High-pressure (HP) veins in eclogites are the products of fluid-rock interaction and provide insight into the composition and evolution of fluids in subduction zones. We here present the Cu and Zn isotope data for different types of HP veins and their host eclogites from Ganghe and Hualiangting in the Dabie Orogen to reveal the behavior of Cu and Zn isotopes during fluid-rock interaction and fluid evolution. The HP veins include omphacite-epidote (Omp-Ep), epidote-quartz (Ep-Qtz), and kyanite-epidote-quartz (Ky-Ep-Qtz) veins. The Omp-Ep veins first crystallized from eclogite-derived, solute-rich fluids with the Ep-Qtz and Ky-Ep-Qtz veins successively crystallizing from the residual fluids after the Omp-Ep vein formation. The early Omp-Ep veins have variably lower δCu (−1.32 to −1.12‰ at Ganghe, −0.70 to −0.66‰ at Hualiangting) but higher δZn (0.36 to 0.38‰ at Ganghe, 0.40 to 0.41‰ at Hualiangting) relative to the host eclogites (δCu: −0.71 vs. 1.84‰, δZn: 0.22 vs. 0.33‰), indicating CuZn isotope fractionation during slab dehydration in subduction zones with the lighter Cu and heavier Zn isotopes preferentially entering the vein-forming fluids from the eclogites. Systematic increase of δCu from the Omp-Ep (−0.70 to −0.66‰) through Ep-Qtz (0.01‰) to Ky-Ep-Qtz veins (0.46 to 0.95‰) can be attributed to isotope fractionation induced by redox changes during the evolution of metamorphic fluids, as revealed by the negative correlations of δCu with redox-sensitive ratios of Fe/ΣFe and (Eu/Eu*) in those veins. Additionally, the higher δZn of the Omp-Ep veins relative to the Ky-Ep-Qtz veins can be explained by equilibrium isotope fractionation between crystallized minerals and evolved metamorphic fluids. Our results thus demonstrate that CuZn isotope fractionation occurred during the evolution of slab-derived metamorphic fluids in subduction zones. Binary mixing calculations show that fluids derived from dehydration of mafic rocks in subducted slabs, represented by the multistage HP veins in the present study, cannot account for the heavier Cu and lighter Zn isotope compositions in most arc magmas than in mid-ocean ridge basalts. This offset can be resolved by addition of forearc serpentinite-derived fluids enriched in heavy Cu and light Zn isotopes into the mantle source of arc magmas.

中文翻译：

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俯冲带流体介导的铜和锌同位素分馏及其对弧火山作用的影响：大别造山带榴辉岩内高压脉的约束


榴辉岩中的高压（HP）脉是流体-岩石相互作用的产物，可以深入了解俯冲带流体的成分和演化。我们在此提供大别造山带冈河和花凉亭不同类型高压脉及其寄主榴辉岩的铜和锌同位素数据，以揭示铜和锌同位素在流体-岩石相互作用和流体演化过程中的行为。 HP 矿脉包括绿辉石-绿帘石 (Omp-Ep)、绿帘石-石英 (Ep-Qtz) 和蓝晶石-绿帘石-石英 (Ky-Ep-Qtz) 矿脉。 Omp-Ep 脉首先从榴辉岩衍生的富含溶质的流体中结晶出来，Ep-Qtz 和 Ky-Ep-Qtz 脉在 Omp-Ep 脉形成后相继从残余流体中结晶出来。早期 Omp-Ep 矿脉的 δCu 值相对较低（岗河为 -1.32 至 -1.12‰，花凉亭为 -0.70 至 -0.66‰），但 δZn 较高（岗河为 0.36 至 0.38‰，花凉亭为 0.40 至 0.41‰）。宿主榴辉岩（δCu：-0.71 vs. 1.84‰，δZn：0.22 vs. 0.33‰），表明俯冲带板片脱水过程中发生了 CuZn 同位素分馏，较轻的 Cu 和较重的 Zn 同位素优先从榴辉岩进入脉形成流体。 δCu 从 Omp-Ep（−0.70 至 −0.66‰）通过 Ep-Qtz（0.01‰）到 Ky-Ep-Qtz 矿脉（0.46 至 0.95‰）的系统性增加可归因于氧化还原变化引起的同位素分馏。变质流体的演化，正如这些矿脉中 δCu 与氧化还原敏感比率 Fe/ΣFe 和 (Eu/Eu*) 的负相关性所揭示的。此外，Omp-Ep 矿脉相对于 Ky-Ep-Qtz 矿脉较高的 δZn 可以通过结晶矿物和演化的变质流体之间的平衡同位素分馏来解释。 因此，我们的结果表明，在俯冲带板片衍生的变质流体的演化过程中发生了铜锌同位素分馏。二元混合计算表明，俯冲板片中基性岩脱水产生的流体（以本研究中的多级高压脉为代表）无法解释大多数弧岩浆中比洋中脊玄武岩中更重的铜和更轻的锌同位素组成。这种偏移可以通过将富含重铜和轻锌同位素的弧前蛇纹岩衍生流体添加到弧岩浆的地幔源中来解决。