Unravelling the deep sulfur cycle: isotopic signatures of 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, and isotope composition and fractionation processes still remain unclear. Most of the S isotopic studies investigate the melt inclusions, which potentially preserve the original budget and isotopic signature of the magma. However, these researches are limited, as melt inclusions are rare. Studying ophiolites represent a valid alternative to investigate contents and isotopic features of S with the aim to reconstruct its cycle in different geodynamic settings. Ophiolites are fragments of ancient oceanic lithosphere that were tectonically emplaced into orogenic belts and, according to the Dilek and Furnes (2014) classification, they can be discriminated as subduction-unrelated and subduction-related magmatic rocks. 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) through 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). This research is therefore essential to understand the global S cycle.

 

References

Dilek Y., Furnes H., 2014. Elements, 10: 93-100.

Moberly R., et al., 2006. Proc. ODP, Sci. Results, 203: 1-36.

Saccani E., et al., 2011. Lithos, 124: 227-242.

Brombin V., et al., 2022. Ofioliti, 47: 85-102.