Resumen de The composition of noble gas and co2 in the european subcontinental lithospheric mantle
This PhD thesis reports an integrated study of petrography and geochemistry of the minerals with noble gases and CO2 (when available) in fluid inclusions (FI) from selected mantle xenoliths erupted in Europe during alkaline magmatism.
The aim of the thesis is to show how the occurrence of partial melting, metasomatism, refertilization, magmatic degassing, volatile recycling, and mixing of volatiles modified the pristine composition of fluids stored in European SCLM and give clues on its geodynamic evolution. Mantle xenoliths from a few key regions where magmatism occurred in different temporal and geodynamic conditions were selected: Lower Silesia (Eger Rift) in SW Poland, Persani Mts. (Transylvania) in Romania, Eifel and Siebengebirge in Germany.
The main outcomes of this thesis are: - The chemistry of FI in mantle xenoliths from the studied areas revealed is dominated by CO2, with N2 as second major species. Noble gas are in trace. Oliv are systematically gas-poor respect to Px from the same xenoliths. Among Oliv from European xenoliths, those from Lower Silesia display the highest CO2 concentrations suggesting carbonated-like metasomatism, as also inferred from trace elements in cpx.
- Ne and Ar concentration and isotopic ratios indicate variable extents of contamination by atmosphere-derived fluids. This contamination is likely derived from the recycling into the mantle of atmospheric-derived material inherited by local/recent or fossil subduction. This is more evident in the xenoliths from Persani Mts., where a recent subduction occurred.
- Oliv and Px display variable extents of partial melting and metasomatism/refertilization, as indicated by the mineral chemistry and He/Ar* in FI. The highest degrees of melting (25-30%) are found in samples from Lower Silesia and Siebengebirge, which are also among the oldest within the suite of samples (>6 Ma). Evidences of low degrees of melting and/or refertilization are found in Persani Mts. and West Eifel, which are Quaternary. This suggests that refertilization process is likely occurred in Quaternary, postponing mantle melting still recorded in Lower Silesia and Siebengebirge.
- The 3He/4He corrected for air contamination (Rc/Ra) is within European SCLM range (6±1 Ra) that is lower than MORB (8±1 Ra). This indicates a widespread recycling of crustal material below Europe if compared to other continental rift (e.g., Antarctica, East African). However, the careful investigation of olivine, cpx, and opx in single localities allowed distinguishing variations of 3He/4He that are related to the local history of the mantle. In detail, Persani Mts. show the lowest 3He/4He values within the dataset that indicate highest degree of crustal recycling due to the recent subduction that plays an important role in contaminating the mantle. Instead, metasomatism involving asthenospheric MORB-like fluids well explains the highest 3He/4He values recognized in all the studied localities.
- Ne-Ar-He isotopic systematics indicates that most of the data fall along a mixing between air and a MORB mantle. The presence of a plume (lower mantle) beneath the studies localities can be excluded.
- I presented the first data of carbon isotope composition of CO2 (δ13C V-PDB) in the Lower Silesia mantle xenoliths, which suffered a metasomatism by carbonated-rich fluids. The δ13C values are about -3.9 ‰ and are within MORB range with <0.1% limestone contamination.
- The comparison of 3He/4He signature measured in mantle xenoliths from the three target areas with that of CO2-dominated gas emissions located within or nearby shows that only gases emitted along Eger Rift are representative of the local mantle signature, suggesting an active magmatic activity below this area. Instead, gases emitted in East Eifel and nearby Persani Mts. are slightly to strongly contaminated by crustal fluids during their rising toward the surface or are released from a cooling and aging magma residing within the crust.
