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

• Mineral chemistry of basic lavas from Marsili volcano, the Tyrrhenian back-arc spreading ridge.
• Deeper working of the volcano plumbing system is revealed by phenocryst features.
• A model of the lower crustal Marsili volcano magmatic plumbing system is proposed.

We investigate the texture and chemical zoning of phenocrysts of six basic lavas (five basalts and one basaltic andesite) from the Marsili volcano, the superinflated spreading ridge of the Marsili back-arc basin (Southern Tyrrhenian). The samples, dredged from different portions of the volcano, were selected in order to represent the two distinct mafic magmas that sourced its plumbing system. Four of the basalts and the basaltic andesite have an Island Arc Basalt (IAB) affinity, dominant amongst the erupted Marsili lavas; the fifth basalt is an Ocean Island Basalt (OIB)-like lava erupted during the late stage of volcano activity. Olivine, clinopyroxene and plagioclase are the prevalent phenocrysts, except for two basalts that lack clinopyroxene. In addition, small amphibole crystals are found in the basaltic andesite sample. Olivine is more forsteritic (Fo91–75) in the basalts than in the basaltic andesite (Fo78–74) and in all samples a proportion of crystals shows Fo compositions in near equilibrium with the respective whole-rock composition. Clinopyroxene phenocrysts from IAB basalts have higher Mg number (Mg# = 89–83) than those from the OIB lava (Mg# = 81–84), implying that clinopyroxene joined the liquidus shortly after olivine during the early stage of IAB magma fractionation whereas the OIB magma saturated in clinopyroxene after a more extensive olivine crystallization. In both IAB and OIB-like lavas, the clinopyroxene phenocrysts record crystallization at Moho depth. A common feature of these clinopyroxenes 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 melting of the Marsili mantle source. An-rich plagioclases joined liquidus after the earlier fractionation assemblage of olivine–clinopyroxene. The An-rich crystals display distinct features, such as a range of textures and the concurrent increase of Sr and Ba contents in the equilibrium melt, consistent with crystallization within a heterogeneous mush zone, that is pervasive beneath Marsili volcano. The finding of plagioclase crystals derived from more evolved magmas in the basaltic lavas dredged from the southern sector of the volcano indicates that the carrier melt interacted with shallow, previously formed, magma reservoirs during its ascent to the surface. The basaltic andesite lava from this sector of the volcano contains small amphibole crystals in chemical equilibrium with the host melt. This finding indicates that “cryptic” amphibole fractionation at the mush zone depth plays a role in the petrogenesis of this lava.