HR: 17:30h
AN: MR14A-07    [Abstracts]
TI: The Spin State of Iron in Minerals of Earth's Lower Mantle
AU: * Sturhahn, W
EM: sturhahn@anl.gov
AF: Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439 United States
AU: Jackson, J M
EM: jjackson@aps.anl.gov
AF: Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439 United States
AU: Lin, J
EM: j.lin@gl.ciw.edu
AF: Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550 United States
AB: The spin state of Fe(II) and Fe(III) at temperatures and pressures typical for the Earth's lower mantle is discussed. We predict an extended high-spin to low-spin crossover region along the geotherm for Fe-dilute systems depending on crystal-field splitting, pairing energy, and cooperative interactions. In particular, spin transitions in ferromagnesium silicate perovskite and ferropericlase, the dominant lower mantle components, should occur in a wide temperature-pressure range. We also derive a gradual volume change associated with such transitions in the lower mantle. The gradual density changes and the wide spin crossover regions seem incompatible with lower mantle stratification resulting from a spin transition. In particular, we explore the temperature effect on spin states for minerals in Earth's lower mantle like ferropericlase and ferro-magnesium perovskite: the change from a high-spin to a low-spin state (or vice versa) can occur as a continuous crossover rather than a sudden transition. We will use crystal field theory and an extended Bragg-Williams mean-field theory to model the effects of the host mineral and of the iron-iron interactions on the 3d energy levels of the iron. If the Fe concentration of the mineral is of the order of 10 atomic% or less, the average iron-iron distance becomes large enough to result in negligible spin-spin interactions between neighboring iron atoms, and a continuous crossover or "spin equilibrium" is expected. The application of these concepts allows us to express the influence of temperature and discuss the implications for a potential stratification of the lower mantle caused by the spin state of iron in minerals.
DE: 3909 Elasticity and anelasticity
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 3939 Physical thermodynamics
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005