Magma-carbonate interaction under dynamic conditions: experimental insights on crystallization kinetics and multiphase rheology

Magma-carbonate interactions within volcanic plumbing systems are pivotal in shaping the chemical and physical properties of erupted magmas, with far-reaching implications for magma transport dynamics, crystallization, and eruption processes. This research focuses on the Somma-Vesuvius volcanic system (Italy), where assimilation of carbonate lithologies (e.g., limestone and dolostone) introduces CaO and MgO into magmatic melts, significantly altering their rheological behavior. Through a comprehensive experimental approach, this study investigates the effects of carbonate assimilation on melt viscosity, rheological evolution, and crystallization kinetics under varying thermal and deformation conditions. In the first phase, viscosity measurements were conducted on a phonotephritic melt from the 472 CE Pollena eruption, doped with CaO and CaO+MgO to simulate carbonate assimilation. High-temperature (1150–1400 °C) and low-temperature (640–760 °C) viscosity experiments were performed using concentric cylinder viscometry, micropenetration techniques, and differential scanning calorimetry. The experimental results revealed a pronounced viscosity reduction with CaO doping, exceeding the effects of CaO+MgO, highlighting the dominant role of CaO in modifying melt rheology. A viscosity/temperature crossover was observed: above 750 °C, doped melts exhibited significantly lower viscosities than the pristine melt, while below this threshold, the trend reversed, with doped melts becoming more viscous than the undoped counterpart. The formation of nanoheterogeneities, detected during low-temperature experiments, emerged as a critical factor influencing rheological behavior. Raman spectroscopy, integrated with viscosity data, provided detailed insights into the structural changes within the melt, allowing for the quantification of the impact of these heterogeneities on viscosity. Additionally, Brillouin spectroscopy was employed to refine the prediction of high-temperature viscosities, offering a more robust alternative to empirical viscosity models, which consistently failed to reproduce the experimental data for melts with higher levels of carbonate contamination. These findings provide a framework for understanding the impact of carbonate assimilation on key processes in volcanic plumbing systems. The changes in viscosity caused by CaO and CaO+MgO doping influence magma mobility and the potential for hybrid magma formation, with implications for the mixing versus mingling behavior of magmas.. The second phase explored the combined effects of carbonate assimilation and shear strain rate on the rheological evolution of phonotephritic melts, incorporating isothermal static and deformation experiments and flash differential scanning calorimetry. The results demonstrated that carbonate assimilation promotes crystallization and the development of a mineral framework under dynamic conditions. Textural analyses revealed that residual glasses from undoped and low-doped melts resembled juvenile materials from Somma-Vesuvius eruptions, while highly doped samples resembled those from skarn xenoliths. Crystallization kinetics and residual glass viscosity were better characterized using a novel calorimetry-based approach, which proved more reliable than chemical-based models. Distinct deformation regimes were identified: viscous flow in undoped melts, indicative of uniform deformation, and non-homogeneous flow in doped melts, marked by shear localization, stress drops, and brittle rupture. Increasing shear rates and carbonate assimilation amplified the development of crystal networks, leading to non-Newtonian behavior. Mechanical heterogeneities introduced by crystal networks disrupted uniform flow, promoting localized stress and premature crack formation. This resulted in narrower viscosity ranges compared to pure viscous models, emphasizing the complexity of rheological behavior under stress conditions. The interplay between carbonate assimilation and deformation facilitated the recycling of skarn shells and enhanced magma contamination at the margins of magma chambers. This dissertation advances the understanding of magma-carbonate interactions by linking compositional, thermal, and rheological properties to physical processes in volcanic systems.