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The study of fluid inclusions (FI) composition (He, Ne, Ar, CO2) integrated with the petrography and mineral chemistry of mantle xenoliths representative of the Sub Continental Lithospheric Mantle (SCLM) is a unique opportunity for constraining its geochemical features and evaluating the processes and the evolution that modified its original composition. An additional benefit of this type of studies is the possibility of better constraining the composition of fluids rising through the crust and used for volcanic or seismic monitoring. In this respect, the volcanic areas of Eifel and Siebengebirge in Germany represent a great opportunity to test this scientific approach for three main reasons. First, these volcanic centers developed in the core of the Central European Volcanic Province where it is debated whether the continental rift was triggered by a plume (Ritter, 2007 and references therein). Second, Eifel and Siebengebirge formed in Quaternary (0.5-0.01 Ma) and Tertiary (30-6 Ma), respectively, thus spanning a wide range of age. Third, Eifel is characterized by the presence of CO2-dominated gas emissions and weak earthquakes that testify that local magmatic activity is nowadays dormant, but not ended (e.g., Bräuer et al., 2013). It is thus important to better constrain the noble gas signature expected in surface gases in case of magmatic unrest.This work focuses on the petrological and geochemical study of mantle xenoliths sampled in the West Eifel and Siebengebirge volcanic areas (Germany) and aims at enlarging the knowledge of the local SCLM. Gautheron et al. (2005) carried out the first characterization of noble gases in FI of crystals analyzed by crushing technique (as in our study) but limited to olivines and to West Eifel eruptive centers. Here, we integrate that study by analyzing olivines, orthopyroxenes and clinopyroxenes from a new suite of samples and by including two eruptive centers from Siebengebirge volcanic field (Siebengebirge and Eulenberg quarries).Xenoliths from the Siebengebirge localities are characterized by the highest Mg# for olivine, clinopyroxene and Cr# for spinel, together with the lowest Al2O3 contents for both pyroxenes, suggesting that the mantle beneath Siebengebirge experienced high degree of melt extraction (up to 30%) while metasomatic/refertilization events were more efficient in the mantle beneath West Eifel.In terms of CO2 and noble gas concentration, clinopyroxene and most of the orthopyroxene show the highest gas content, while olivine are gas-poor. The 3He/4He varies between 5.5 and 6.9 Ra. These values are comparable to previous measurements in West Eifel, mostly within the range proposed for European SCLM (6.3±0.4 Ra), and slightly below that of MORB (Mid-Ocean Ridge Basalts; 8±1Ra). The Ne and Ar isotope ratios fall along a binary mixing trend between air and MORB-like mantle. He/Ar* in FI and Mg# and Al2O3 content in minerals confirm that the mantle beneath Siebengebirge experienced the highest degree of melting, while the metasomatic/refertilization events largely affected the Eifel area.ReferencesBräuer, K., et al. 2013. Chem. Geol. 356, 193–208.Gautheron, C., et al. 2005. Chem. Geol. 217, 97–112.Ritter, J.R.R., 2007. In: Ritter, J.R.R., Christensen, U.R. (Eds.), Mantle Plumes: A Multidisciplinary Approach. Springer-Verlag, Berlin Heidelberg, pp. 379–404.

The appearance of alkali- and volatile-rich melts often marks the opening of major magmatic cycles, always reflecting the partial melting of heterogeneously enriched mantle domains. In these cases the study of highly alkaline, H2O-CO2-rich magmatic pulses provide important insights on the composition and behavior of the sub-continental lithospheric mantle (SCLM) prior to rift initiation. The camptonitic dykes cropping out at Predazzo (Dolomitic Area, NE Italy) are among the oldest examples of lamprophyric rocks in Italy, and were historically related to the orogenic-like Middle Triassic magmatism of the Southern Alps. A detailed petrological, geochemical and geochronological characterization of these rocks was developed to frame them inside the articulated geodynamic evolution of the Southern Alps domain during Triassic. Whole-rock and mineral phase geochemistry, together with 40Ar/39Ar data showed that Predazzo lamprophyres represent an alkaline-carbonatitic magmatic event temporally isolated (~220 Ma) from the major Ladinian orogenic-like magmatism of the Southern Alps (~238 Ma). Lamprophyres can thus be attributed to the volumetrically limited alkaline magmatic phase that infiltrated several portions of the Southern Alps lithosphere between 225 and 190 Ma. Partial melting models and Sr-Nd isotopes demonstrate that Predazzo lamprophyres were produced by low partial melting degree of a garnet-amphibole-bearing mantle source interacting with a significant asthenospheric contribution. In the light of these new findings, they are interpreted as the geochemical/geochronological bridge between the orogenic-like Ladinian magmatism and the rifting phase related to the opening of the Alpine Tethys. This study highlights the paramount importance of alkaline magmas for tracking the volatiles cycle in the SCLM and the potential lithosphere-asthenosphere interactions during large-scale geodynamic processes.