COHN fluid inclusions in garnet clinopyroxenite of Góry Sowie (SW Poland)

Subduction is an efficient process that allows the transfer of elements and interaction between the crust and the mantle. During subduction elements concentrated in the crust, such as volatiles (e.g., H₂O, CO₂, Cl, F, S and N₂) and other incompatible elements, can be released in melts and fluids that then interact with the overlying mantle. The most direct way to investigate crust-mantle interaction is to target melts and fluids responsible for the elements transfer. In this contribution, we report the occurrence of micrometric volatiles-bearing fluid inclusions in clinopyroxene and garnet of a mantle body in Góry Sowie (SW Poland).

In Góry Sowie, migmatitic gneisses with subordinate granulites host bodies of garnet peridotites and metabasites (Kryza and Pin, 2002). The garnet clinopyroxenite hosting the inclusions is associated with other ultramafic rocks and it mainly contains garnet, clinopyroxene, amphibole, and locally orthopyroxene porphyroblasts in a fine-grained matrix.

The inclusions in clinopyroxene are randomly distributed in the inner part of the crystal, thus they are primary, and they were trapped while the host was growing in the presence of a fluid phase. The main phase assemblage in the inclusions was determined with micro-Raman spectroscopy and it includes two carbonates (dolomite and calcite), N₂, cristobalite, CH_4, and pyrophyllite. In garnet, the inclusions are primary/pseudosecondary (i.e., distributed along fractures occurring during garnet growth) and they contain CO₂, dolomite, pyrophyllite, N_2, and CH₄. The presence of carbonates, OH-bearing phases, and N₂ suggests that the trapped fluid is COHN; hence the garnet clinopyroxenite formed in the presence of such fluid.

Further studies will allow us to better constrain the nature of the fluid and quantify the concentration of carbon in the different C-bearing phases and N₂. COH(N) fluids in subduction zones can interact with the mantle metasomatizing it and our data will help to better constrain their importance for volatiles mobilization and transfer to the mantle during subduction.

This research is part of the project No. 2021/43/P/ST10/03202 co-funded by the National Science Centre of Poland and the European Union Framework Programme for Research and Innovation Horizon 2020 under the Marie Skłodowska-Curie grant agreement No. 945339.

Kryza and Pin (2002). Mafic rocks in a deep-crustal segment of the Variscides (the Góry Sowie, SW Poland): evidence for crustal contamination in an extensional setting. Int J Earth Sci (Geol Rundsch), 91, 1017-1029.