Viscosity and structure of Na2O-enriched haplogranitic melts: a DSC shift-factor calibration for peralkaline rhyolites

The influence of excess Na2O on the structure, thermal properties and viscosity of haplogranitic melts was systematically investigated by integrating micropenetration viscometry, Raman spectroscopy (including Boson peak analysis) and differential scanning calorimetry (DSC). This approach reveals fundamental structure-property relationships in these felsic melt analogs. We demonstrate that Na2O-induced network depolymerization, clearly evidenced by Raman spectroscopy, provides a direct rationale for the observed dramatic, non-linear decrease in melt viscosity and a concurrent systematic increase in melt fragility. Strong correlations are established between melt fragility and both nanoscale vibrational dynamics (Boson peak frequency, ωBP) and macroscopic thermodynamics (configurational heat capacity change at the glass transition temperature, ∆CpconfTg), validating ωBP and ∆CpconfTg as proxies for estimating melt fragility in experimentally challenging systems, such as those prone to nanostructuration. Furthermore, we provide DSC-viscosity shift factors Konset, Kpeak, Kendset applicable to peralkaline compositions. By addressing the role of thermal lag and thermal inertia in DSC analyses, our study refines the experimental accuracy of viscosity–temperature relationships and reinforces the robustness of the shift-factor approach across compositionally diverse silicate systems. This work thus provides an integrated framework and predictive insights into the rheology of Na2O-rich haplogranitic melts, relevant to understanding evolved magmatic systems.