Viscosity of Trachytic Melt Constrained by Integrated Rheological and In Situ Spectroscopic Analyses: Insights From the Agnano‐Monte Spina Eruption (Campi Flegrei, Italy)

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Quantifying the viscosity of hydrous volcanic melts is essential for understanding ascent dynamics, degassing efficiency, and fragmentation, yet laboratory measurements may be compromised by syn‐analytical evolution in melt structure at the nanoscale, thus yielding viscosity values not representative of a homogeneous melt phase. This issue remains poorly explored in highly polymerized trachytic magmas, for which robust viscosity constraints are scarce despite their relevance to explosive volcanism. We develop a composition‐specific viscosity model for the Agnano‐Monte Spina (AMS) trachyte using an integrated approach, combining differential scanning calorimetry (DSC), micropenetration viscometry (MP), Brillouin light scattering, and in situ high‐temperature Raman spectroscopy. Glass transition temperatures () decrease from 632°C to 349°C, over 0–4.45 wt.% H 2 O, and melt fragility (a measure of the temperature‐sensitivity of viscosity) increases systematically with hydration, consistent with independent configurational heat‐capacity data. In situ Raman measurements show that nanoscale structural reorganization begins within minutes once the temperature exceeds . This leads to viscosity increase of up to ∼1 log unit in MP measurements over a period of one‐hour. Instead, single‐cycle DSC measurements preserve the melt structure homogeneity and provide reliable constraints for viscosity parameterization. The resulting viscosity model captures the strong, hydration‐dependent rheology of the AMS trachyte while avoiding biases introduced by syn‐analytical melt structure evolution. Our spectroscopically guided approach offers a transferable strategy for determining pure‐liquid viscosity in volatile‐bearing magmas, improving the quantitative basis for modeling magma ascent and eruptive behavior.