Program > Browse abstracts by speaker > Décossin François

Alteration of volcanic rocks by reactive fluids in shallow hydrothermal systems promotes the formation of secondary minerals, such as silica-rich phases that may pressurize the dome by reducing fluid permeability, and clay minerals that may also create slippery discontinuities within the volcanic edifice. Gas overpressure and detachment plane processes have the capability to generate flank collapses, 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 the lava dome are determined using three approaches: mineralogical, experimental, and numerical modelling. Firstly, we characterized the chemical and physical properties of a suite of non-altered to highly-altered andesites from the lava dome and flows. Chemical investigations of the samples show that plagioclases are replaced by secondary minerals such as kaolinite, Na-alunite, and amorphous silica. Secondly, we performed thermodynamic modelling using GEM-Selektor software to better constrain the alteration conditions (temperature, pressure, fluid composition, water/rock ratio) prevailing in the natural system. Thirdly, based on the P-T-pH-fluid composition conditions obtained from thermodynamic simulations, we performed batch-reaction experiments to better understand the secondary mineral assemblage seen in the natural products. We also performed time-series fluid flow-through experiments, in which fluids were circulated through a pristine and porous andesite core. Rock permeability was measured in-situ and the mineralogical changes were characterized using various methods (SEM, EDS, X-ray microtomography, and XRD). A sharp decrease in permeability of four orders of magnitude (from 10-14 to 10-18 m2) during the first 3-6 days was observed for all solutions. Finally, we carried out kinetic numerical modelling using Phreeqc code to explore the alteration kinetics for timescales longer than those attained experimentally.