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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]
Copyright of Geochimica et Cosmochimica Acta is the property of Pergamon Press - An Imprint of Elsevier Science and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
Database: Engineering Source
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Header DbId: egs
DbLabel: Engineering Source
An: 194522232
AccessLevel: 6
PubType: Academic Journal
PubTypeId: academicJournal
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  Data: 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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  Data: <searchLink fieldCode="JN" term="%22Geochimica+et+Cosmochimica+Acta%22">Geochimica et Cosmochimica Acta</searchLink>. Jun2026, Vol. 422, p459-470. 12p.
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  Data: <searchLink fieldCode="DE" term="%22Iron+isotopes%22">Iron isotopes</searchLink><br /><searchLink fieldCode="DE" term="%22Metasomatism%22">Metasomatism</searchLink><br /><searchLink fieldCode="DE" term="%22Island+arcs%22">Island arcs</searchLink><br /><searchLink fieldCode="DE" term="%22Earth's+mantle%22">Earth's mantle</searchLink><br /><searchLink fieldCode="DE" term="%22Subduction+zones%22">Subduction zones</searchLink>
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  Data: <searchLink fieldCode="DE" term="%22China%22">China</searchLink>
– Name: Abstract
  Label: Abstract
  Group: Ab
  Data: 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]
– Name: AbstractSuppliedCopyright
  Label:
  Group: Ab
  Data: <i>Copyright of Geochimica et Cosmochimica Acta is the property of Pergamon Press - An Imprint of Elsevier Science and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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RecordInfo BibRecord:
  BibEntity:
    Identifiers:
      – Type: doi
        Value: 10.1016/j.gca.2025.10.031
    Languages:
      – Code: eng
        Text: English
    PhysicalDescription:
      Pagination:
        PageCount: 12
        StartPage: 459
    Subjects:
      – SubjectFull: Iron isotopes
        Type: general
      – SubjectFull: Metasomatism
        Type: general
      – SubjectFull: Island arcs
        Type: general
      – SubjectFull: Earth's mantle
        Type: general
      – SubjectFull: Subduction zones
        Type: general
      – SubjectFull: China
        Type: general
    Titles:
      – TitleFull: 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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          Name:
            NameFull: Ma, Li-Tao
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            NameFull: Dai, Li-Qun
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            NameFull: Yao, Zi-Wei
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            NameFull: Zhao, Zi-Fu
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            NameFull: Hu, Ye-Dan
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            NameFull: Chen, Zhen-Wu
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            NameFull: Sun, Guo-Chao
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            – D: 01
              M: 06
              Text: Jun2026
              Type: published
              Y: 2026
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              Value: 422
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