HR: 11:50h
AN: MR22A-07 INVITED     [Abstracts]
TI: Compressibility and Structural Evolution of Germanate and Silicate Post-Perovskite Phases
AU: * Duffy, T S
EM: duffy@princeton.edu
AF: Dept. of Geosciences, Princeton U., Princeton, NU 08544 United States
AU: Kubo, A
EM: akubo@princeton.edu
AF: Dept. of Geosciences, Princeton U., Princeton, NU 08544 United States
AU: Shieh, S R
EM: sshieh@uwo.ca
AF: Dept. of Earth Sciences, National Cheng Kung U., Tainan, 701 Taiwan
AU: Shen, G
EM: gshen@hpcat.aps.anl.gov
AF: CARS, U. of Chicago, Chicago, IL 60439 United States
AU: Prakapenka, V
EM: prakapenka@cars.uchicago.edu
AF: CARS, U. of Chicago, Chicago, IL 60439 United States
AU: Kiefer, B
EM: bkiefer@nmsu.edu
AF: Dept. of Physics, New Mexico State U., Las Cruces, NM 88003 United States
AU: Shim, S
EM: sangshim@mit.edu
AF: Dept. EAPS, MIT, Cambridge, MA 02319 United States
AB: The post-perovskite (ppv) phase transition has important implications for interpretation of the D" layer at the base of Earth's mantle. Here we report equation of state (EoS) and structural data on the ppv phase (CaIrO₃-type) in MgGeO₃ and (Mg,Fe)SiO₃. Germanate and silicate ppv phases were synthesized from orthopryoxene and glass starting materials at high pressures and temperatures using laser-heated diamond cells with Ar or NaCl pressure medium. Angle-dispersive X-ray diffraction experiments were conducted at the GSECARS sector of the Advanced Photon Source. For MgGeO₃, EoS data were obtained over the range of 7-200 GPa. The bulk modulus, its pressure derivative, and room pressure unit cell volume were constrained to be 207(5) GPa, 4.4, 179.2(7) ų, respectively. We successfully carried out Rietveld refinements using data obtained between 80 and 110 GPa with both Ar and NaCl pressure media, including high temperature (1400-1700 K) data at ~90 GPa. The structural response appears to be generally smooth over the pressure range investigated. Average Mg-O and Ge-O bond lengths at room temperature are consistent with those determined for other germanates and silicates. It is also noted that trends of structure parameters obtained by Rietveld refinement are qualitatively consistent with results of theoretical (DFT) calculations. The temperature effect on average Mg-O bond length is large while average Ge-O bond length is insensitive to temperature. Our measured pressure-volume data for the post-perovskite phase of (Mg,Fe)SiO₃ yield a bulk modulus of 219(5) GPa and a zero-pressure volume of 164.9(6) ų when K_0'=4. Direct comparison of volumes of coexisting perovskite and CaIrO₃-type phases at 80-106 GPa demonstrates that the post-perovskite phase has a smaller volume than perovskite by 1.1(2)%. Using measured volumes together with the bulk modulus calculated from EOS fits, we find that the bulk sound velocity is expected to decrease by 2.3(2.1)% across this transition.
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 7208 Mantle (1212, 1213, 8124)
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005