Hydrothermal sealing and erratic explosive behaviour at Turrialba, Poás, and Rincón de Vieja volcanoes (Costa Rica)

Reductions to the permeability of a volcanic system can increase pore fluid pressure and, in turn, promote volcanic hazards such as erratic explosive behaviour and slope failure. Hydrothermal alteration, ubiquitous at volcanoes worldwide, is one mechanism thought to reduce permeability and therefore increase volcanic hazard potential. Turrialba, Poás, and Rincón de Vieja, active stratovolcanoes in Costa Rica, are characterised by erratic explosive behaviour thought to be the result of the formation, maturation, and rupture of hydrothermal seals that clog the conduit. To better understand this process, we present here a systematic study in which we assessed the textural, mineralogical, and physical properties of hydrothermal seals from Turrialba, Poás, and Rincon de Vieja volcanoes, ejected as ballistics following recent explosive activity. We first document the type and intensity of the hydrothermal alteration preserved in the collected ballistics using X-ray powder diffraction and scanning electron microscopy. All samples are characterised by pervasive acid-sulphate alteration. Prior to sample preparation, the permeability of these ballistics was first estimated using a TinyPerm III, a portable handheld air permeameter. We find that the hydrothermal seal is characterised by low values of permeability, between 10⁻¹⁵ and 10⁻¹³ m². Cylindrical samples were then prepared for a systematic physical property (porosity, permeability, P-wave velocity, thermal properties, and mechanical strength) characterisation in the laboratory. These laboratory data, together with fluid flow modelling, highlight how alteration can create a low-permeability hydrothermal seal that promotes cyclic, but erratic, explosive volcanic behaviour.