Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmmr23a..06r&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #MR23A-06
Physics
3924 High-Pressure Behavior, 3934 Optical, Infrared, And Raman Spectroscopy, 3939 Physical Thermodynamics
Scientific paper
The distribution of iron-isotopes between and within planetary bodies provides constraints on their histories of accretion and differentiation. Equilibrium isotopic signatures should become heavier with increasing pressure as bonds are compressed and vibrational frequencies increase. Because the ionic radius of low-spin iron is smaller than high-spin iron, iron-isotope signatures also may be made heavier by the spin transition for Fe2+, which, in ferropericlase, is predicted to occur within the Earth's mantle. We perform B3LYP density functional calculations of the equilibrium 57Fe/54Fe ratios for ferrous iron dissolved in periclase and MgSiO3 perovskite at the pressures and temperatures of the Earth's lower mantle. The coordination environments for iron are represented with autocompensated clusters terminated to conserve Pauling bond strength on the outer rind of oxygen atoms. The vibrational modes of the core atoms within the cavity that displace the iron center are assumed to approximate the vibrational modes that govern equilibrium isotopic fractionation in the crystal. Pressure increases the partitioning of 57Fe into both phases by a factor of three from the Earth's surface to the core-mantle boundary. In ferropericlase, a large contribution to this increase comes from the electronic transition from high-spin to low-spin iron. Our calculations do not indicate a spin-crossover for Fe2+ in ferroperovskite. Although heavy iron is partitioned into Fe-perovskite more strongly than Fe-periclase below the spin transition in Fe-periclase, at pressures above the spin transition, the equilibrium isotopic composition of the two phases should be approximately equal. Our calculations demonstrate that the spin transition can play an important role in determining planetary iron-isotope composition, potentially creating a reservior of isotopically heavy iron below the spin transition. Such a reservior could shield the heavy iron from dispersal during impact events, leaving larger planets with heavier iron isotope signatures.
Rustad James R.
Sauceda Maryali P.
Yin Quan
Zarzycki Piotr
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