Insights into S recycling in the mantle from high-precision isotope analysis of pyroxenite-hosted sulfides

Large-scale tectonic processes introduce a variety of crustal lithologies into the Earth’s mantle, leading to increasing mantle heterogeneity over time. During subduction in arc environments, recycled mantle pyroxenites may retain pristine geochemical and isotopic signatures that reflect their crustal evolution. Therefore, pyroxenites can be considered representatives of the materials transferred from deeply subducted slabs to the mantle sources of oceanic basalts, providing insights into the recycling of sulfur into the deep Earth interior. However, no S isotopic data are available in literature for recycled pyroxenite-hosted sulfides. Here, we focus on garnet clinopyroxenites from the External Ligurian (EL) mantle sequences (N Apennines, Italy) and on selected samples from the world-famous orogenic massifs of Ronda and Beni Bousera (Betic-Rif Belt, Spain and Morocco) with the aim of integrating new sulfur isotope data into the existing pyroxenite petrogenetic models, shedding light on mechanisms of sulfur geochemical cycling in the subduction factory. Previous studies, based on major and trace elements and Nd-Hf-O isotope systematics, have shown that the EL pyroxenites derived from MORB-type gabbroic precursors, which underwent a long-term evolution into the mantle (> 1 Ga). The Ronda and Beni Bousera massifs display significant pyroxenite diversity, including the occurrence of UHP (>4.5 GPa) garnet clinopyroxenites with graphitized diamond pseudomorphs and corundum-bearing garnet clinopyroxenites. For these rocks, a wide range of elemental and Sr-Nd-Hf-Pb-O isotopic signatures have been documented. This heterogeneity has been ascribed to an origin from continental lower crust (Varas Reus et al., 2018), or from oceanic crustal protoliths that experienced variable degrees of seafloor hydrothermal alteration (Pearson et al., 1993) and mixing with pelagic sediments (Lorand et al., 2021). Our study targets the sulfur isotopic composition of primary Fe-Ni-Cu sulfides (BMS) from these localities to provide new constraints on their protoliths and sulfur sources. In situ measurements of δ34S have been carried out by SIMS (Secondary-ion mass spectrometry). The sulfide assemblage in the EL garnet clinopyroxenites consists of Ni-free pyrrhotite, pentlandite, and chalcopyrite reflecting subsolidus exsolution from high-temperature Fe–Ni–Cu MSS. Their sulfur isotope compositions range from δ34S = - 2.84 to + 0.83 ‰ compared to VCDT. A garnet clinopyroxenite from Ronda, with a sulfide assemblage of chalcopyrite and pentlandite+pyrrhotite intergrowths, yields a larger interval of prevailing negative δ34S values, ranging from -3.59 to +0.59. Conversely, Cu-Ni sulfides from a Beni Bousera corundum-bearing garnet pyroxenite have δ34S shifted towards positive values (from -0.24 to + 4.80). Both the Ronda and Beni Bousera corundum-bearing pyroxenites belong to the Group A as defined by Varas Reus et al. (2018) based on radiogenic isotopes. They were interpreted as ancient recycled oceanic gabbros, possibly mixed with low amounts of pelagic sediments. Whereas the sulfur isotope signature of the EL garnet clinopyroxenites overlaps depleted mantle values (−1.4 ±0.5, Labidi et al., 2012), in agreement with the lack of a oxygen isotope signature distinctive of hydrothermal alteration (Montanini et al., 2012), the entire range of δ34S from the Betic-Rif Belt significantly deviates from depleted mantle values, possibly reflecting preservation of the S signature of altered oceanic crust. The sulfides of a diamond-facies Beni Bousera garnet clinopyroxenite (Group-B of Varas Reus et al., 2018) consist of troilite with negative δ34S ranging from – 6.03 to -1.48. Desulfidation of a sedimentary pyrite component during prograde metamorphism of subducted oceanic crust under strongly reducing conditions was proposed by Lorand et al. (2021) for the origin of troilite in these samples. The recorded δ34S signature is permissive (although not exclusive) of such a derivation. 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