Primary Sulfur Isotopes of Intraplate Basalts and Implications for Deep S Recycling of Altered Oceanic Crust

Abstract

Altered oceanic crust (AOC) is the largest contributor to the subducted sulfur (S) budget and its recycling modulates the redox evolution and S distribution in the mantle. However, the role of AOC in the deep cycling of S remains poorly constrained. Here we probe the primary S isotopes of Cenozoic intraplate basalts in eastern China by investigating sulfide inclusions in magmatic clinopyroxene megacrysts. These basalts were derived from the deep mantle metasomatized by melts derived from recycled AOC but show MORB-like S isotopes (−0.9–0.9‰), suggesting that AOC-derived melts transfer negligible sulfate and hardly change the δ³⁴S and redox state of the deeper mantle. This contrasts with the generally high δ³⁴S values of mantle wedge peridotites and primary arc magmas that reflect the slab addition of sulfate, indicating that S species and isotopes released from the subducted slab and associated fO₂ are not constant and vary with subduction depth.