Magma evolution and ascent mechanisms at inflated spreading ridges in backarc settings: The Marsili seamount (Tyrrhenian Sea, Italy)

Marsili axial volcano is the largest submarine volcanic complex in the Euro-Mediterranean area and represents an inflated spreading ridge built up in a slow spreading backarc setting. An updated geochemical dataset (major oxides, trace elements, and Sr, Nd, and Pb isotopes) unveils that the Marsili rocks evolve from prevailing island arc and oceanic island basalts to trachytes by fractional crystallization. The heterogeneous parent Marsili magmas are cogenetic to those of the neighbor Aeolian volcanic arc, both of which lie within the Tyrrhenian basin off southern Italy. Petrological and independent geophysical models reveal the depth of the Marsili magma storage zones and the temperature, pressure, oxygen fugacity, and H2O conditions of the magmas. The Marsili plumbing system develops from the upper mantle to the shallower oceanic crust. Silica-poor magmas accumulate in reservoirs at the boundary between the upper and lower oceanic crust, whereas SiO2-rich magmas preferentially accumulate in sill-like reservoirs at the Moho depth. Results of geochemical, geophysical, and thermo-rheological models reveal a reversed plumbing system in which the more viscous SiO2-rich magmas are more limited in reaching the surface with respect to the less viscous SiO2-poor suspensions. The different cooling rate of the two end-member magmas also plays a role. The apparently unusual structure of the Marsili plumbing system could serve as a model for understanding other slow spreading ridges in backarc settings.