HR: 08:15h
AN: MR21A-02    [Abstracts]
TI: In Situ X-ray Diffraction Study of MgSiO$_3$ Perovskite to the Core-Mantle Boundary Conditions
AU: * Shim, S
EM: sangshim@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 United States
AU: Duffy, T S
EM: duffy@princeton.edu
AF: Princeton University, Washington Road, Princeton, NJ 08544 United States
AU: Jeanloz, R
EM: jeanloz@uclink.berkeley.edu
AF: University of California, 307 McCone Hall, Berkeley, CA 94720 United States
AU: Shen, G
EM: shen@cars.uchicago.edu
AF: University of Chicago, 5640 South Ellis, Chicago, IL 60637 United States
AB: In order to understand the stability and crystal structure of MgSiO$_3$ perovskite in the deep mantle, in situ angle-dispersive X-ray diffraction measurements have been performed at 90-144 GPa and 300-2900 K in double-sided laser-heated diamond-anvil cells at the GSECARS sector of the Advanced Photon Source. MgSiO$_3$ glass was used as a starting material. Mixtures of MgSiO$_3$ and platinum (pressure standard) were loaded in 50 micron holes in rhenium gaskets and compressed with beveled diamond anvils (100 micron size cullet). In our study, the samples were surrounded by argon which serves as both pressure medium and thermal insulation. Together with double-sided laser heating, this helps to reduce temperature and stress gradients in the sample during laser heating. The occurrence of a new diffraction peak at 2.57-2.62 \AA, which we previously reported based on energy-dispersive X-ray diffraction measurements, was confirmed at 90-144 GPa. The pressure-induced d-spacing shift of this new peak is consistent with that expected for MgSiO$_3$ perovskite lines, indicating that the new diffraction line is associated with either a modification of the crystal structure of MgSiO$_3$ perovskite at 88 GPa or another structure with very similar compressibility. During 25 minutes of heating at 2000 K and 144$\pm$10 GPa, only the diffraction peaks of MgSiO$_3$ perovskite were observed together with the new line at 2.57-2.62 \AA\ and the lines from the pressure scale and the pressure medium. When we increase temperature to 2500 K, however, significant changes were observed: several new peaks and peak splittings, as well as intensity changes of some peaks. The major lines of MgSiO$_3$ perovskite were still observed in the diffraction patterns. Dominant peaks among the new diffraction features can be explained by those of the proposed post-perovskite phase (CaIrO$_3$-type: Murakami et al., 2004). Our results therefore support the possibility of a crystal structure modification in MgSiO$_3$ perovskite at 1800-2000-km depth conditions and a major phase transition in MgSiO$_3$ near the base of the mantle.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 3924 High-pressure behavior
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting