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

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.