Outgassing of mantle fluids across an tectonically active crustal segment in between two volcanic systems (Etna and Aeolian arc): the Nebrodi-Peloritani case

Abstract

Mantle-degassing occurs primarily through active volcanic systems and young oceanic lithosphere. Mantle-volatiles are also released by tectonically active continental regions, but the magnitude of this phenomenon is far less characterised. Helium (He) is a powerful tracer to track deep volatile degassing, because the mantle contains more 3He than the crust/atmosphere systems, which are dominated by radiogenic 4He produced in the crust. Here, we studied the volatiles in thermal manifestations discharged along the Nebrodi- Peloritani chain in north-eastern Sicily, with the aim of investigating the origin of thermalism and the related fluids. Thisseismically active region connects the African-SicilianMaghrebides to the SouthernApennines, and its geological evolution has been controlled by the convergence between the European and African plates, as well as by the subduction of the Ionian slab beneath the Tyrrhenian crust. Water and gas samples,collected throughout the study area, exhibit a 3He excess that is evidence for an active outgassing of mantle-derived volatiles. The computed mantle-derived He fluxes are up to 3 orders of magnitude higher than those typical of stable continental areas.We argue that these high fluxescannot be sustained by diffusion through the crust, but rather require deep fluid transport via advection through the regional tectonic discontinuities. The area, despite being a chain, is located between two most active volcanic systems, Mt.Etna to the south and the Aeolian arc to the north. Geophysical observations and models support the existence of toroidal flows in the mantle that bypass the subduction plate and produce a mantle rise below the studied area, and are thus consistent with the presence of magma accumulation at the mantle-crust interface, or even in the crust. Our study suggests a) the possible presence of magmatic intrusions below this sector of the Maghrebian-Apenninic chain, and b) the active role played by regional discontinuities in transferring mantle fluids towards the surface.