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Abstract Aiming at understanding the source of the fluids that mineralizing within seismically active fault zones, we investigate the noble gas isotopes (i.e., helium (He), neon (Ne), and argon (Ar)) in the fluid inclusions (FIs) trapped in the calcite veins sampled along high‐angle fault zones of the Contursi hydrothermal basin, southern Italy. The latter basin lies in close vicinity of the MW = 6.9, 1980 Irpinia earthquake and exposes numerous fault scarps dissecting Mesozoic shallow‐water carbonates. The analyses of noble gases (He, Ne, Ar) are conducted to identify the origin of the volatiles circulating along the faults at the time of calcite precipitation. Then, outcomes of this discussions are compared with currently outgassing of deep‐sourced CO2 coupled to mantle‐derived He in that area, whose output is larger than those from some volcanic areas worldwide. The results indicate that He in FIs is dominated by a crustal radiogenic component (4He), and by an up to 20% of a mantle‐derived component (3He), with a highest isotopic signature of 1.38 Ra. This value is consistent with the highest percentage of mantle‐derived He associated to high‐flux CO2 gas emission in the investigated area (1.41 Ra). We propose that the variability of the He isotopic signature measured in primary FIs can result from early trapping of fluid inclusions or post trapping processes and seismic activity that modify the pristine He isotopic signature (i.e., derived from the crust and/or mantle) in groundwater along the faults during periods of background seismicity. Such investigations are fundamental to understand fluid migration in fault systems and the role of fluids in processes of earthquake nucleation.

Among the volcaniclastic products of melilitite–carbonatite eruptions, pelletal lapilli are often found, resulting in them being particularly useful for characterising the interface between the erupting magma and its volatile component. Pelletal lapilli, which were erupted during the most recent melilitite–carbonatite volcanic activity of the Mt. Vulture volcano, are characterised by a predominantly wehrlitic core with CO2-rich fluid inclusions and a Ca-rich outer portion composed of fine-grained xenocrystic debris of olivine and clinopyroxene, with microcrysts of haüyne and melilite laths (± calcite). The chemical composition of the olivine reflects the interaction with a proto-melilitite–carbonatite melt, which is the main metasomatic agent. The whole-rock analyses of the external portion of pelletal lapilli show values that are comparable with those of extrusive carbonatites. This evidence supports the hypothesis that the primary carbonatite melt was a significant contributor to the CO2-rich magma source that transported the lapilli to the surface. The modelling of the geometric data of the pelletal lapilli structure, together with inferences regarding the role of the CO2 gas phase, the main propellant in an ascending gas-dominated medium, allowed for the reconstruction of a possible scenario where the CO2 expansion and the fluidised spray granulation process are crucial during the volcanic conduit dynamics.

El estudio petrográfico de los minerales de calcita en los diques carbonatíticos del Complejo Basal de Fuerteventura (Ajuy, Punta de la Nao) muestra la presencia de inclusiones fluidas primarias aisladas, corroborando su origen magmático más que un origen tardío. Los estudios microtermométricos en calcita muestran un intervalo de temperatura eutéctica aparente (Te*= -21.7 – -21.0 °C) y de fusión final del hielo (Tim= -3.2 – -2.1. °C) muy restringido. Las temperaturas y las presiones de homogenización están en un intervalo de 140-210 °C y 4-16 bares respectivamente. Nuestros resultados sugieren el sistema binario NaCl-H2O como sistema químico que caracteriza las inclusiones fluidas estudiadas, con la presencia de flúidos de baja salinidad (≈ 5 % en peso NaCleq), probablemente relacionados con interacciones flúido/roca.

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).