Sediment-derived deep forearc fluid caused coupled sulfur-Fe3+-enrichment in eclogite from the subducted Farallon plate

• Sr-Nd-S-O isotopes for pyrite-bearing UHP eclogite and omphacitite xenoliths. • Xenoliths sample the Farallon oceanic crust shallowly subducted beneath NE Arizona. • Coupled omphacite-pyrite addition traced to pulsed sediment-derived fluid ingress. • Heavy S in pyrite and increase in bulk Fe 3+ /ΣFe indicative of sulphate reduction. • Later Mg metasomatism by serpentinite fluid linked to mantle-δ 18 O, lower Fe 3+ /ΣFe. Sulfur is a key element in geochemical cycles and ore formation, but where, how and in which speciation it is mobilised in subduction zones remains unclear. Here, we investigate the petrography, geochemistry and Sr-Nd-S-O isotopic composition of pyrite-bearing eclogite and omphacitite xenoliths from the Navajo Volcanic Field, which likely sample the oceanic crust of the Cretaceous Farallon Plate at sub-arc depths inaccessible to tectonically exhumed eclogites. After eclogitisation (∼2.4–3.3 GPa, 400-<550 °C), the Farallon oceanic crust experienced two well-documented enrichment events: (1) Ingress of siliceous sediment-derived fluid converting eclogite to omphacitite under deep forearc conditions (∼3–3.5 GPa, >500-<600 °C) and (2) interaction with serpentinite-derived fluid with discrete formation of high-MgO garnet rims under sub-arc conditions (>3.5 GPa, >550 °C). This provides the exceptional opportunity to study the behaviour of sulfur during multi-stage metasomatism in deep subduction zones. High-MgO garnet rims, with mantle-like δ 18 O values (down to +5.62‰) that contrast with higher δ 18 O values in the garnet cores (up to +9.63‰), have distinctly lower Fe 3+ /ΣFe than the cores and are devoid of sulphide inclusions, documenting reduction – rather than oxidation – by serpentinite-derived fluid. Omphacitites have crustal Sr-Nd isotopic compositions, and pyrite is significantly co-enriched with omphacite, both showing marked oscillatory zoning. Supra-mid-ocean ridge basalt sulfur isotopic compositions in four pyrite grains (δ 34 S V-CDT = +0.62±0.05 to +9.03±0.09‰) and highly variable δ 34 S V-CDT (-5.60 to +5.86‰) in another grain indicate some incorporation of sulphate-derived sulfur in omphacitites, which also have higher bulk Fe 3+ /ΣFe than their precursor eclogites. In the absence of carbon-bearing minerals, the distinct enrichment of fluid-immobile Fe 3+ in omphacitites combined with isotopically heavy S in pyrite are ascribed to iron-sulfur redox interactions via ingress of metasediment-derived fluid. The average fraction oxidised S needed to explain the Fe 3+ enrichment in the omphacitites is lower than the total S that was added (corresponding to S 6+ /Σ S = 0.30±0.27, n = 15), adding to increasing evidence for substantial mobility of both reduced and oxidised S within subducted oceanic crust. The subduction-modified Farallon oceanic crust may become an oxygen source upon recycling into more reduced mantle regions and impart an 87 Sr- 34 S-enriched signature without the direct involvement of metasedimentary components.