Reconstructing the plumbing system of Marsili seamount (southern Tyrrhenian Sea): a combined petro-chemical and experimental petrology approach

This PhD thesis investigates the magmatic processes and architecture of the trans-crustal plumbing system beneath the Marsili volcano (MV), a submarine volcano located in the southern Tyrrhenian back-arc basin. The MV represents not only a key site to explore magma generation and differentiation mechanisms in oceanic back-arc settings, but also a relevant case study for assessing the tsunami hazards that is related to the volcanic activity of this submarine volcano. To provide a comprehensive view of the behavior of the Marsili plumbing system, a multidisciplinary petro-chemical and experimental approach was adopted, integrating the whole-rock and mineral chemistry, with the textural analysis of volcanic products, thermobarometic analysis based on mineral-melt equilibria, and phase equilibria investigated through high pressure-high temperature experiments. Lava samples, representative of the three different sectors of the volcanic edifice (northern, axial, and lateral sectors) were examined, and their crystal cargo was exploited to extract information on the spatial and temporal evolution of magma storage and differentiation throughout the trans-crustal plumbing system. Particular attention was devoted to the study of glomerocrysts, aggregates of minerals representing fragments of a magmatic crystal mush that are periodically remobilized and entrained by magmas ascending towards the surface. Two distinct mineralogical associations of glomerocrysts, one composed by olivine + plagioclase, and one composed by olivine + clinopyroxene + plagioclase, permit to identify two chemically distinct parental magmas, respectively a low-Ca and a high-Ca basalt. These two basaltic magmas likely derived from partial melting of a heterogeneous mantle source. The petrologic diversity of glomerocrysts contained in basaltic lavas collected from all three sectors of MV provides further evidence of a spatially heterogeneous, mush-dominated plumbing system extending from the lithospheric mantle to the upper crust and characterized by physical and chemical interactions between crystal mush and ascending basaltic magmas. Collectively, these observations support the model of a trans-crustal mush system for MV, where crystal accumulation, rejuvenation and recycling determine the overall evolution of the magmatic system. The crystallization conditions of the most primitive basaltic magma erupted from the northern sector is constrained by phase equilibria determined experimentally and showing the best correspondence between natural and experimental mineral assemblages and composition under conditions of 200–300 MPa, 1150–1175 °C, 2–3 wt% dissolved H2O, and fO2 near the NNO buffer. These results confirmed that differentiation of basaltic magmas mostly occurred at lower- to mid-crustal depths (~7–10 km depth) and that relatively large degree of crystallization are required to produce andesitic magma compositions erupted in the axial sector of the volcano, in accord with mass balance modeling and the petrographic evidence derived from the analysis of the glomerocrysts. Importantly, the results of this study increase the knowledge on how new oceanic crust forms at an active oceanic back-arc spreading center providing, for the first time, a framework for crustal accretion that can represent a model for back-arc basins elsewhere.