Magmatic processes within the plumbing system of the Marsili seamount (Southern Tyrrhenian Sea): constraints from the phenocryst cargo of the sampled lavas

Abstract

This dissertations represents the work developed during the three years of the PhD program. The aim of the project was the study of the Marsili basalts and their phenocryst cargo to further characterise the plumbing system of this large volcano and consequently to give further insights into the dynamics occurring in a back-arc basin context. A variable number of magmatic reservoirs was identified for each sample and the complexity of these magmatic structures is described through the interactions between the products of each reservoir (i.e. magmatic crystals and magmas). The samples showing a larger compositional range for both olivine and clinopyroxene crystals suggest that the magmatic plumbing systems are made of heterogeneous mushes, where the crystals record the input of magmas with variable differentiation degrees and within variable timescales. Most primitive olivine crystals overlap the compositions related to a peridotitic source, while only few crystals show deviated compositions towards the pyroxenitic end-member. Overall, the variations found for the Marsili olivine crystals overlap those from the Aeolian Arc (Zamboni et al. 2017). However, magma composition and diffusional reequilibration due to magma mixing may have significantly influenced the olivine compositions. The trace element compositions from the clinopyroxene crystals overlap the previous published data, with one sample showing OIB-like compositions, and the other three samples overlapping with the IAB compositions. However, two of the IAB basalts contain clinopyroxene crystals with variable LREE/HREE, suggesting that the two compound may have fed these two magmatic systems, although in significantly different proportions. Geospeedometry tests show that most of the samples are lavas that carried olivine crystals with variable residence times. Samples show variable re-equilibrium periods, where the larger ranges are found for samples within the ridge’s summit (from few days to a decade). Moreover, the olivine crystals from the samples with more differentiated compositions have compositional profiles with similar zoning patterns and lower variable re-equilibrium periods, varying from 100 to 300 days, suggesting that in these relatively acid systems the magma inputs are less frequent.