A multidisciplinary strategy based on geophysicaland geochemical data to investigate hydrothermalcirculation along the rift zones of Mt. Etna volcano(Italy)

We surveyed the South Rift and Northeast Rift of Mt. Etna volcano in order to assess the presence and the spatial extent of shallow hydrothermal activity linked with magma degassing along the main feeder systems of the volcano. Surveys were carried out across the two rift zones at altitude between 1500 and 2500 m a.s.l. Soil CO2 effluxes, soil heat fluxes, self‑potential intensity, simultaneously with magnetic surveys, were measured for the first time together in 2017, both along profiles of measurement points and over networks of points. Several convective hydrothermal cells were found close together, particularly on the South Rift where they occupied a large portion of this rift. This suggested the existence of a complex plexus of volcano‑tectonic structures, highly permeable to volcanic fluids, that are all potential pathways for new magma intrusions, as they appear well connected with the portions of the central feeder conduits that allow for efficient magma degassing. Gravity measurements acquired successively along the South Rift strongly support this hypothesis. The Northeast Rift showed only one major hydrothermal convective cell, centered along the 2002 eruptive fissure, and a minor one a few hundred meters west of it. These surveys were repeated in 2018, although only partially. The 2018 measurements showed a strong decrease in the overall intensity of hydrothermal circulation along the Northeast Rift area and in most of the South Rift areas surveyed in 2017. This was explained as indication of strong migration of high‑enthalpy fluids from the two rift zones following magma transfer along the main conduits of the volcano towards its summit. Similar conclusions were drawn from the observed variations in gravity data from 2020 to 2021. The hypothesized magma transfer episodes seem to be coherent with the subsequent occurrence of the December 2018 flank eruption and of the 2020‑2021 sequence of paroxysmal summit eruptions.