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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.

Los diques carbonatíticos que afloran en la parte centro-occidental del Complejo Basal de Fuerteventura (Ajuy, Punta de la Nao), fueron afectados por el evento intrusivo mioceno responsable del metamorfismo de contacto y fusión parcial de parte del complejo alcalino-carbonatítico. Las características texturales, los procesos de formación de monacita enriquecida en LREE (La, Ce, Nd) en microfracturas que afectan a los cristales de zircón, evidencian interacciones de las carbonatitas con fluidos hidrotermales y removilización parcial de REE. El pirocloro es el principal mineral reservorio de tierras raras (suma REE=216694 ppm), con valores elevados de LREE (LaN=185816 y CeN=165209) respecto a las HREE (YbN=2589 y LuN=1814) con un espectro bastante inclinado (La/Yb)N=72 en el diagrama de abundancia de REE. La paragénesis mineral representada por los interestratificados illita/esmectita, clorita y vermiculita permite encuadrar el evento hidrotermal en facies de las ceolitas (Ta ≈ 100-200 °C), Carbonatitic dykes that crop out in the central western part of Fuerteventura Basal Complex (Ajuy, Punta de la Nao), were affected by the Miocene intrusive event responsible for contact metamorphism and partial melting of part of the alkaline-carbonatitic complex. Textural characteristics, monazite enriched in LREE (La, Ce, Nd) formation processes in microfractures that affect zircon crystals, show carbonatite interactions with hydrothermal fluids and partial REE remobilization. Pyrochlore is the main mineral carrier of rare earth elements (sum REE=216694 ppm), with high LREE (LaN=185816 y CeN=165209) with respect to HREE (YbN=2589 y LuN=1814) with a rather steep pattern (La/Yb)N=72 in REE diagram. The mineral paragenesis represented by the interstratified illite/smectite, chlorite and vermiculite allows to frame the hydrothermal event in zeolite facies (T° ≈ 100-200 °C)

Carbonatites complexes are very rare in oceanic environments and in the Atlantic Ocean they can be only found at Cape Verde islands and Fuerteventura. Fuerteventura is the second largest island in the Canary Archipelago and is located on a transitional, continental to oceanic, crust. It consists essentially of Mesozoic sediments, submarine volcanic rocks, subaerial basaltic and trachytic series, ultramafic, mafic to felsic intrusives (clinopyroxenites, melteijites-ijolites, nepheline-monzogabbros, nepheline-sienites) and carbonatitic dike swarms (age 25 Ma). Carbonatite dike (Ca-carbonatites) mineralogy consists of calcite, aegirine-augite, albite, K-feldspar, biotite, apatite, Fe-Ti oxides and accessory minerals, such as zircon, barite, monazite and pyrochlore. The degree of alteration of carbonatites (evaluated by DTA and XRD) is generally low, with the occurrence of illite-montmorillonite mixed layers, vermiculite and chlorite. Whole rock samples (XRF) are high in CaO (> 50 %) and SrO (> 2-3 %) and very low in MgO (< 1-2 %). Trace elements were determined by LA-ICP-MS on calcite and accessory minerals (pyrochlore, monazite, zircon). Results show in calcite phenocrysts REE patterns highly enriched in all REEs (sum REE= 1186-2943 ppm) and particularly in LREE: LaN = 2178 (chondrite normalized), CeN = 1499, with respect to HREE (YbN = 37, LuN = 34) and rather fractionated pattern (La/Yb)N = 58. Small negative Eu anomalies do also occur (Eu/Eu* = 0.77 - 0.94) and these are coupled with high Sr anomalies (Sr/Sr* = 28.3 - 58.7). REE in pyrochlore are extremely enriched in REE (sum REE= 12.8 wt %) with very high LREE (LaN = 71214 and CeN = 62803) and HREE (YbN = 997, LuN = 695) with a slightly steeper pattern if compared to calcite: (La/Yb)N = 71. No significative Eu anomalies were found (Eu/Eu* = 1.06), while there is a conisistent Sr positive anomaly in spiderdiagrams (Sr/Sr* = 1.74). Preliminar results show that pyrochlore is the main mineral repository of REEs and its occurrence, even in trace amounts, gives the fingerprint to trace element to whole rock.