Aluminum coordination in rhyolite and andesite glasses and melts: Effect of temperature, pressure, composition and water content

The coordination state of the network formers Si and Al is one of the primary parameters describing the structure of silicate glasses and melts. In this experimental study, we use ²⁷Al MAS NMR spectroscopy to investigate the effect of temperature, pressure and composition on the coordination of Al in dry and hydrous rhyolite and andesite glasses, quenched from pressures of up to 3.5GPa. The spectra were collected at a high magnetic field of 20 Tesla to increase spectral resolution and allow for a direct quantification of the different Al coordinations. The disorder around [⁴]Al increases with increasing fictive temperature and pressure and with decreasing water content or peralkalinity. Al is predominantly (>98%) in fourfold coordination in all investigated samples. Nevertheless, systematic trends in Al coordination have been observed as a function of pressure, composition and water content. For the investigated pressure range, the proportion of [⁵]Al increases with increasing pressure, but decreases with increasing water content and proximity to the metaluminous join for rhyolite glasses. Within analytical uncertainty, the amount of [⁵]Al is independent of fictive temperature. The addition of water to andesite glasses does not decrease the amount of [⁵]Al, in contrast to the rhyolite glasses. Based on the glass data, we conclude that the majority of Al and Si will be present in fourfold coordination in metaluminous dry and hydrous rhyolite magmas at the temperature and pressure conditions relevant for the crust and the subducting slab (up to 3.5GPa). However, consistent with previous predictions from model systems, more higher-coordinated Al may be present in andesite and peralkaline or peraluminous rhyolite melts, although the exact concentrations cannot be predicted accurately from the available data. Compared to more depolymerized melts, rhyolite melts can accommodate a higher degree of densification without a significant increase in Al coordination.