Heavy Copper Isotopes in Arc‐Related Lavas From Cold Subduction Zones Uncover a Sub‐Arc Mantle Metasomatized by Serpentinite‐Derived Sulfate‐Rich Fluids

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Abstract Serpentinite dehydration during subduction releases fluids into the sub‐arc mantle, yet the oxidizing capability of these fluids remains controversial. Here, redox‐sensitive Cu isotopes and in situ S isotopes of magmatic sulfides in fresh lavas from three subduction zones in the western Pacific were analyzed to determine the speciation of multi‐valence elements in the serpentinite‐derived fluids. Notably, the basaltic lavas from the Mariana Arc, eastern Manus Basin, and southern Okinawa Trough have isotopically heavy Cu (δ 65 Cu = +0.26 to +0.67‰) compared to the mantle‐like δ 65 Cu values of the middle Okinawa Trough and Mariana Trough basalts (+0.11 to +0.14‰). In addition, positive correlations between δ 65 Cu and slab fluid indicators (e.g., B/Nb, Ba/Th, and Ba/La) combined with positive δ 34 S values (∼+4‰) of magmatic sulfides in the Mariana Arc lavas, suggest that fluids with isotopically heavy Cu and S metasomatized their mantle sources. Theoretically, the bearing fluids released by serpentinite dehydration have extremely heavy Cu and S isotope compositions. Consequently, the sulfate‐rich fluids infiltration into the sub‐arc mantle produces heavy Cu and S isotopes in arc‐related magmas. Comparatively, sulfate‐rich fluids are absent at back‐arc depths due to the complete breakdown of serpentine minerals during the early stages of subduction, which results in the mid‐ocean ridge basalts‐like δ 65 Cu and δ 34 S values (∼−1‰) observed in the middle Okinawa Trough and Mariana Trough basalts. Therefore, heavy Cu isotopes in arc‐related magmas uncover that the sub‐arc mantle was metasomatized by serpentinite‐derived sulfate‐rich (oxidizing) fluids, which accounts for the oxidized nature of arc magmas in cold subduction zones.

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