The heterogeneous nature of the Southern Tyrrhenian mantle: Evidence from olivine-hosted melt inclusions from back-arc magmas of the Marsili seamount

Abstract

We present major and trace element data of olivine (Fo 86–91) — hosted melt inclusions from five Island Arc Basalt (IAB) lavas from Marsili seamount, a large volcano in the active Marsili back-arc basin of the Southern Tyrrhenian. The geochemical composition of this suite of melt inclusions varies widely, extending the range of variation of the host lavas and providing further insights into the mantle components entering the melting region under the Marsili volcano. The trace element trends defined by melt inclusions with compositions similar to the host rock confirm the involvement of two different mantle domains, affected by different degree of subduction-related metasomatism. Melt inclusions generated from the ambient MORB mantle wedge are depleted in incompatible trace elements and show a pronounced subduction signature, resembling the Marsili IAB- group 2 lavas. Melt inclusions derived from heterogeneous mantle, where small blobs of an OIB enriched component are also involved in the melting, are less depleted in incompatible trace element, and are geochemically similar to the Marsili IAB- group 1 lavas. The olivine-hosted melt inclusions with a composition different to that of the host rock reveal two further distinct geochemical signatures. One signature resembles that of the OIB-like lavas from the nearby Ustica Island, and is also alike to that of the few OIB-like lavas so far recovered from the Marsili volcano. However, the Marsili OIB-like melt inclusions are richer in SiO 2, TiO 2, Na 2O, Zr, Y and HREE and depleted in CaO relative to these OIB-like lavas, suggesting an origin by melting of trachyandesitic bodies in the lower lithospheric mantle, which are themselves formed by high pressure fractional crystallization of parental OIB-like melts. The other detected signature is similar to that of the MORB lavas recording the early ocean-formation stage of the Southern Tyrrhenian back-arc basin. Nevertheless, the high CaO contents of these melts suggest that they probably resulted from melting of clinopyroxene-rich veins related to this early ocean-forming stage.