A review of the differential scanning calorimetry shift–factor approach: Application to Colli Albani melt viscosity and implications for mafic Plinian eruptions

The differential scanning calorimetry (DSC) shift factor has recently been applied to model the viscosity of volcanic melts, revealing that such melts are often less viscous and more prone to nanoscale phase separation and crystallization of nanolites (i.e., nanostructuration) than previously thought. In this study, we investigate the melt viscosity and structural evolution of the tephri-phonolite "Pozzolane Nere" (PNR) magma from the Colli Albani volcanic district (Rome, Italy), which fed one of the largest Plinian eruptions of this volcanic system. By combining viscometry, conventional and flash DSC, and spectroscopic techniques - Raman, Mossbauer, and Brillouin - we examine the melt viscosity and structure both under anhydrous and hydrous conditions. Our results demonstrate that the PNR melt is highly susceptible to nanocrystallization, particularly during viscometry, leading to a significant increase in viscosity compared to previous estimates. Additionally, the data suggest that under pre-eruptive conditions (1050 degrees C and H2O = 5 wt%), the melt exhibits a viscosity one log unit lower than predicted by models. Upon dehydration, the viscosity increases remarkably, by up to 4300-fold. These findings imply that the low-viscosity behavior of PNR melt at depth could facilitate rapid magma storage and transfer through deep transcrustal magma pathways, supporting the rapid ascent needed for explosive mafic eruptions. Our results also imply that, despite this lower viscosity, the strong tendency of the PNR melt towards nanostructuration is likely to play a critical role in influencing magma rheology, as well as degassing and outgassing processes in the conduit. These findings provide new insights into the magma storage and ascent dynamics, shedding light on how these processes may facilitate relatively rapid accumulation of mafic magmas in shallow reservoirs prior to highly explosive eruptions driven by rapid and significant crystallization.