HR: 10:20h
AN: MR22A-01 INVITED    [Abstracts]
TI: Intermediate-spin ferrous iron in lower mantle perovskite
AU: * McCammon, C
EM: catherine.mccammon@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, 95440, Germany
AU: Kantor, I
EM: innokenty.kantor@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, 95440, Germany
AU: Narygina, O
EM: Olga.Narygina@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, 95440, Germany
AU: Rouquette, J
EM: jerome@lpmc.univ-montp2.fr
AF: Universite de Montpellier II, LPMC, Montpellier, F-34095, France
AU: Ponkratz, U
EM: ulrich.ponkratz@esrf.fr
AF: European Synchrotron Radiation Facility, BP 220, Grenoble, F-38043, France
AU: Sergueev, I
EM: sergueev@esrf.fr
AF: European Synchrotron Radiation Facility, BP 220, Grenoble, F-38043, France
AU: Mezouar, M
EM: mezouar@esrf.fr
AF: European Synchrotron Radiation Facility, BP 220, Grenoble, F-38043, France
AU: Prakapenka, V
EM: prakapenka@cars.uchicago.edu
AF: Center for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637, United States
AU: Dubrovinsky, L
EM: Leonid.Dubrovinsky@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, 95440, Germany
AB: The lower mantle is dominated by (Mg,Fe)(Si,Al)O₃ perovskite, where the ability of iron to adopt multiple valence and spin states can affect a broad spectrum of physical and chemical properties. A high- to low-spin transition of Fe²⁺ in (Mg,Fe)O is now well established by both experimental and computational data to occur near 50 GPa at room temperature for lower mantle compositions. However in the Earth's most abundant phase the picture is not so clear. Previous X-ray emission (XES) and nuclear forward scattering (NFS) data present conflicting results on the location, number and sharpness of the transition(s), and whether Fe²⁺ or Fe³⁺ or both are involved. To reconcile these observations, we undertook the first high-pressure high- temperature study of iron-containing silicate perovskite using combined Mössbauer and NFS techniques to determine the spin state of iron in the dominant lower mantle phase. We collected 119 ⁵⁷Fe Mössbauer and 32 NFS spectra of Mg_0.88Fe_0.12SiO₃ and Mg_0.86Fe_0.14Si_0.98Al_0.02O₃ perovskite using a resistively-heated diamond anvil cell at pressures up to 110 GPa and temperatures up to ca. 1000 K, combined with high-resolution X-ray diffraction of several of the same sample loadings. Spectra show the appearance of a new quadrupole doublet above ca. 30 GPa with extremely high quadrupole splitting and narrow linewidth, which can be explained only as intermediate- spin Fe²⁺. The pressure dependence of the average spin number calculated from the Mössbauer data is in excellent agreement with all previous XES data for both Al-free and Al-containing silicate perovskite. Our high- temperature data show that elevated temperatures stabilise the intermediate-spin state; hence Fe²⁺ in silicate perovskite is inferred to be predominantly in the intermediate-spin state throughout most of the lower mantle.
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3630 Experimental mineralogy and petrology
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting