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Iron isotope fractionation at the core–mantle boundary by thermodiffusion

Abstract

The D” layer at the base of the Earth’s mantle exhibits anomalous seismic properties, which are attributed to heat loss from and chemical interaction with the underlying molten Fe-rich outer core. Here we show that mass transfer due to temperature variations within the D” layer could lead to resolvable fractionation of iron isotopes. We constrain the degree of isotope fractionation by experiments on core-forming Fe alloy liquids at 2100–2300 K and 2 GPa, which demonstrate that heavy Fe isotopes preferentially migrate towards lower temperature and vice versa. We find that this isotope fractionation occurs rapidly due to the high mobility of iron, which reaches 0.013 ± 0.002‰ (2σ) per degree per amu at steady state. Numerical simulations of mantle convection capturing the evolution of a basal thermal boundary layer show that iron isotope fractionation immediately above the core–mantle boundary can reach measurable levels on geologic timescales and that plumes can entrain this fractionated material into the convecting mantle. We suggest that such a process may contribute to the heavy Fe isotope composition of the upper mantle inferred from mantle melts (basalts) and residues (peridotites) relative to chondrites. That being the case, non-traditional stable isotope systems such as Fe may constrain the interactions between the core and mantle.

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Fig. 1: δ57Fe (in per mil) versus temperature (in K) for experiments reaching steady-state isotope fractionation by thermodiffusion.
Fig. 2: Solutions to the Chapman–Enskog relation for Fe isotopes for steady-state experiments.
Fig. 3: Observations compared with predictions from the ASPECT simulation for thermodiffusion in the thermal boundary layer above the CMB.

Data availability

All data generated during this study are included in this article and its supplementary information and supplementary data files.

Code availability

The version of ASPECT we used to compute the geodynamic models is available online (https://github.com/jdannberg/aspect/commits/iron_fractionation), and all input files required to reproduce our computations, together with instructions for how to run them, are provided in a separate repository (https://github.com/jdannberg/SI-data-thermodiffusion).

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Acknowledgements

This work was supported by NSF EAR-1019887 and the Danish National Research Foundation–Niels Bohr Professorship (26-123/8) to C.E.L., by NSF EAR-1250331 to D.J.L. and NSERC Discovery Grants to J.M.B. J.D. was partially supported by the Computational Infrastructure for Geodynamics initiative through NSF EAR-0949446, NSF EAR-1550901 and the Deep Carbon Observatory. The authors acknowledge the Texas Advanced Computing Center, University of Texas at Austin, for providing high-performance computing resources. We thank P. Sossi for very constructive comments and suggestions that helped improve the presentation.

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Authors

Contributions

This project was conceived and managed by C.E.L. N.R.B. and J.M.B. conducted experiments, G.H.B., J.J.G.G. and C.E.L. performed analyses and J.D. developed the ASPECT code, executed the simulations and documented the modelling effort presented in the Supplementary Information. D.J.L. contributed to the theoretical underpinnings of the kinetic theory of thermodiffusion. C.E.L. drafted the manuscript. All authors contributed to interpretations and revisions of the manuscript.

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Correspondence to Charles E. Lesher.

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Peer review information Primary Handling Editors: Tamara Goldin; Stefan Lachowycz.

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Supplementary information

Supplementary Information

Supplementary Discussion, Figs. 1–10 and Tables 1–3.

Supplementary Data 4

ASPECT model output for plotting.

Supplementary Data 5

ASPECT model output for plotting.

Supplementary Data 7

ASPECT model output for plotting.

Supplementary Data 8

ASPECT model output for plotting.

Supplementary Data 10

ASPECT model output for plotting.

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Lesher, C.E., Dannberg, J., Barfod, G.H. et al. Iron isotope fractionation at the core–mantle boundary by thermodiffusion. Nat. Geosci. 13, 382–386 (2020). https://doi.org/10.1038/s41561-020-0560-y

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