Heavy Copper Isotopes in Arc‐Related Lavas From Cold Subduction Zones Uncover a Sub‐Arc Mantle Metasomatized by Serpentinite‐Derived Sulfate‐Rich Fluids. Issue 10 (20th October 2022)

• Record Type: Journal Article
• Authors: Chen, Zuxing; Chen, Jiubin; Tamehe, Landry Soh; Zhang, Yuxiang; Zeng, Zhigang; Xia, Xiaoping; Cui, Zexian; Zhang, Ting; Guo, Kun
• Is Part Of: Journal of geophysical research. Volume 127:Issue 10(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-10-20
• Subjects: Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2022JB024910
• Languages: English
• ISSNs: 2169-9313

Abstract

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 C u 2 + − S O 4 2 – ${\mathrm{C}\mathrm{u}}^{2+}-{\mathrm{S}\mathrm{O}}_{4}^{2\mbox{--}}$ 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. Plain Language Summary: Serpentinite dehydration is essential for water recycling in subduction zones; however, whether the released fluid is oxidized or reduced remains hotly debated. To trace serpentinite‐derived fluids and identify their redox state, we analyzed the Cu and S isotopic compositions of fresh lavas from the Mariana, Ryukyu, and Manus subduction zones. We find that arc lavas have heavier Cu and S isotopic compositions relative to the mid‐ocean ridge basalts and back‐arc lavas, and their δ 65 Cu values are positively correlated with geochemical proxies for slab‐derived fluids. These features provide strong evidence for the addition of serpentinite‐derived sulfate‐rich (oxidizing) fluids into the sub‐arc mantle. Moreover, this study shows that redox‐sensitive Cu isotopes can be used to assess the redox state and transport of serpentinite‐derived fluids in subduction zones. Key Points: Arc‐related lavas from cold subduction zones display heavier Cu and S isotopic compositions compared to the mid‐ocean‐ridge basalts Elevated δ 65 Cu and δ 34 S values of arc‐related lavas uncover serpentinite‐derived sulfate‐rich fluids modified the sub‐arc mantle Cu isotopes can provide a novel perspective on the Cu mobility and oxidized fluids transport during subduction