The carbon cycle in the mantle below intra-continental rift settings

The investigation of mantle-derived products coming from Sub Continental Lithospheric Mantle (SCLM) is crucial for understanding its geochemical features and evolution, the mantle-crust interaction, and the volatiles composition. In this respect, mineral-hosted fluid inclusions (FI) in mantle xenoliths play a fundamental role, as their composition provides useful insights about the extent and timing of mobilization of volatiles during melt extraction and melt/fluid-rock reactions in the mantle, especially when their composition is combined with the information extracted from mineral chemistry and texture.

Peridotite xenoliths sampled by the intra-continental rift magmatism at West Eifel (Germany) and northern Victoria Land (Antarctica) are extremely rich in FI, and bear witness to multiple metasomatic modifications taking place in the local SCLM (Rizzo et al. 2021; Casetta et al. 2022). In this study, the concentration and isotopic signature of CO₂ in mineral-hosted FI in peridotite rocks was coupled to mineral chemistry and thermo-oxy-barometric modelling, with the aim of exploring if, and how, the provenance and mobilization of C-bearing species are related to the main melt extraction and metasomatic processes that took place in the local SCLM domains or to the recycling into the mantle of old crustal material. Our findings show that the concentration of CO₂ in FI varies from 0 up to 162 µg/g, being higher in West Eifel than in Antarctica samples, and also higher in pyroxenes- than in olivine-hosted inclusions. A correlation between the CO₂ content in FI and the Mg#, Al₂O₃   and TiO₂ concentrations in mineral phases is observed. The δ¹³C ratio of CO₂ in pyroxene-hosted FI spans a wide range, from typical mantle values of -6‰ to -4‰ in peridotites from Antarctica up to higher values (-2‰ to +2‰) in peridotites from West Eifel that overlap the range of carbonates. Interestingly, a clear correlation between the δ¹³C ratio of the FI and the Al₂O₃ concentration of their host pyroxenes is displayed by all xenoliths, indicating that the signature of fluids is related to the chemical evolution of the host mineral phases. Consistently, the δ¹³C ratio is positively correlated to the temperature recorded by both olivine-spinel and orthopyroxene-clinopyroxene pairs (T = 850-1200°C) in xenoliths from both localities.

Besides potentially widening the range of δ¹³C ratios of mantle-derived products, our results confirm that coupling the chemistry of FI to that of the host mineral phases in mantle peridotites is one of the best ways to explore the cause and effects of the melt/fluid-rock reactions taking place in the SCLM.

REFERENCES

Casetta, F., Rizzo, A. L., Faccini, B., Ntaflos, T., Abart, R., Lanzafame, G., ... & Coltorti, M. (2022). CO2 storage in the Antarctica Sub-Continental Lithospheric Mantle as revealed by intra-and inter-granular fluids. Lithos, 416, 106643.

 Rizzo, A. L., Faccini, B., Casetta, F., Faccincani, L., Ntaflos, T., Italiano, F., & Coltorti, M. (2021). Melting and metasomatism in West Eifel and Siebengebirge Sub-Continental Lithospheric Mantle: Evidence from concentrations of volatiles in fluid inclusions and petrology of ultramafic xenoliths. Chemical Geology, 581, 120400.