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Oxidation extent of the upper mantle by subducted slab and possible oxygen budget in deep Earth inferred from redox kinetics of olivine
  • Chengcheng Zhao,
  • Takashi Yoshino,
  • Baohua Zhang
Chengcheng Zhao
Institute for Planetary Materials

Corresponding Author:chengzhao@zju.edu.cn

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Takashi Yoshino
Institute for Planetary Materials
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Baohua Zhang
Zhejiang University
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Redox input by subducting slab into deep mantle is of vital importance for deep cycle and isotopic evolution of volatile elements, whose chemically stable forms are controlled by redox state. Lithospheric mantle is crucial in redefining redox state of the Earth’s deep mantle. To constrain to which extent subducted slab can modify redox state of the upper mantle and how much oxygen slab can carry into deep Earth, we investigated redox kinetics of olivine adopting diffusion couple methods at 1 GPa and 1373-1573 K in a piston cylinder apparatus. It is found that redox process in olivine is diffusion-controlled, and diffusing on the order of 10-12 m2/s at 1473 K. The oxidation process in initially reduced olivine is oxygen fugacity (fO2)-independent with activation enthalpy of 235±56 kJ/mol, while the reduction process in initially oxidized olivine is fO2-dependent with an exponent of 2/5. Diffusion profile analysis reveals that redox state of starting material plays decisive role in determining redox mechanism. Below ΔFMQ+1, redox process in olivine is controlled by oxygen grain boundary diffusion, while above ΔFMQ+1, it is rate-limited by faster diffusion species which might be hydrogen related Mg vacancy. The extremely slow redox rate limits the homogenization of the slab and its surrounding mantle as redox state of the upper mantle remains unchanged for over past 3.5 Gyrs. The subducted slab has the ability to efficiently transport oxidized components to the region deeper than the mantle transition zone. A highly underestimated oxygen reservoir may have formed in the deep Earth.
Feb 2022Published in Journal of Geophysical Research: Solid Earth volume 127 issue 2. 10.1029/2021JB022977