HR: 12:05h
AN: V42A-08    [Abstracts]
TI: Valence state of iron in mantle phases at high pressures and high temperatures
AU: * Shen, G
EM: shen@cars.uchicago.edu
AF: Consortium for Advanced Radiation Sources, University of Chicago, Chicago, 60637 United States
AU: Sturhahn, W
EM: sturhahn@aps.anl.gov
AF: Advanced Photon Source, Argonne National Lab, Argonne, 60439 United States
AU: Prakapenka, V
EM: prakapenka@cars.uchicago.edu
AF: Consortium for Advanced Radiation Sources, University of Chicago, Chicago, 60637 United States
AU: Jackson, J
EM: jmjackso@uiuc.edu
AF: Department of Geology, University of Illinois, Urbana-Champaign, 61810 United States
AU: Zhao, J
EM: jzhao@aps.anl.gov
AF: Advanced Photon Source, Argonne National Lab, Argonne, 60439 United States
AU: Fei, Y
EM: fei@gl.ciw.edu
AF: Carnegie Institution of Washington, 5251 Broad Branch Rd, Washington DC, 20015 United States
AB: Starting from orthopyroxene with composition Mg$_{0.90}$Fe$_{0.10}$SiO$_3$, the valence states of iron in its high pressure polymorphs (clinopyroxene, ringwoodite, ilmenite, and perovskite) have been studied using synchrotron M”ssbauer spectroscopy (SMS) at pressures to 31 GPa and temperatures to 2000 K. The orthopyroxene sample was loaded in a diamond anvil cell. The high-pressure phases were synthesized with laser heating at 1500-2000 K and at pressures of 12 GPa (clino-pyroxene), 19 GPa (ringwoodite), 22 GPa (ilmenite), and 31 GPa (perovskite). M”ssbauer spectra were recorded in situ before, during, and after laser heating at each pressure with the newly developed SMS techniques. All phases were identified by x-ray diffraction at room temperature under high pressures. Data for the orthopyroxene starting material show only Fe$^{2+}$ in two octahedral sites, in agreement with literature data. After compressing the orthopyroxene to 12 GPa, SMS data can be interpreted by Fe$^{2+}$ in a single octahedral site. During heating at 1400 K, M”ssbauer spectra show stark difference from that at room temperature. One possible explanation for the high temperature data is that an intermediate spin state for Fe$^{2+}$ could be formed because the high-spin and low-spin configurations could be in the thermally accessible range. After temperature quench at 12.5 GPa, the transformed clinopyroxene shows Fe$^{2+}$ in a single octahedral site. Data for ringwoodite at 19 GPa after heating show small amount, yet detectable, Fe$^{3+}$. Significant amount of Fe$^{3+}$ is observed at higher pressures for ilmenite and perovskite in which Si is six-coordinated. Implications for the oxidation state of the upper mantle, the transition zone and the lower mantle will be discussed.
DE: 8125 Evolution of the Earth
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting