Sulfate-dominant apatite in the deep arc crust indicates that high oxidation state promotes copper fluxing in arcs

Abstract Sulfide saturation during arc magma evolution depletes ascending magmas in chalcophile elements (e.g., Cu) and promotes metal recycling into the mantle. Although sulfide saturation is posited to be delayed by high oxygen fugacity (ƒO2) in the deep crust of arcs, the timing and controls on sulfide saturation in arcs remain disputed. Here, we report new constraints on S speciation in the deep arc crust using measurements of the relative abundance of S species (S2–, S4+, and S6+) in apatite in deep crustal arc cumulates from the Sierra Valle Fértil–La Huerta (Argentina). Sulfide is rare in primitive cumulates (<0.01%), only becoming abundant in evolved cumulates (Mg# <0.65), indicating that sulfide saturation was delayed in the magmatic evolution. Apatite yields consistently high S6+/ΣS (0.8–1.0), giving an fO2 of 1.4 ± 0.4 log units above the fayalite-magnetite-quartz (FMQ) redox buffer. Delayed sulfide saturation was therefore the result of the parental magma having elevated fO2 acquired from its mantle source. We use modeling to show that magma differentiation at FMQ+1.4 would be sufficient to delay sulfide saturation and produce mafic to intermediate melts with elevated Cu concentrations. Oxidized, hydrous intermediate magmas may therefore play a key role in delivering elevated sulfur and Cu fluxes to magmatic-hydrothermal systems and the atmosphere, contributing to the Cu deficit of modern continental crust.