Chemically controlled nanostructuration in alkaline silicate melts

Understanding the chemical and physical behaviour of silicate melts requires direct constraints on melt structure at the nanoscale. Recent studies suggest that variations in melt nanostructure exert a primary control on magmatic properties (e.g., melt viscosity), yet their evolution at deep undercooling remains poorly constrained. Here, we investigate nanoscale melt structure dynamics in alkaline and alkaline-earth–rich volcanic melts using a combination of synchrotron-based small- and wide-angle X-ray scattering (SAXS–WAXS) and Raman spectroscopy. All investigated compositions are X-ray amorphous at room temperature. However, upon heating above the glass transition temperature (Tg), SAXS–WAXS data reveal the rapid development of nanoscale heterogeneities rich in Fe and Ti. The onset and evolution of this nanostructuration are strongly dependent on the initial redox conditions and melt composition. Mössbauer spectroscopy indicates that rapid nanostructuration correlates with higher proportions of tetrahedrally coordinated Fe3+, which is in turn interpretable as originating from differences in the network modifiers content of the melts. Our findings have important implications for the interpretation of viscosity measurements and for the understanding of the non-equilibrium evolution of magmatic liquids.