| Summary: | 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 (δ
34S) 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
δ
34S values are between -0.5h and +4.9h. Sulfur contents and δ
34S 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 δ
34S between -1h and +6h, becoming enriched in 34S
as degassing progresses. We estimate initial melt δ
34S values for Fuego, Cerro Negro, and Turrialba
to be +0.7±1.4h, +2.2±1.0h, and +1.6±0.8h (two standard errors), respectively. All these values
are elevated compared to MORBs (-0.9h). Addition of oxidised slab material enriched in 34S to the
mantle wedge can explain elevated arc primary melt δ
34S and the oxidising conditions observed in
arc magmas globally. Based on mass balance, a slab component with δ
34S between +2h to +5h 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.
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