Lower crustal differentiation processes beneath a back-arc spreading ridge (Marsili seamount, Southern Tyrrhenian Sea).

We investigate the texture and chemical zoning of phenocrysts of six basic lavas (five basalts and one basalticandesite) from the Marsili volcano, the superinflated spreading ridge of the Marsili back-arc basin (SouthernTyrrhenian). The samples, dredged from different portions of the volcano, were selected in order to representthe two distinct mafic magmas that sourced its plumbing system. Four of the basalts and the basaltic andesitehave an Island Arc Basalt (IAB) affinity, dominant amongst the erupted Marsili lavas; the fifth basalt is anOcean Island Basalt (OIB)-like lava erupted during the late stage of volcano activity. Olivine, clinopyroxene andplagioclase are the prevalent phenocrysts, except for two basalts that lack clinopyroxene. In addition, smallamphibole crystals are found in the basaltic andesite sample. Olivine is more forsteritic (Fo91–75) in the basaltsthan in the basaltic andesite (Fo78–74) and in all samples a proportion of crystals shows Fo compositions innear equilibrium with the respective whole-rock composition. Clinopyroxene phenocrysts from IAB basaltshave higher Mg number (Mg# = 89–83) than those from the OIB lava (Mg# = 81–84), implying thatclinopyroxene joined the liquidus shortly after olivine during the early stage of IAB magma fractionationwhereasthe OIB magma saturated in clinopyroxene after a more extensive olivine crystallization. In both IAB and OIB-likelavas, the clinopyroxene phenocrysts record crystallization at Moho depth. A common feature of theseclinopyroxenes is the intra-crystal trace element variability, indicative of melt mixing during crystal growth.The mixing process involved chemically variable mantle melts derived from incremental fractional meltingof the Marsili mantle source. An-rich plagioclases joined liquidus after the earlier fractionation assemblage ofolivine–clinopyroxene. The An-rich crystals display distinct features, such as a range of textures and the concurrentincrease of Sr and Ba contents in the equilibriummelt, consistentwith crystallizationwithin a heterogeneousmush zone, that is pervasive beneath Marsili volcano. The finding of plagioclase crystals derived from moreevolved magmas in the basaltic lavas dredged from the southern sector of the volcano indicates that the carriermelt interacted with shallow, previously formed, magma reservoirs during its ascent to the surface. The basalticandesite lava from this sector of the volcano contains small amphibole crystals in chemical equilibrium with thehost melt. This finding indicates that “cryptic” amphibole fractionation at the mush zone depth plays a role inthe petrogenesis of this lava.