structures of multi-component silicate melts at ambient and high pressure provide insight into the macroscopic properties of natural magmas and has implication for magmatic processes in the Earth’s interior, the structure of most multi-components melts has not been fully described. This is primarily because of a usual increase in inhomogeneous broadening of the spectra with increasing number of components and with pressure. Advent of high-resolution NMR techniques and synchrotron inelastic scattering allow us to yield previously unknown details of pressure-induced structural changes in divers oxide melts (e.g. Lee SK et al. Phys. Rev. Lett. 2009, 103, 095501; J. Phys. Chem. B. 2009, 113, 5162; Proc. Nat. Aca. Sci. 2008, 105, 7925), shedding light on microscopic origins of their thermodynamic properties. Multi-nuclear high-resolution NMR spectra for quaternary, Ca-Mg and Ca-Na aluminosilicate glasses, a model system for primary basaltic magmas, show the presence of a substantial fraction of five coordinated Al and Al-O-Al at 1 atm. The NMR results also suggest a considerable extent of mixing between network modifying cations around non-bridging oxygen, and increases in the topological entropy with the Ca content in those quaternary silicates. The non-linear variation of O-17 NMR parameters for diverse oxygen clusters implies that Na plays a preferential role as a charge-balancing cation, while Ca can act as a network-modifying. With increasing pressure up to 8 GPa, the degree of polymerization (NBO/T) in multi-component glasses decreases with pressure while high-coordinately aluminum are dominant at 8 GPa. Na-O bond length also decreases with pressure. Through-bond and space correlation NMR spectroscopy reveals differential proximity among framework cations and anions in oxide glasses at high pressure. The Al-O-Al cluster is apparently stable up to 8 GPa, suggesting a moderate degree of chemical disorder in the multi-components melts. The core and valence electron excitation spectroscopy for oxide glasses using non-resonant inelastic x-ray scattering also reveal the pressure-induced structural changes around Ca, Fe, O, and B in complex oxide glasses and reports the changes in cation-oxygen bond length and confirms the formation of tri-coordinated oxygens with pressure. While the total entropy of melt is expected to decrease with pressure, the current spectroscopic results rather suggest an increase in the extent of diverse aspects of chemical and topological disorder in the melts with pressure. A decrease in Si-O-Si fraction at high pressure also implies a decrease in activity coefficient of silica. The formation of silica-rich melts is, thus, expected in the multi-component melts formed at elevated pressure ranges. Because the fraction of small-ring clusters and oxygen tricluster increase with pressure, the crystal-melt partitioning coefficient is likely to decrease with pressure.