Iron isotope fractionation in the Earth’s lower mantle

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Understanding the distribution of iron isotopes within planetary bodies can help constrain their histories of accretion and differentiation1,2,3. A large fraction of the iron in the silicate Earth is dissolved in ferropericlase and ferroperovskite—mineral phases that make up the bulk of the lower mantle4. These phases have distinct crystallographic structures and iron resides in them in multiple spin states; they are therefore likely to partition iron isotopes differently. Here we use density functional methods to calculate the equilibrium iron isotope composition of these phases at a range of temperatures and pressures, including those thought to exist near the core–mantle boundary. We find that the iron isotopic composition of ferropericlase strongly depends on the spin state of iron. At pressures near the base of the mantle, the low-spin state is enriched in heavy isotopes relative to the high-spin state. In contrast, for ferroperovskite, our calculations suggest that the isotopic composition is almost independent of spin state. Our results warrant a careful search for a pressure-dependent isotopic signature in samples brought up by mantle plumes and in materials subjected to lower mantle pressures and temperatures in the laboratory.

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Figure 1: Core atoms of molecular clusters.
Figure 2: Development of an equilibrium lower-mantle isotopic profile.
Figure 3: Cumulative value of 103lnβ as a function of vibrational frequency.


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This work was financially supported by the US Department of Energy, Division of Basic Energy Sciences, grant DE-FG02-04ER15498 to J.R.R, and NASA Cosmochemistry (NNX08AG57G) and Origins of Solar Systems (NNX09AC93G) grants to Q.-Z.Y. We thank F. Poitrasson for helpful suggestions on the manuscript and V. B. Polyakov for many discussions and sharing his data presented in ref. 5.

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The research problem was identified by Q.-Z.Y. and J.R.R. J.R.R. designed the protocol and carried out the electronic structure calculations. The paper was written by J.R.R. and Q.-Z.Y.

Correspondence to James R. Rustad or Qing-Zhu Yin.

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