Large silicic magma chambers at the Moho depth characterize the multi-level plumbing system of back-arc spreading ridges

The plumbing system of volcanoes in crustal convergent settings consists of vertically superimposed, sill-like reservoirs. The shallower reservoirs are generally filled by less dense and SiO2-richer magmas. The plumbing systems of spreading ridges are supposed to be simpler and characterized by prevailing sub-parallel vertical dykes. Here, we present the results of thermo-barometric determinations from minerals and modelling of potential field data at the Marsili spreading ridge in the Southern Tyrrhenian Sea back-arc oceanic basin. The Marsili plumbing system consists of sill-like, large SiO2-rich magma reservoirs located at the Moho depth and sub-vertical basaltic dykes crossing the whole oceanic crust. The formation of deep-seated silicic reservoirs is associated with a decrease in the back-arc spreading rate. The geometry of the plumbing system of spreading ridges may be not dissimilar from that of volcanoes at convergent margins.