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

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Title: Decoupling of Fe isotope and Fe3+/ΣFe in arc igneous rocks: Implications for crustal metasomatism and its contribution to mantle redox.
Authors: Ma, Li-Tao1 (AUTHOR), Dai, Li-Qun1 (AUTHOR) lqdai@ustc.edu.cn, Yao, Zi-Wei1 (AUTHOR), Zhao, Zi-Fu1 (AUTHOR), Hu, Ye-Dan1 (AUTHOR), Chen, Zhen-Wu1 (AUTHOR), Sun, Guo-Chao1 (AUTHOR)
Source: Geochimica et Cosmochimica Acta. Jun2026, Vol. 422, p459-470. 12p.
Subjects: Iron isotopes, Metasomatism, Island arcs, Earth's mantle, Subduction zones
Geographic Terms: China
Abstract: The redox state of subduction zones is important 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 isotopes and Fe3+/ΣFe ratios for Early Triassic arc igneous rocks from the West Qinling Orogen, China. The studied mafic igneous rocks display arc-type trace element characteristics as well as increasing (87Sr/86Sr) i and decreasing ε Nd (t) and ε Hf (t) values, suggesting that their mantle source was metasomatized by substantial sediment-derived hydrous melts, with minor slab-derived aqueous fluids. 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. Melt extraction involves the preferential removal of isotopically heavy Fe3+, leaving a reduced residue with lighter Fe and lower Fe3+/ΣFe. However, this is inconsistent with the higher Fe3+/ΣFe ratios (0.19–0.33) observed in our samples compared to MORB (0.16 ± 0.01), leading to a significant decoupling of the Fe3+/ΣFe ratios and δ56Fe values. Notably, the strong correlations between Fe3+/ΣFe values and sediment melt proxies (Th/Yb, Th/Nd, Th/La, and Th/Ce) suggest that the addition of sediment melts plays a key role in elevating the oxidation state of the subarc mantle. The decoupling likely results from prior melt depletion of the mantle source, followed by metasomatism by sediment-derived hydrous melts rich in oxidants. 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. [ABSTRACT FROM AUTHOR]
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Database: Engineering Source
Description
Abstract:The redox state of subduction zones is important 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 isotopes and Fe3+/ΣFe ratios for Early Triassic arc igneous rocks from the West Qinling Orogen, China. The studied mafic igneous rocks display arc-type trace element characteristics as well as increasing (87Sr/86Sr) i and decreasing ε Nd (t) and ε Hf (t) values, suggesting that their mantle source was metasomatized by substantial sediment-derived hydrous melts, with minor slab-derived aqueous fluids. 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. Melt extraction involves the preferential removal of isotopically heavy Fe3+, leaving a reduced residue with lighter Fe and lower Fe3+/ΣFe. However, this is inconsistent with the higher Fe3+/ΣFe ratios (0.19–0.33) observed in our samples compared to MORB (0.16 ± 0.01), leading to a significant decoupling of the Fe3+/ΣFe ratios and δ56Fe values. Notably, the strong correlations between Fe3+/ΣFe values and sediment melt proxies (Th/Yb, Th/Nd, Th/La, and Th/Ce) suggest that the addition of sediment melts plays a key role in elevating the oxidation state of the subarc mantle. The decoupling likely results from prior melt depletion of the mantle source, followed by metasomatism by sediment-derived hydrous melts rich in oxidants. 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. [ABSTRACT FROM AUTHOR]
ISSN:00167037
DOI:10.1016/j.gca.2025.10.031