New determination of deep mantle sulfur isotopic composition from Galápagos submarine glasses

Whether high 3 He/ 4 He ratios in some ocean island basalts (OIB) reflect the remnant of an undegassed reservoir or the core interaction with the deep mantle is still debated. In this study, we bring additional constraints on the S origin in the high 3 He/ 4 He reservoir. We investigated submarine basaltic glasses from Fernandina volcano, Galápagos Archipelago, and from the Galápagos Spreading Center (GSC) to estimate the S-isotopic composition of the primitive mantle (high 3 He/ 4 He reservoir). We specifically extracted dissolved sulfide and sulfate, whose S-isotopic compositions reveal that these species are not in isotopic equilibrium within the glasses, producing higher δ 34 S bulk values than expected, even though bulk and sulfide ∆ 33 S remain quite similar. Sulfate S-isotope analysis thus becomes essential. Fernandina OIB and GSC MORB show δ 34 S ∼ 0 ± 0.5‰ and ∆ 33 S ∼ 0.015 ± 0.005‰ (1σ), higher than the depleted MORB mantle. After excluding samples significantly affected by S degassing and assimilation of altered oceanic crust, we propose that this 33 S and 34 S-enrichment relative to MORB in the Fernandina mantle source could result from a mixing between an ambient upper mantle (depleted mantle ± 33 S- 34 S-enriched recycled components), represented by the GSC composition, and a primitive mantle (high 3 He/ 4 He reservoir) having δ 34 S and ∆ 33 S of ∼ 0‰. This “chondritic” S-isotopic composition, in the trend of Iceland OIB, could thus reflect either the remnant of an undegassed reservoir assuming a negligible S-isotopic fractionation during core segregation, or the influence of the core (δ 34 S ∼ 0‰) on the mantle S budget.