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<p>Ocean island basalts exhibit heavier and more variable iron isotopic compositions (δ<sup>56</sup>Fe) than mid-ocean ridge basalts, but the origin of these signatures remains unresolved. Here we investigate whether Fe-rich ultralow-velocity zones at the core-mantle boundary can provide a deep-mantle source of heavy Fe isotopes. High-pressure nuclear resonant inelastic X-ray scattering and first-principles calculations show that two candidate phases therein, ferropericlase and pyrite-type FeO<sub>2</sub>H<sub><i>x</i></sub> (<i>x</i> ≤ 1), have stiffer Fe-O bonding than metallic Fe alloys and Fe-bearing bridgmanite, favoring heavy Fe isotope enrichment. Mass-balance modelling indicates δ<sup>56</sup>Fe enrichments of + 0.07 to + 0.17‰ at 2500–3500 K and 135 GPa in such domains, depending on mineralogy and local equilibration. Geodynamic simulations further support the entrainment of dense ultralow-velocity-zones materials into rising mantle plumes. These results identify Fe-rich ultralow-velocity zones as potential high-δ<sup>56</sup>Fe reservoirs and plausible deep-mantle endmember contributing to the iron isotope heterogeneity observed in ocean island basalts.</p>

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Evidence for heavy iron isotopic enrichment at Earth’s core-mantle boundary

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年8月5日

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  • Chaojia Lv,
  • Wenzhong Wang,
  • Tianyang Wang,
  • Mingming Li,
  • Haotian Chen,
  • Wendy L. Mao,
  • Ho-kwang Mao,
  • Yongjun Tian,
  • Jin Liu

摘要

Ocean island basalts exhibit heavier and more variable iron isotopic compositions (δ56Fe) than mid-ocean ridge basalts, but the origin of these signatures remains unresolved. Here we investigate whether Fe-rich ultralow-velocity zones at the core-mantle boundary can provide a deep-mantle source of heavy Fe isotopes. High-pressure nuclear resonant inelastic X-ray scattering and first-principles calculations show that two candidate phases therein, ferropericlase and pyrite-type FeO2Hx (x ≤ 1), have stiffer Fe-O bonding than metallic Fe alloys and Fe-bearing bridgmanite, favoring heavy Fe isotope enrichment. Mass-balance modelling indicates δ56Fe enrichments of + 0.07 to + 0.17‰ at 2500–3500 K and 135 GPa in such domains, depending on mineralogy and local equilibration. Geodynamic simulations further support the entrainment of dense ultralow-velocity-zones materials into rising mantle plumes. These results identify Fe-rich ultralow-velocity zones as potential high-δ56Fe reservoirs and plausible deep-mantle endmember contributing to the iron isotope heterogeneity observed in ocean island basalts.