Abstract :
Recently, X-ray spectroscopy indicated that the low-spin (LS) electronic configuration of Fe2+ and Fe3+ ions substituting in silicate perovskite of various compositions, and also in magnesiowüstite is stable at room temperature above pressures of ∼49 to 120 GPa [J. Badro, G. Fiquet, F. Guyot, J.-P. Rueff, V.V. Struzhkin, G. Vanko, G. Monaco, Iron partitioning in Earthʹs mantle: toward a deep lower mantle discontinuity, Science 300 (2003), 789–791.; J. Badro, J.-P. Rueff, G. Vanko, G. Monaco, G. Figuet, F. Guyot, Electronic transition in perovskite: possible non-convecting layers in the lower mantle, Science 305 (2004) 383–386.; J. Li, V.V. Struzhkin, H.K. Mao, J. Shu, R.J. Hemley, Y. Fei, B. Mysen, P. Dera, V. Prakapenka, G. Shen, Electronic spin state of iron in lower mantle perovskite, Proc. Natl. Acad. Sci. 101 (2004) 14027–14030.; J.M. Jackson, W. Sturhahn, G. Shen, J. Zhao, M.Y. Hu, D. Errandonea, J.D. Bass, Y. Fei, A synchrotron Mössbauer study of (Mg,Fe)SiO3 perovskite up to 120 GPa, Am. Mineral. 90 (2005) 199–205.; J.F. Lin, V.V. Struzhkin, S.D. Jacobsen, M.Y. Hu, P. Chow, J. Kung, H. Liu, H.K. Mao, R.J. Hemley, Spin transition of iron in magnesiowüstite in the Earthʹs lower mantle, Nature 436 (2005) 377–380.]. Simple thermodynamic relationships combined with crystal field theory provide a minimum Clausius–Clapeyron slope of ∼0.23 to 0.4 GPa/K for the high-spin (HS) to low-spin transition of Fe ions in perovskite, consistent with lower mantle temperatures stabilizing the disordered HS state. This computation of ∂P / ∂T utilizes experimentally determined parameters only (P and T at the transition), and is supported by microstructural analysis of the change in volume and entropy considerations. Similarly, (∂P / ∂T)min ∼0.18 to 0.31 GPa/K for magnesiowüstite. High spin Fe2+ should be stable in silicates throughout Earthʹs mantle for compositions of Fe / (Mg + Fe) ∼0.1, consistent with disorder prevailing at high temperature, although partial conversion of octahedral Fe3+ to LS may occur. Similarly, partial conversion of octahedral Fe2+ in magnesiowüstite to the LS is possible for Fe-rich compositions, which transform at relatively low pressures.
