Decoupling of Fe isotope and Fe3+/ΣFe in arc igneous rocks： Implications for sediment recycling and its contribution to mantle redox

The redox state of subduction zones is pivotal for understanding the geochemical and geodynamic processes of Earth's interior. However, the mechanism behind high oxidation state of arc magmas remains controversial. The possible mechanism is indicated by a combination of Fe isotope and Fe³⁺/ΣFe ratio for Early Triassic arc igneous from the West Qinling Orogen, China. The studied mafic igneous rocks display arc-type trace element characteristics and enriched Sr–Nd–Hf isotopes, suggesting that their mantle source was metasomatized by substantial sediment-derived hydrous melts, with minor slab-derived aqueous solutions. Additionally, these mafic igneous rocks exhibit lighter Fe isotopes (δ⁵⁶Fe = -0.05 to 0.09‰) compared to MORB (0.11 ± 0.06‰). While dehydration of subducting serpentinites at subarc depths can release isotopically light Fe fluids, the lack of co-variation between δ⁵⁶Fe and slab-fluid tracers (e.g., Ba/La, Th/Yb, Nd-Hf isotopes) suggests that an alternative mechanism, such as prior melt extraction, is more plausible. This process involves the preferential removal of isotopically heavy Fe³⁺, leaving a reduced residue with lighter Fe and low Fe³⁺/ΣFe. However, this is inconsistent with the higher Fe³⁺/ΣFe ratios (0.19–0.33) observed in the mafic igneous rocks compared to MORB (0.16 ± 0.01), leading to a significant decoupling of the Fe³⁺/ΣFe ratio and δ⁵⁶Fe value. Notably, the strong correlations between Fe³⁺/ΣFe values and sediment melt proxies (Th/Yb, Th/Nd, Th/La, and Hf isotopes) suggest that the addition of sediment melts plays a key role in controlling the high oxidation state of the subarc mantle. Accordingly, the decoupling of Fe³⁺/ΣFe and δ⁵⁶Fe likely results from prior melt depletion of the mantle source, followed by slab fluid metasomatism dominated by sediment melts with high oxidant content (e.g., S⁶⁺). Therefore, the observed decoupling of Fe³⁺/ΣFe and δ⁵⁶Fe in arc magmas provides critical insights into the geodynamic controls on subarc mantle melting regimes, sediment recycling, and their effects on mantle oxidation states.

俯冲带的氧化还原状态是解析地球内部地球化学与地球动力学过程的核心要点。然而，弧岩浆高氧化状态的成因机制迄今仍存在广泛争议。针对中国西秦岭造山带早三叠世弧火成岩，结合铁同位素（Fe isotope）与三价铁与全铁比值（Fe³⁺/ΣFe ratio）的综合分析，可为其潜在成因机制提供有效指示。本次研究的镁铁质火成岩展现出典型的弧型微量元素地球化学特征，且Sr-Nd-Hf同位素组成显著富集，表明其地幔源区曾受到大量沉积物来源的含水熔体交代，仅伴随少量板片来源的水溶液交代作用。此外，与洋中脊玄武岩（MORB，0.11 ± 0.06‰）相比，这些镁铁质火成岩的铁同位素组成更轻（δ⁵⁶Fe = -0.05 ~ 0.09‰）。尽管弧下深度的俯冲蛇纹岩脱水作用可释放同位素轻铁流体，但δ⁵⁶Fe与板片流体示踪剂（如Ba/La、Th/Yb、Nd-Hf同位素）之间并未呈现协变关系，这暗示先期熔体抽取等其他机制更为合理。该先期熔体抽取过程会优先移除同位素较重的三价铁，使残留相呈现还原状态，且铁同位素更轻、三价铁与全铁比值更低。但这一结论与本次观测到的镁铁质火成岩三价铁与全铁比值（0.19 ~ 0.33）高于洋中脊玄武岩（0.16 ± 0.01）的结果相悖，进而导致三价铁与全铁比值与δ⁵⁶Fe值出现显著解耦。值得注意的是，三价铁与全铁比值与沉积物熔体代用指标（Th/Yb、Th/Nd、Th/La及Hf同位素）之间存在极强的相关性，这表明沉积物熔体的加入对调控弧下地幔的高氧化状态起到了关键作用。据此，弧岩浆中观测到的三价铁与全铁比值与δ⁵⁶Fe解耦现象，可能源于地幔源区先期熔体亏损，随后受到以高氧化剂含量（如六价硫S⁶⁺）的沉积物熔体为主的板片流体交代作用。因此，弧岩浆中三价铁与全铁比值与δ⁵⁶Fe的解耦现象，为理解弧下地幔熔融体系、沉积物循环及其对地幔氧化状态的地球动力学控制作用提供了关键约束。