Petrology and geochemistry of peri-Mediterranean carbonatite magmatism: case studies from Fuerteventura (Canary Islands) and Mt. Vulture volcano (southern Italy)

Carbonatites are rare magmatic rocks of great scientific and economic importance, and carbonatite magmatism is mainly associated with intraplate continental tectonic settings, with some rare occurrences in oceanic contexts. Despite their importance, many aspects of carbonatite petrogenesis and evolution processes remain still poorly constrained. In order to further constrain the mantle source and the storage system of carbonatite magmas, Fuerteventura (Canary Islands) was taken as a representative case study of oceanic carbonatites, while Mt. Vulture (southern Italy) was taken as a representative case study of intra-continental carbonatites, with a two-fold aim: to understand (i) the role of the carbonatite primary melts in metasomatizing the mantle source in different geodynamic settings with possible implications in terms of volcanic hazard, and (ii) the role of the infiltrating fluids in the transport and concentration of Rare Earth Elements (REEs) with the processes involved in the carbonatite-related hydrothermal mineralization. The first noble gases study on intrusive oceanic Ca-carbonatites from Fuerteventura reflects a sub-continental lithospheric mantle (SCLM) signature in their petrogenesis, corroborating that, also in the rare context of oceanic lithosphere, a contribute of a sub-continental lithosphere is needed. As regards the REEs characterization, detailed petrographic and micro-thermometric studies on the same carbonatites show how REEs can be mobilised locally by low-temperature hydrothermal fluids with a process known as autometasomatism (for intrusive carbonatites). At sub-solidus temperatures (T > 600 °C) and the brine-melt stage (600 ≤ T ≤ 400 °C), where REEs are sufficiently concentrated in the residual brine-melt to form REE-minerals, the infiltrating fluids play an important role in the transport and concentration of REEs, as testified by the presence of REEs-rich filled microfractures in accessory minerals. As regards the Mt. Vulture case study, a suite of several pelletal lapilli (enclosing ultramafic mantle xenoliths), mantle xenoliths and loose olivine and clinopyroxene xenocrysts, brought to the surface by the last melilitite-carbonatite explosive volcano activity, was characterized. The melilitite-carbonatite matrix and carbonatite-rich layers within the matrix in the ash-rich tuff deposits, show whole rock compositions comparable with those of the average values of extrusive carbonatites, suggesting a possible contribution of a carbonatite melt in the trace elements enrichment processes. Furthermore, petrographic evidences of wehrlitization processes reflects the direct evidence of carbonatite metasomatism beneath the Mt. Vulture. Detailed petrographic, micro-thermometric, and geothermometric studies provide insights into the P-T history of the mineral-melt-fluid interaction processes in the mantle and within the Mt. Vulture magma storage system, identifying evidences of wehrlitization processes and two different magma ponding stages at the local crust-mantle boundary and at a shallower crustal level. Calculations on magma dynamics show how the ascent rates of carbonatite magmas can be comparable with kimberlite magmas, with the important role of a pure CO2 gas phase as principal propellent in an upward moving elutriated gas-dominated medium. High calculated oxygen fugacity supports the geochemical evidence that Mt. Vulture xenoliths would have formed by interaction with an oxidized CO2-rich metasomatic fluid, according to the presence of pure CO2 fluid inclusions within the mantle xenoliths-forming minerals.