Understanding the structure and transport properties of depolymerized silicate melts at high pressure and temperature is important for modeling melt transport and segregation throughout the Earth’s mantle.  We have undertaken a comprehensive study to determine the pressure dependence of short-range structure in highly depolymerized MgSiO₃ (enstatite) glass that is often considered a proxy for basaltic liquids.  The ²⁹Si magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectrum of an enstatite glass quenched from 500•C and 10 GPa indicates that a small fraction of the Si atoms (~1%) in the glass are 6-fold coordinated.  The corresponding peak in the ²⁹Si MAS NMR spectrum is centered at –185ppm.  The most intense peak in the ²⁹Si MAS NMR spectrum of the pressurized glass corresponds to the Q² species and is centered at ~-77 ppm.  This peak position is deshielded by ~4 ppm compared to the uncompressed glass (~-81 ppm); however, the width of this peak is unaffected by pressurization.  The downfield shift in the Q² peak position with pressure suggests that densification of enstatite glass to at least 10 GPa occurs primarily by the reduction in Si-O-Si bond angles as found for vitreous SiO₂.  This result supports previous conclusions that highly depolymerized silicate glass/melt structure responds to pressure in a fashion very similar to that observed for fully polymerized systems.  6-fold coordinated Si may play an important role as a transition-state species by facilitating Q-species exchange, thereby controlling configurational entropy and viscous flow in depolymerized liquids at high pressure.  The corresponding structural and dynamical models of enstatite melt at high pressure and implication for thermodynamic and transport properties will be discussed. 

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