Probing the structure of Fe-free model basaltic glasses: A view from a solid-state 27Al and 17O NMR study of Na-Mg silicate glasses, Na2O-MgO-Al2O3-SiO2 glasses, and synthetic Fe-free KLB-1 basaltic glasses

Abstract

The extent of structural disorder in multi-component basaltic (mafic) melts with varying compositions (particularly, Mg content) and the formation depth has not been well understood. Here, we explore the structure of Mg-rich silicate glasses including Na-Mg silicate glasses, NaO-MgO-Al2O3-SiO2 (NMAS) glasses in a nepheline (NaAlSiO4)-forsterite (Mg2SiO4)-quartz (SiO2) eutectic composition, and synthetic equivalents of basaltic glasses generated by partial melting of upper mantle peridotite (KLB-1) in a multi-component CaO-MgO-Na2O-K2O-Al2O3-TiO2-SiO2 system using high-resolution solid-state nuclear magnetic resonance (NMR). The 2-dimensional (2D) 17O 3Q (triple quantum) magic angle spinning (MAS) NMR spectra for those multi-component glasses revealed the nature of diverse aspects of structural disorder. Non-bridging oxygens (NBOs) apparently prefer proximity toward Ca2+ and Mg2+, whereas bridging oxygens (BOs) prefer to be near Na+, confirming a stronger interaction between NBOs and high field-strength cations in multi-component basaltic melts. The changes of oxygen configurations with increasing Mg contents provide implications for the structural evolution of basaltic melts with varying formation depth. Thus, while the pressure effect remains to be explored, considering the changes in melt composition alone, the NBO fraction of the KLB-1 basaltic melts increases with increasing melting depth of KLB-1 peridotite.The 27Al 3QMAS NMR spectra for multi-component KLB-1 basaltic glasses reveal non-negligible fraction of [5]Al that increases with increasing Mg content. The fraction of [5]Al in natural basaltic magma is expected to increase with melting depth of KLB-1 peridotite. The 27Al NMR quadrupolar coupling constant for [4]Al in multi-component silicate glasses systematically increases with Mg content, indicating an increase of the topological distortion (variation of bond angle and length distribution and network distortion) around [4]Al in the basaltic glasses. The observed increase in the extent of structural disorder in basaltic melts with Mg content may increase the overall configurational and topological entropy. While further confirmation is necessary, the changes in melt polymerization observed from 17O NMR spectra and network disorder around Al imply the decrease in melt viscosity in the complex silicate melts with varying formation depth of KLB-1 basaltic melts.