Near-zero 33S and 36S anomalies in Pitcairn basalts suggest Proterozoic sediments in the EM-1 mantle plume

•Submarine Pitcairn glasses show evidence for concomitant sulfur and water degassing during eruption.•Degassing accounts for the 1.5‰ variation of the 34S/32S ratios of the glasses.•Δ33S and Δ36S signatures remain unaffected by magmatic degassing.•Uniformly high Δ33S values, by +0.02‰ compared to MORB, while Δ36S values are strictly MORB-like.•The contribution of Proterozoic sediments to the Pitcairn mantle source may account for the S isotope data.