Contribution of the subducting slab on sulfur recycling: Insight from sulfur isotopic composition of lower crustal arc cumulates

Sulfur is an important volatile and its cycle is of great significance to the evolution of life. The role of the subducting slab as a key component in the sulfur cycle remains a topic of ongoing debate. Here, we report whole-rock sulfur isotope and chalcophile element content of lower crustal cumulates from the Kohistan complete arc section in order to investigate this cycle in terms of the subduction products, i.e. arc magmas. Results show that the Kohistan garnet-free pyroxenites have heavy sulfur isotopic compositions with δ34S values of +11.84 to +15.24‰, while the garnet-bearing ultramafic cumulates show lighter isotopic signatures with δ34S values of +3.08 to +6.35‰. The occurrence of well-preserved primitive magmatic sulfide inclusions dominated by pyrrhotite in clinopyroxene suggests that the heavy δ34S values of clinopyroxenite and websterite were inherited from the magma source. The sub-arc mantle source is thought to be modified by the 34S-rich fluid released by the subducting slab. Although the garnet-bearing ultramafic cumulates have higher Cu concentrations (up to ∼530 ppm), most of the sulfides are decomposed and distributed along mineral boundaries and fractures through interaction with CO2-rich metamorphic fluid before and/or during the crystallization of garnet. The sulfur isotopic compositions of the garnet-bearing cumulate can be explained by a simple Rayleigh fractionation between metamorphic fluid and sulfide. Our study identifies the heaviest sulfur isotopes (δ34S up to +15‰) of primitive arc magma with a perspective of lower crustal cumulate. These findings suggest that the subducting slab-derived sulfur recycles into the mantle wedge, highlighting the critical role of subduction zone in the global volatile cycle. •Kohistan pyroxenites possess significantly heavy sulfur isotopic compositions;•Slab-derived sulfur enters the sub-arc mantle resulting in the heavy sulfur isotopes;•Contribution of the subducting slab on sulfur recycling is confirmed in the terms of lower crustal cumulates.