The rheological evolution of a phonotephritic melt upon variable degrees of carbonate assimilation and deformation regimes

The dynamic interplay between magmas and carbonate wall rocks within volcanic plumbing systems heavily influences the chemical and physical properties of erupted magmas. In this study we present results from isothermal static experiments (ISEs) and isothermal deformation experiments (IDEs) aimed at investigating the rheological evolution of a phonotephritic melt from Somma-Vesuvius (Italy) under variable shear rates and CaO and CaO + MgO doping levels. Flash differential scanning calorimetry is also used to determine the viscosity of interstitial melt, allowing for the first direct assessment of how crystallization affects melt rheology without relying on empirical viscosity models. Two distinct rheological scenarios emerge from IDEs: 1) a viscous deformation, characterized by uniform flow, and 2) a non-homogeneous deformation, featuring shear localization and viscous/brittle rupture of the magma. As both shear rate and doping level increase, a non-Newtonian melt behavior is observed due to stress localization and rupture, facilitated by the development of the crystal network. The narrower viscosity range measured from IDEs, compared to models of pure viscous behavior, suggests that shear localization and fracturing reduce the resistance to flow in doped, crystal-rich samples. The complex rheology of doped melts reflects the disaggregation of skarn shells at the margins of magma chambers, thereby facilitating skarn recycling and enhancing magma contamination.