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

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 (δ65Cu = +0.26 to +0.67‰) compared to the mantle‐like δ65Cu values of the middle Okinawa Trough and Mariana Trough basalts (+0.11 to +0.14‰). In addition, positive correlations between δ65Cu and slab fluid indicators (e.g., B/Nb, Ba/Th, and Ba/La) combined with positive δ34S 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 normalCnormalu2+−normalSnormalO42– ${\mathrm{C}\mathrm{u}}^{2+}-{\mathrm{S}\mathrm{O}}_{4}^{2\mbox{--}}$ 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 δ65Cu and δ34S 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.

Using Apatite to Track Volatile Evolution in the Shallow Magma Chamber below the Yonaguni Knoll IV Hydrothermal Field in the Southwestern Okinawa Trough

“…In contrast, the X F /X Cl ratios of apatite strongly increased during H 2 O-saturated crystallization (Figure 4b). The decreased trend in the X Cl /X OH and X F /X OH ratios of the studied apatites was consistent with the mode of H 2 O-undersaturated crystallization [48] (Figure 4b). Therefore, the rhyolitic magma in a shallow magma chamber was H 2 O-unsaturated during the period of apatite crystallization.…”

Deciphering Contribution of Recycled Altered Oceanic Crust to Arc Magmas Using Ba‐Sr‐Nd Isotopes

“…Major element, trace element, Sr‐Nd isotopic compositions, and petrographic features of these samples have been reported by Z. Chen et al. (2022).…”

Copper isotope evidence for sulfide fractionation and lower crustal foundering in making continental crust

“…The Cu-depleted lower crustal rocks are plotted for comparison. (B) Comparison of δ 65 Cu values among lower crustal rocks, continental arc cumulates, basalt and basaltic andesites in southern Tibet, sulfides in magmatic Cu ore deposits (19,20,78), and various types of basalts (16,17,27,28,79). The equilibrium Cu isotope fractionation between sulfide and silicate melts is from the experimental work of (21).…”

“…The δ 65 Cu data for granulite xenoliths from Hannuoba in the North China craton are not plotted because these rocks suffered strong oxidative sulfide breakdown and associated Cu mobilization (26). All data for arc basalts and andesites, including high-Mg andesites are from (16,27,28,79). The horizontal yellow band represents the δ 65 Cu of BSE (16).…”