The primary magmatic δ34S of the Troodos Ophiolite and evidence for early and late sulfide saturation

Sulfur cycling in the vicinity of subduction zones is an important control on chalcophile element mobility and the formation of massive sulfide ore deposits such as those exposed in the Cretaceous-aged Troodos Ophiolite in Cyprus. Here, we assess the sources and fates of magmatic S in the Troodos Ophiolite using in-situ S isotope measurements of quenched volcanic glasses along with petrographic and geochemical evidence for sulfide saturation in both primitive and evolved melts. Primitive glasses (MgO ≥ 8 wt%) define the primary S isotope composition of the ophiolite: δ 34 S = +0.14 ± 0.56 ‰ (1 s.d., V-CDT, n = 12), which overlaps with mid-ocean ridge basalts and is low compared to bulk δ 34 S values measured in altered rocks and sulfide ore deposits in the ophiolite that interacted with 34 S-enriched seawater. The Troodos glasses have geochemical and isotopic evidence for slab influence, however these parameters (e.g., H 2 O, oxygen fugacity, Ba/La, 87 Sr/ 86 Sr) do not correlate with δ 34 S values, suggesting that the flux of slab-derived S was low. Magmatic pyrrhotite inclusions in Mg-rich olivine and chromite phenocrysts indicate that some primitive Troodos mantle melts were reduced and saturated with an immiscible Ni-rich sulfide liquid. During ascent, melt aggregation and decompression resulted in sulfide-undersaturation and a wide range of dissolved S contents in lavas. The S contents of lavas require a heterogeneous mantle source and cannot be explained solely by magmatic S degassing. In evolved magmas, saturation of Cu-rich sulfide accompanied exsolution of a fluid phase and Fe Ti oxide saturation, and in some cases this may have occurred during rapid cooling upon eruption at the seafloor. Collectively, these observations confirm that magmatic S in the Troodos Ophiolite was derived from a reduced, depleted and variably sulfide-saturated mantle and that slab fluids involved in flux melting were sulfur-poor. • Sulfur isotopes were measured in volcanic glasses from the Troodos Ophiolite. • Magmatic sulfur in the ophiolite was derived mostly from the pre-existing depleted mantle and not from slab fluids or melts. • We observe petrographic and geochemical evidence for sulfide saturation in Troodos mantle melts and in evolved magmas.