The influence of hydrothermal alteration on permeability: A field study

Hydrothermal alteration can modify the permeability of volcanic rocks, influencing the movement of fluids and volcanic hazard potential. Permeability increases could facilitate outgassing, promoting effusivity, and permeability decreases could result in overpressurisation, promoting explosivity and instability. We measured the permeability of 573 variably altered rocks from La Soufrière de Guadeloupe (Eastern Caribbean) using a calibrated field permeameter. Our data show a wide range of alteration (from pristine to extremely altered) and permeability (10^–18–10^–11 m²), and that more altered rocks have a higher average permeability, and a wider permeability range, than less altered rocks. We also find that alteration and permeability are spatially heterogeneous. Microstructural analysis reveals evidence for dissolution in plagioclase phenocrysts and void-filling precipitation. For high-permeability rocks, such alteration did not affect the well-connected flow-path: these rocks were simply altered without modifying their permeability. For low-permeability rocks, dissolution in isolated plagioclase phenocrysts did not decrease flow-path tortuosity, but precipitation blocked important pathways for flow: alteration reduced their permeability. Therefore, although the more altered rocks have a higher average permeability, we conclude that alteration reduces permeability. Alteration sites are, therefore, sites of high permeability undergoing permeability reduction. Downsampling our large dataset to test whether smaller datasets reproduce the observed trend highlights the problems associated with using small datasets to understand the influence of alteration on permeability. The results of our study can be used to improve volcano monitoring and hazard mitigation.