Mid-term thesis – François DECOSSIN

Abstract

Volcano unrest associated to ascent of magmatic fluids (magma, brines, gases) at shallow depths in the presence of a very active hydrothermal system, can promote or enhance extensive hydrothermal rock alteration and form fragile discontinuities within the edifice. A process that can favour flank instability and culminate in partial flank collapse, engendering significant risks to the surrounding population. In the MYGALE ANR project, we focus on hydrothermal alteration timescales of andesitic rocks to better assess the hazard of volcano flank instability at La Soufrière de Guadeloupe (Eastern Caribbean, France). The conditions and kinetics of hydrothermal alteration reactions of the volcanic rocks of La Soufrière lava dome are determined by three approaches: mineralogical, experimental, and by numerical modelling. Firstly, we characterized the natural alteration sequence of 20 samples from the lava dome and lava flows showing different degrees of alteration and porosity. SEM and XRD analyses of the samples show that the plagioclases are replaced by secondary minerals such as kaolinite, Na-alunite, and amorphous silica. Secondly, we performed thermodynamic numerical modelling using Phreeqc code, in order to better constrain hydrothermal alteration conditions in terms of P-T-pH and equilibrium fluid composition. Thirdly, we performed batch-reaction and fluid flow-through experiments, in which fluids are circulated through a pristine and porous andesite core (representative of the unaltered state of the present-day lava dome at La Soufrière). We varied temperature (150-250 °C), pressure (100-150 bar), duration (from days to months), and fluid composition (H₂O-HCl mixtures). Rock permeability is measured in-situ and the mineralogical changes are characterized by post-experiment using various methods (SEM, EDS, X-ray microtomography, and XRD). Finally, we performed kinetic numerical modelling using Phreeqc software to explore the alteration kinetics for timescales longer than those attained experimentally.