Experimental and numerical thermo-kinetic modelling of hydrothermal alteration of volcanic rocks - Example of La Soufrière de Guadeloupe (Eastern Caribbean, France)

Volcano unrest associated to ascent of magmatic fluids at shallow depths in the presence of a very active hydrothermal system can promote or enhance extensive hydrothermal rock alteration and form low-strength layers within the edifice. This process can favour flank instability and culminate in partial flank collapse with emplacement of debris-avalanches and pyroclastic-density-currents from potential laterally-directed explosions, 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 volcano (Eastern Caribbean, France). We characterised the mineral chemistry and the 3D porosity of a suite of unaltered to highly altered andesite samples from the volcanic dome of La Soufrière. Chemical and textural investigation shows the progressive deterioration of plagioclase into kaolinite and Na-alunite, the dissolution of the volcanic glass, and silica precipitation. By comparing the natural products with geochemical computations, we suggest that hydrothermal alteration prevailing at the base of the dome is primarily dominated by the reactive fluid composition (H2O, HCl, and H2SO4) and to a lesser extent by water/rock (W/R) ratio (from 0.5 to 10), pressure close to 100 bars, and temperature (from 150 to 250 °C). We propose that acidic aqueous solutions containing 0.1 mmol/L HCl and 0.15–0.5 mmol/L H2SO4, corresponding to pH of 3.0–3.5, are mandatory to co-precipitate kaolinite, Na-alunite, and silica. We also investigate the alteration of the volcanic dome of La Soufrière under these acidic hydrothermal conditions, using both batch and reactive percolation experiments, combined with kinetic modelling using PHREEQC. The experimental results highlight a strong reactivity of the rhyolitic residual glass and the plagioclase phenocrysts, leading to the formation of clay minerals such as illite, while the pyroxenes and the Fe-Ti oxides remain largely unaltered. Spatial mineralogical heterogeneities develop along the reacted cores, with intense dissolution and secondary mineral precipitation near the fluid inlet and preservation of primary phases toward the outlet. Kinetic simulations of plagioclase dissolution in either pure water or HCl highlight the influence of temperature, W/R ratio, grain-size, and pH on silicon release. Increased temperature and a lower W/R ratio enhance the dissolution rate, while larger grain-sizes reduce the reactive surface area and slow reaction progress. Anorthite dissolution kinetics and alteration extent are also strongly pH-dependent, remaining negligible at pH ≥ 2, becoming rapidly self-limited at pH~1, and evolving continuously under extremely acidic conditions (pH = 0). The alteration sequence simulated following the conditions of the reactivation of La Soufrière since 2018 (i.e., temperature increase, fluid acidification, and rainfall reduction) predicts the preferential formation of Na-alunite and silica precipitation, which reduces the formation of slippery argillic discontinuities and imparts some internal cohesion on the dome rocks. In contrast, an increase in the W/R ratio, as predicted by global warming, would result in preferential formation of phyllosilicates that would serve to increase dome instability. The combined experimental and modelling approach provides a detailed view of the controls on hydrothermal alteration sequence in volcanic systems.