HR: 14:40h
AN: MR33B-05    [Abstracts]
TI: Sound Velocities of MgSiO₃ Perovskite to Megabar Pressure and the Mineralogy of Earth's Lower Mantle
AU: * Murakami, M
EM: mmurakam@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551 Japan
AU: Sinogeikin, S V
EM: sinogeik@uiuc.edu
AF: Department of Geology, University of Illinois, 245, 1301 W. Green St., Urbana, IL 61801 United States
AU: Hellwig, H
EM: hhellwig@uiuc.edu
AF: Department of Geology, University of Illinois, 245, 1301 W. Green St., Urbana, IL 61801 United States
AU: Bass, J D
EM: jaybass@uiuc.edu
AF: Department of Geology, University of Illinois, 245, 1301 W. Green St., Urbana, IL 61801 United States
AB: Knowledge of the pressure dependence of the elastic properties of deep Earth materials is of fundamental importance for constraining mineralogical models of Earth's deep interior through comparisons of the acoustic properties of candidate mineral phases with global seismological observations. It is widely accepted that MgSiO₃ perovskite (space group: Pbnm) is the primary constituent phase in the Earth's lower mantle, thus making it the most abundant mineral in the Earth. Since the discovery of this phase in 1976, its elastic properties have been extensively investigated at lower mantle pressures by static and dynamic experiments. However, the few experimental acoustic measurements on MgSiO₃ perovskite are limited to 9 GPa, which is far below its stability field. Extrapolations of such low pressure acoustic measurements on metastable samples to mantle pressures can result in highly uncertain estimates of elastic parameters at lower mantle pressures, and therefore are poor constraints on the mineralogy of the deep mantle. To resolve this problem we measured the shear wave velocities of fine-grained polycrystalline MgSiO₃ perovskite within its stability field to pressures approaching 1 Mbar. These are the first reported Brillouin measurements performed to megabar pressures. A pre-pressed pellet of MgSiO₃ gel sandwiched between two layers of NaCl was loaded into a DAC. The gel was transformed to the perovskite structure by CO₂ laser heating of the sample in the perovskite stability field. At all pressures the sample was heated and annealed with a CO₂ laser prior to Brillouin measurements, thus relaxing deviatoric stresses in the sample and pressure medium. Thus, up to the highest pressure in these experiments, the pressure gradient across the sample showed a reasonably small value. Raman spectroscopic measurements were conducted for phase identification and checking the crystallization of the sample. Raman spectra obtained after annealing exhibited sharp and clear bands even at the highest pressures, and all bands could be assigned to the perovskite phase. The shear velocity of perovskite extrapolated to ambient pressure is in excellent agreement with earlier room pressure measurements. Using static compression measurements in combination with current shear velocity measurements we determined the compressional velocity of MgSiO₃ perovskite up to the lowermost mantle pressures. Our new data allow us to put strong constraints on the mineralogy and bulk chemistry of the Earth's lower mantle.
DE: 3900 MINERAL PHYSICS
DE: 3909 Elasticity and anelasticity
DE: 3924 High-pressure behavior
DE: 3934 Optical, infrared, and Raman spectroscopy
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005