Solving the sulfur isotope discrepancy in Central America

Abstract

Sulfur has considerable leverage on the redox budget of subducted materials due to its presence as both sulfide (S²⁻) and sulfate (S⁶⁺). Recent work has revealed a discrepancy: arc magmas have positive δ³⁴S values, while much of the S entering subduction zones should be hosted in pyrite, which is likely to have negative sulfur isotopic values. To address this discrepancy, we focus on the Central American subduction zone, where previous work has revealed positive δ³⁴S values in parental arc magmas. We report the first comprehensive study of the sedimentary S input to any subduction zone with S concentration and sulfur-isotopic measurements of ODP Site 1040 sediments. Sedimentary S input for Central America is greater than previously thought (1.71 ± 0.38 × 10⁸ mol S/yr/100 km), although insufficient to supply the S output from the arc without contributions from subducting oceanic crust. Furthermore, ~90 % of the sedimentary S input is hosted in pyrite, leading to a bulk sedimentary δ³⁴S of −19.5 ‰. In assessing the mass balance for Central America, we find that selective removal of sulfate does not provide enough S to support arc output and thus, additional S must be mobilized by the oxidation of pyrite with several per mil rock-fluid δ³⁴S fractionation to positive values. Our results agree with existing evidence that subduction zones act as efficient redox reactors, with oxidized portions of the slab (e.g., altered oceanic crust) providing the oxidizing power to supply sulfate to the arc, while a reduced and ³²S-enriched complement sinks into the deeper mantle.