The role of degassing in eruptive dynamics and its influence on magma rheology

Abstract

Volatiles have a strong influence on volcanic and magmatic processes, both as dissolved and exsolved species. Water (H2O) is the most dominant species and greatly affects (even at low concentration) a variety of thermodynamic and physical properties (reaction kinetics, element diffusivities, electrical conductivity, heat capacity, partial melting). Furthermore, bulk properties such as viscosity and density of the melt can vary by several orders of magnitude depending on the dissolved and exsolved water content. Such properties control the entirety of magmatic and volcanic processes from melt generation, magma rise, decompression and, ultimately, and the style of volcanic eruptions. Knowledge of the rheological properties of magmas is required for accurate modeling of the volcanic processes. For instance, rheological data are needed to quantify volatile exsolution, bubble growth rates and explosivity of volcanic eruptions. In this thesis the role of degassing in silicic melts is investigated, combining different techniques, involving the study of both dissolved and exsolved volatiles in volcanic materials. The study of degassing process requires an accurate quantification of dissolved H2O. Analytical studies of water distribution in natural glasses are here performed by Raman spectroscopy techniques. A calibration model for water content estimation of natural glassy samples is provided for a wide range of compositions (from basalt to rhyolite including alkali- and iron-rich). In the second part of this thesis, H2O exsolution is investigated with a new experimental approach, in order to better constrain the relationship between viscosity and porosity. The proposed viscosity models highlight the importance of different frameworks phase for modeling magma rheology. The last part of this thesis regards the study of H2O resorption. The rehydration of melt could occurs after degassing and exsolution, when the slow cooling of the magma allows H2O to “return” in the melt. Dome extrusions or ignimbrites and lava emplacement represent some possible volcanic scenarios in which this phenomenon could alter the physical characteristic of the material. In conclusion, this thesis provides an in depth investigation of H2O both as dissolved and exsolved species in order to better understand degassing processes and how they affect the rheological behavior of magmas.