Mantle-derived fluids in Central Mediterranean: Geochemical and geophysical constrains on sources of fluids and migration

The geodynamics of the central Mediterranean is characterized by the interaction between the European plate and the African’s. In this setting Sicily is a sector of the Appenine-Maghrebide accretionary prism, which is located between two areas affected by extensional tectonics (Sicily Channel to the south and the Thyrrenian back arc basin to the north). Significant mantle-derived helium (0.4<R/Ra<2.8; R=3He/4He in the sample, Ra in atmosphere) is found in the CH4 and N2-CO2 rich fluids released in central western Sicily, a region without evidence of recent magmatism. CH4- dominated gases are released from mud volcanoes localized in an area of both low heat flow and seismicity. On the contrary CO2 is mainly associated to the thermal groundwater circulating mainly in Mesozoic limestone over an area characterized by high seismicity and heat flow anomaly. Total carbon dissolved in thermal water is a mixture of mantlederived and crustal inorganic CO2, while CH4-dominated fluids show a mixing between a 3He rich and CH4-poor term and a CH4-rich and 3He-poor one typical of crustal reservoir of gases. The computed mantle derived He, much higher than stable continental areas, indicates that the transfer of fluids is controlled by tectonic mechanism through the crust. Finally, recent geophysical investigations discovered the occurrence of active lithospheric faults that could control the transfer of mantle derived fluids from the sources to the crust and throughout this towards the surface.