Iron isotopic fractionation in mineral phases from Earth's lower mantle: Did terrestrial magma ocean crystallization fractionate iron isotopes?

Hong Yang, Jung Fu Lin, Michael Y. Hu, Mathieu Roskosz, Wenli Bi, Jiyong Zhao, Esen E. Alp, Jin Liu, Jiachao Liu, Renata M. Wentzowitch, Takuo Okuchi, Nicolas Dauphas

Research output: Contribution to journalArticle

Abstract

Iron is the most abundant transition metal in the Earth's interior, yet considerable uncertainties remain as to why mantle-derived rocks have diverse iron isotopic compositions. In particular, the isotopic fractionation behavior of iron in the lower-mantle minerals bridgmanite and ferropericlase are largely unexplored. The reason is that it is challenging to study isotopic fractionation at the high pressures relevant to the deep mantle. Here we report in situ measurements of the mean force constants of iron bonds in these minerals pressurized in diamond anvil cells using the technique of nuclear resonant inelastic X-ray scattering (NRIXS). We find that the transition from high- to low-spin iron in ferropericlase ((Mg0.75Fe0.25)O) at approximately 60 GPa drastically stiffens its iron bonds in the low-spin state. The mean force constant of iron bonds in both Fe-bearing and (Fe,Al)-bearing bridgmanite exhibits softening by 21% at approximately 40–60 GPa, which seems to be associated with changes in the iron local environment during the transition from low to high quadrupole splitting states. These results indicate that in the lower mantle, low-spin ferropericlase is enriched in heavy iron isotopes relative to bridgmanite and metallic iron by +0.15‰ and +0.12‰ respectively. Based on these results, we investigate whether terrestrial magma ocean crystallization could have fractionated iron isotopes. We conclude that this process cannot be responsible for the heavy iron isotope enrichment measured in terrestrial basalts.

LanguageEnglish
Pages113-122
Number of pages10
JournalEarth and Planetary Science Letters
Volume506
DOIs
Publication statusPublished - Jan 15 2019

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Iron Isotopes
iron isotopes
isotopic fractionation
lower mantle
Fractionation
Crystallization
fractionation
magma
Minerals
oceans
Earth mantle
crystallization
Iron
Earth (planet)
minerals
isotope
iron
ocean
mineral
Bearings (structural)

Keywords

  • iron isotopic fractionation
  • magma ocean crystallization
  • nuclear resonant spectroscopy
  • spin transition

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

Cite this

Iron isotopic fractionation in mineral phases from Earth's lower mantle : Did terrestrial magma ocean crystallization fractionate iron isotopes? / Yang, Hong; Lin, Jung Fu; Hu, Michael Y.; Roskosz, Mathieu; Bi, Wenli; Zhao, Jiyong; Alp, Esen E.; Liu, Jin; Liu, Jiachao; Wentzowitch, Renata M.; Okuchi, Takuo; Dauphas, Nicolas.

In: Earth and Planetary Science Letters, Vol. 506, 15.01.2019, p. 113-122.

Research output: Contribution to journalArticle

Yang, Hong ; Lin, Jung Fu ; Hu, Michael Y. ; Roskosz, Mathieu ; Bi, Wenli ; Zhao, Jiyong ; Alp, Esen E. ; Liu, Jin ; Liu, Jiachao ; Wentzowitch, Renata M. ; Okuchi, Takuo ; Dauphas, Nicolas. / Iron isotopic fractionation in mineral phases from Earth's lower mantle : Did terrestrial magma ocean crystallization fractionate iron isotopes?. In: Earth and Planetary Science Letters. 2019 ; Vol. 506. pp. 113-122.
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AU - Yang, Hong

AU - Lin, Jung Fu

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AU - Bi, Wenli

AU - Zhao, Jiyong

AU - Alp, Esen E.

AU - Liu, Jin

AU - Liu, Jiachao

AU - Wentzowitch, Renata M.

AU - Okuchi, Takuo

AU - Dauphas, Nicolas

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N2 - Iron is the most abundant transition metal in the Earth's interior, yet considerable uncertainties remain as to why mantle-derived rocks have diverse iron isotopic compositions. In particular, the isotopic fractionation behavior of iron in the lower-mantle minerals bridgmanite and ferropericlase are largely unexplored. The reason is that it is challenging to study isotopic fractionation at the high pressures relevant to the deep mantle. Here we report in situ measurements of the mean force constants of iron bonds in these minerals pressurized in diamond anvil cells using the technique of nuclear resonant inelastic X-ray scattering (NRIXS). We find that the transition from high- to low-spin iron in ferropericlase ((Mg0.75Fe0.25)O) at approximately 60 GPa drastically stiffens its iron bonds in the low-spin state. The mean force constant of iron bonds in both Fe-bearing and (Fe,Al)-bearing bridgmanite exhibits softening by 21% at approximately 40–60 GPa, which seems to be associated with changes in the iron local environment during the transition from low to high quadrupole splitting states. These results indicate that in the lower mantle, low-spin ferropericlase is enriched in heavy iron isotopes relative to bridgmanite and metallic iron by +0.15‰ and +0.12‰ respectively. Based on these results, we investigate whether terrestrial magma ocean crystallization could have fractionated iron isotopes. We conclude that this process cannot be responsible for the heavy iron isotope enrichment measured in terrestrial basalts.

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