Constraining trachytic melt viscosity by integrating rheological and in situ spectroscopic analyses: Insights from the Agnano-Monte Spina eruption (Campi Flegrei, Italy)

The viscosity of hydrous volcanic melts exerts a primary control on magma ascent, degassing efficiency, and fragmentation, yet its experimental determination is often affected by time-dependent melt structure changes at the nanoscale during measurements. This issue remains poorly constrained for highly polymerised, alkali-rich trachytic magmas, despite their key role in explosive volcanism at caldera systems such as Campi Flegrei (Italy).

We investigate the anhydrous and hydrous viscosity of a trachytic melt from the Agnano–Monte Spina (AMS) eruption (Campi Flegrei) by combining differential scanning calorimetry (conventional and flash DSC), micropenetration viscometry (MP), Brillouin light scattering (BLS), and in situ high-temperature Raman spectroscopy. This integrated approach allows us to directly link viscosity behaviour to nanoscale structural evolution during thermal treatments. Glass transition temperature (T_g) decreases from ~632 to ~349 °C with increasing water (0–4.45 wt.%), while melt fragility increases systematically with hydration, independently constrained from BLS elastic moduli measurements.

In situ Raman spectroscopy reveals that nanoscale melt structure reorganisation initiates within minutes, slightly above T_g = 632 °C. These processes lead to a viscosity overestimate of up to ~1 log unit in standard viscometry experiments. Using the glass transition temperatures derived from DSC measurements and BLS-derived melt fragilities, we develop a composition-specific viscosity model for the AMS trachytic magma that avoids nanostructuration-induced artefacts.

As the AMS trachytic magmas are crystal-poor, its rheology is dominated by the melt phase, making melt viscosity a primary control on magma ascent. Our results show that viscosity is highly sensitive to dehydration, with relatively low initial viscosity at high water content, followed by rapid rheological stiffening during ascent. The new model indicates a 10⁵-fold increase in melt viscosity associated with dehydration from 5 to 0 wt.% H₂O, relative to the 10⁴-fold increase calculated using previous experimental and empirical models. As a result, commonly used empirical viscosity laws likely underestimate both the magnitude and rate of viscosity evolution during decompression of hydrous trachytic melts, with significant consequences for degassing efficiency, fragmentation depth, and eruptive style. The spectroscopically guided approach developed in this study is readily applicable to other volatile-bearing magmas and offers a more physically robust rheological framework for modelling viscosity in volcanic systems.