Vein networks in hydrothermal systems provide constraints for the monitoring of active volcanoes

Vein networks affect the hydrothermal systems of many volcanoes, and variations in their arrangement
may precede hydrothermal and volcanic eruptions. However, the long-term evolution of vein networks
is often unknown because data are lacking. We analyze two gypsum-filled vein networks affecting
the hydrothermal field of the active Lipari volcanic Island (Italy) to reconstruct the dynamics of the
hydrothermal processes. The older network (E1) consists of sub-vertical, N-S striking veins; the younger
network (E2) consists of veins without a preferred strike and dip. E2 veins have larger aperture/length,
fracture density, dilatancy, and finite extension than E1. The fluid overpressure of E2 is larger than that
of E1 veins, whereas the hydraulic conductance is lower. The larger number of fracture intersections
in E2 slows down the fluid movement, and favors fluid interference effects and pressurization. Depths
of the E1 and E2 hydrothermal sources are 0.8 km and 4.6 km, respectively. The decrease in the fluid
flux, depth of the hydrothermal source, and the pressurization increase in E2 are likely associated to a
magma reservoir. The decrease of fluid discharge in hydrothermal fields may reflect pressurization at
depth potentially preceding hydrothermal explosions. This has significant implications for the longterm
monitoring strategy of volcanoes.