Advanced argillic alteration at Cave di Caolino, Lipari, Aeolian Islands (Italy): Implications for the mitigation of volcanic risks and the exploitation of geothermal resources.

Four sites in the western sector of Lipari Island with still active hydrothermal activity are here considered. The petrography (mesoscopic observations and XRPD) and geochemistry (major, minor and trace elements chemistry) of ten representative and extremely altered volcanic rocks were characterized. Two types of parageneses of altered rocks are discriminable, one rich in silicate phases (opal/cristobalite, montmorillonite, kaolinite, alunite and hematite) and one in sulphates (gypsum, plus minor amounts of anhydrite or bassanite). The altered silicate-rich rocks are rich in SiO2, Al2O3, Fe2O3 and H2O, and depleted in CaO, MgO, K2O and Na2O, while the sulphate-rich ones are extremely enriched in CaO and SO4 in comparison with local unaltered volcanic rocks. The content of many incompatible elements is similar in altered silicate-rich rocks and lower in sulphate-rich ones with respect to the pristine volcanic rocks; conversely, almost all REEs are markedly enriched in silicate-rich rocks and heavy REEs are enriched in sulphate-rich altered rocks compared to unaltered volcanic rocks. Reaction path modelling of basaltic andesite dissolution in local steam condensate predicts the production of amorphous-silica, anhydrite, goethite, and kaolinite (or smectites and saponites) as stable secondary minerals and alunite, jarosite, and jurbanite as ephemeral minerals. Considering possible post-depositional reactions and admitting that the presence of two distinct parageneses is apparent, since gypsum is prone to form large crystals, it can be concluded that there is an excellent agreement between the alteration minerals occurring in nature and those predicted by geochemical modelling. Consequently, the modelled process is the main responsible for the production of the advanced argillic alteration assemblage of "Cave di Caolino" on Lipari Island. Since rock alteration is sustained by the H2SO4 solution produced by hydrothermal steam condensation, there is no need to invoke the involvement of SO2-HCl-HF-bearing magmatic fluids, in line with the absence of fluoride minerals.

One-Year Seismic Survey of the Tectonic CO2-Rich Site of Mefite d'Ansanto (Southern Italy): Preliminary Insights in the Seismic Noise Wavefield.

A passive seismic experiment is carried out at the non-volcanic highly degassing site of Mefite d'Ansanto located at the northern tip of the Irpinia region (southern Italy), where the 1980 MS 6.9 destructive earthquake occurred. Between 2020 and 2021, background seismic noise was recorded by deploying a broadband seismic station and a seismic array composed of seven 1 Hz three-component sensors. Using two different array configurations, we were allowed to explore in detail the 1-20 Hz frequency band of the seismic noise wavefield as well as Rayleigh wave phase velocities in the 400-800 m/s range. Spectral analyses and array techniques were applied to one year of data showing that the frequency content of the signal is very stable in time. High frequency peaks are likely linked to the emission source, whereas at low frequencies seismic noise is clearly correlated to meteorological parameters. The results of this study show that small aperture seismic arrays probe the subsurface of tectonic CO2-rich emission areas and contribute to the understanding of the link between fluid circulation and seismogenesis in seismically active regions.

Seismic signature of active intrusions in mountain chains.

Intrusions are a ubiquitous component of mountain chains and testify to the emplacement of magma at depth. Understanding the emplacement and growth mechanisms of intrusions, such as diapiric or dike-like ascent, is critical to constrain the evolution and structure of the crust. Petrological and geological data allow us to reconstruct magma pathways and long-term magma differentiation and assembly processes. However, our ability to detect and reconstruct the short-term dynamics related to active intrusive episodes in mountain chains is embryonic, lacking recognized geophysical signals. We analyze an anomalously deep seismic sequence (maximum magnitude 5) characterized by low-frequency bursts of earthquakes that occurred in 2013 in the Apennine chain in Italy. We provide seismic evidences of fluid involvement in the earthquake nucleation process and identify a thermal anomaly in aquifers where CO2 of magmatic origin dissolves. We show that the intrusion of dike-like bodies in mountain chains may trigger earthquakes with magnitudes that may be relevant to seismic hazard assessment. These findings provide a new perspective on the emplacement mechanisms of intrusive bodies and the interpretation of the seismicity in mountain chains.

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 long-term monitoring strategy of volcanoes.