HR: 16:00h
AN: U34A-01 INVITED     [Abstracts]
TI: Stability and Crystal Structure of MgSiO$_3$ Perovskite in the Earth's Deep Mantle
AU: * Shim, S
EM: sangshim@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MS 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: Experiments at high pressures and temperatures indicate that (Mg,Fe)SiO$_3$ perovskite is likely the dominant mineral of the lower mantle. However, many contradicting results have been reported regarding its stability and structure. We have measured X-ray diffraction from MgSiO$_3$ in double-sided laser-heated diamond cells at 50-144 GPa and 1600-2900 K using four different starting materials (glass, MgO+SiO$_2$ mixture, enstatite, perovskite) at the GSECARS sector of APS. The platinum pressure scale was used, and the samples were insulated by an argon pressure medium. We have confirmed the stability of Mg-perovskite to at least 2500-km depth conditions. A new diffraction line appears at 88 GPa and 2000 K together with those of Mg-perovskite. The shift of the new line on compression is consistent with those of the Mg-perovskite lines. These observations may indicate a modification of the perovskite structure at 88 GPa. However, other possibilities still remain to be investigated, such as chemical reactions and impurity. At 107 GPa after heating, we observed a weak line which can be assigned to the most intense SiO$_2$ diffraction line. However, the peak was observed only at 10 microns distance from the laser-heated spot, where extreme thermal gradients exist, consistent with the proposal that observations of the dissociation may have been caused by the presence of large thermal gradients at these conditions. At 144$\pm$10 GPa and above 2500$\pm$200 K, together with the major diffraction lines of Mg-perovskite, we observed new lines which are consistent with the recently proposed post-perovskite phase [Murakami et al., 2004]. Our data confirm that perovskite may transform at the bottom of the lower mantle. If so, this new transition may affect the pattern of mantle convection near the core-mantle boundary, perhaps influencing plume formation and the underlying geodynamo. However, important issues that remain to be resolved include the effects of minor elements on the depth and sharpness of the transition, and the uncertainty in pressure scales.
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: Union [U]
MN: 2004 AGU Fall Meeting