Micro-Raman spectroscopy for a comprehensive understanding of the structural evolution of Basaltic-Andesite and Trachybasalt multiphase systems

The liquid viscosity of multiphase systems is usually retrieved by the use of combined chemical analysis and empirical chemistry-based models. Here we present an alternative approach to study the glassy pools in highly crystallized basaltic-andesite and trachybasalt samples obtained from isothermal crystallization experiment at different shear rates. Polarized micro-Raman spectra were collected at room temperature on the glassy portions. Results of residual glass structures from the high frequency region of Raman spectra show an increasing polymerization degree. The fragility, derived from the evolution of the boson peak, decreases according to the experimental time evolution. Also, the presence of nanolites in residual glasses adds a further degree of complexity that needs to be considered in the frame of melts polymerization and rheological behavior of volcanic systems. On this concern, the MYEGA formalism approach allows to retrieve the residual melt viscosity of the partly crystallized samples, avoiding its direct measurement and reducing the errors induced when general viscosity models, based only on chemical data, are used. We validated our prediction by determining the low-temperature viscosity data converting, through known shift factors, the thermograms obtained by differential scanning calorimetry. This study provides a further route in characterizing multiphase systems, also including the effect of nanosized crystals, and offers the possibility of increasing the spatial resolution through microscopy and ridding viscosity parametrizations of invasive and time-consuming operations.