Sulfur isotopic signature of subduction-unrelated and subduction-related ophiolitic rocks

Sulfur (S) is one of the key volatiles in Earth’s chemical cycles as it affects biological, climate, ore-deposits, and redox processes. It is known that S stored in the crust is recycled into the mantle at subduction zones. However, some aspects of the S cycle in the deep Earth such as S speciation, flux, isotope composition and fractionation processes still remain unclear. The study of ophiolites could provide information about contents and isotopic features of S in subduction-unrelated and subduction-related geodynamic settings. In this work we compiled a global dataset of both subduction-unrelated and subduction-related ophiolitic basalts, and we measured their whole rock S contents and the relative S isotopic ratio (34S/32S) using an elemental analyzer coupled with a mass spectrometer (EA-IRMS). The considered samples are Mid-Ocean Ridge Basalts (MORBs) from Corsica, Romania, Albania, and North Macedonia; ii) Island Arc Tholeiites (IAT) from Albania and Greece; iii) Calc-Alkaline Basalts (CAB) from Greece, Romania, North Macedonia, and Iran already constrained from a petrological and geochemical point of view by different studies (Moberly et al., 2006; Saccani et al., 2011; Brombin et al., 2022). In the studied basalts, the S contents range from 200 and 300 ppm. Despite the different areas of provenance, for most of the samples the S isotopic signatures are similar in rocks having similar geochemical affinity. The average S isotopic ratios are –0.7‰, +5.8, and +7.4‰, for MORBs, IATs, and CABs, respectively. It is evident that only MORBs preserved the typical S signature of the Earth mantle (i.e., from –2‰ to 0‰). The subduction related magmatic rocks (i.e., IATs and CABs) show positive S isotopic values, probably due to the contamination of i) enriched-34S subducting sediments in the magma sources or ii) fluids released by serpentinized rocks of the slab, which typically have comparatively more positive S signature. In summary, this work allowed the definition of: i) the S isotope compositions in both subduction-unrelated and subduction-related magmatic rocks; ii) the possible causes which modify the original S signature (e.g., contamination by subducting sediments). Research like this are therefore essential to unravel the global S cycle. REFERENCES Dilek Y., Furnes H., 2014. Ophiolites and Their origins. Elements, 10: 93-100. Moberly, R., Ishii, T., Garcia, M.O., Ross, K., Artita, K., 2006. Enriched, transitional, and normal mid-ocean-ridge basalt glass, ODP Leg 203. In Schultz, A., Orcutt, J.A., and Davies, T.A. (Eds.), Proc. ODP, Sci. Results, 203, 1–36 Saccani E., Beccaluva L., Photiades A., Zeda O., 2011. Petrogenesis and tectono-magmatic significance of basalts and mantle peridotites from the Albanian–Greek ophiolites and subophiolitic mélanges. New constraints for the Triassic–Jurassic evolution of the Neo-Tethys in the Dinaride sector. Lithos, 124: 227-242. Brombin V., Barbero E., Saccani E., Precisvalle N., Lepitkova S., Milevski I., Ristovski I., Milcov I., Dimov G., Bianchini G., 2022. Subduction signature of the Vardar ophiolite of North Macedonia: new constraints from geochemical and stable isotope data. Ofioliti, 47: 85-102.