Sulfur from the subducted slab dominates the sulfur budget of the mantle wedge under volcanic arcs

Sulfur is of a crucial importance in the Earth system influencing biological, climate, ore-forming, and redox processes. Subduction zones play a key role in the global sulfur cycle. Arc magmas have higher sulfur contents and are more oxidised than mid-ocean ridge basalts (MORBs) due to either an oxidised mantle source or magma differentiation. Melt oxidation state and sulfur content may interrelate, as sulfur is a potential oxidising agent during slab-mantle interaction. Here, we use melt inclusions (MIs) to determine the sulfur isotopic composition (δ34S) of primary arc magmas from three volcanic centres along the Central American Volcanic Arc (CAVA): Fuego (Guatemala), Cerro Negro (Nicaragua), and Turrialba (Costa Rica). These three locations sample much of the global arc magma trace element variability: Ba/La ratios range from 22 (Turrialba) to 118 (Cerro Negro). Melt δ34S values are between -0.5‰ and +4.9‰. Sulfur contents and δ34S values of homogenised and naturally quenched MIs overlap, indicating post-entrapment processes do not affect sulfur contents and sulfur isotope ratios in the studied MIs. Degassing causes limited sulfur isotope fractionation; calculated gas-melt isotope fractionation factors are between 0.998-1.001. Our model calculations predict that most volcanic gases along the CAVA have δ34S between -1‰ and +6‰, becoming enriched in 34S as degassing progresses. We estimate initial melt δ34S values for Fuego, Cerro Negro, and Turrialba to be +0.7±1.4‰, +2.2±1.0‰, and +1.6±0.8‰ (two standard errors), respectively. All these values are elevated compared to MORBs (-0.9‰). Addition of oxidised slab material enriched in 34S to the mantle wedge can explain elevated arc primary melt δ34S and the oxidising conditions observed in arc magmas globally. Based on mass balance, a slab component with δ34S between +2‰ to +5‰ is present in the mantle wedge under the CAVA, elevating local arc mantle S contents to 360±30 ppm at Fuego, 462±11 ppm at Cerro Negro. Modelling suggests that 40-70% of sulfur in the mantle wedge originates from a slab-derived component. Slab subduction is expected to have major control on the evolution of Earth's sulfur cycle and mantle oxidation state over its geological history.