Fluids composition in the mantle beneath the Eastern Transylvanian Basin inferred from mineral chemistry and noble gases in fluid inclusions of ultramafic xenoliths

The investigation of noble gases (He, Ne, Ar) and CO₂ in fluid inclusions (FI) of mantle-derived rocks from the Sub Continental Lithospheric Mantle (SCLM) is crucial for constraining its geochemical features and evolution as well as the volatiles cycle, and for better evaluating the information arising from the study and monitoring of volcanic and geothermal gases. Eastern Transylvanian Basin in Romania is one of the places in Central-Eastern Europe where mantle xenoliths are brought to the surface by alkaline magmatism, offering the opportunity for applying the above-mentioned approach. Moreover, this locality is one of the few places on Earth where alkaline eruptions occurred contemporaneously with calc-alkaline activity, thus being a promising area for the investigation of subduction influence on the magma sources and volatiles composition.

In this work, we studied petrography, mineral chemistry and noble gases in FI of mantle xenoliths found in Perşani Mts. alkaline volcanic products. Our findings reveal that the local mantle recorded two main events. The first was a pervasive, complete re-fertilization of a previously depleted mantle by a calc-alkaline subduction-related melt, causing the formation of very fertile, amphibole-bearing lithotypes. Fluids involved in this process and trapped in olivine, opx and cpx, show ⁴He/⁴⁰Ar* ratios up to 1.2 and among the most radiogenic ³He/⁴He values of the European mantle (5.8 ± 0.2 Ra), reflecting the recycling of crustal material in the local lithosphere. The second event is related to a later interaction with an alkaline metasomatic agent similar to the host basalts, that caused slight LREE enrichment in pyroxenes and crystallization of disseminated amphiboles, with FI showing ⁴He/⁴⁰Ar* and ³He/⁴He values up to 2.5 and 6.6 Ra, respectively, more typical of magmatic fluids.

Although volcanic activity in the Perşani Mts. is now extinct, strong CO₂ degassing (8.7 × 10³ t/y) in the neighbouring Ciomadul volcanic area may indicate that magma is still present at depth (Kis et al., 2017; Laumonier et al., 2019). The gas manifestations present from Ciomadul area are the closest to the outcrops containing mantle xenoliths for comparison of the noble gas composition in FI. ³He/⁴He values from Stinky Cave (Puturosul), Doboşeni and Balvanyos are up to 3.2, 4.4 and 4.5 Ra, respectively, indicating the presence of a cooling magma (Vaselli et al., 2002 and references therein). In the same area and more recently, Kis et al. (2019) measured ³He/⁴He ratios up to 3.1 Ra, arguing that these values indicate a mantle lithosphere strongly contaminated by subduction-related fluids and post-metasomatic ingrowth of radiogenic ⁴He. Our findings consider more likely that magmatic gases from Ciomadul volcano are not representative of the local mantle but are being released from a cooling and aging magma that resides within the crust. Alternatively, crustal fluids contaminate magmatic gases while they are rising to the surface.

 

Kis et al. (2017). Journal of Volcanology and Geothermal Research 341, 119–130.

Kis et al. (2019) Geochem. Geophys. Geosyst. 20, 3019-3043.

Laumonier et al. (2019) Earth and Planetary Science Letters, 521, 79-90.

Vaselli et al. (2002) Chemical Geology 182, 637–654.