HR: 1340h
AN: MR23B-0067    [Abstracts]
TI: Post-Perovskite Phase Transition in the MgSiO3 - Al2O3 system
AU: * Kubo, A
EM: akubo@princeton.edu
AF: Department of Geosciences, Princeton University, Guyot Hall, Washington Road, Princeton, NJ 08544 United States
AU: Duffy, T S
EM: duffy@princeton.edu
AF: Department of Geosciences, Princeton University, Guyot Hall, Washington Road, Princeton, NJ 08544 United States
AU: Shen, G
EM: shen@cars.uchicago.edu
AF: Consortium for Advanced Radiation Sources, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637 United States
AU: Prakapenka, V B
EM: prakapenka@cars.uchicago.edu
AF: Consortium for Advanced Radiation Sources, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637 United States
AB: Aluminum is an important element in the Earth's interior, and the lower mantle is thought to contain ~5 mol% Al₂O₃. It is well known that the Al₂O₃ content can affect phase transition pressures and elastic properties of mantle phases. We have studied the effects of aluminum content on transition pressure and equation of state of post-perovskite (ppv) phase in MgSiO₃ using laser-heated diamond cells. Powdered glasses along the MgSiO₃ - Al₂O₃ join were selected as starting materials. They were mixed with Pt powder that served as both a pressure standard and a laser absorber. Either LiF or NaCl was used as a pressure medium. In situ x-ray diffraction experiments at high pressure and temperature were conducted at GSECARS, Argonne National Lab, using angle dispersive diffraction and double-sided laser heating. We synthesized ppv phases in 90 mol% MgSiO₃ + 10 mol% Al₂O₃ (= En90Co10) and En80Co20 bulk compositions at ~2000 K, ~140 GPa and at ~2400 K, ~170 GPa, respectively. In both runs, weak and broad peaks of perovskite (pv) coexisted with the ppv. In addition, two additional peaks were observed that might be assigned to Al₂O₃ ppv. During heating of the En80Co20 composition at 148 GPa on decompression, we recognized growth of pv 112 peak. These observations suggest that both bulk compositions at these conditions may be characterized by both a pv + ppv coexistence region as well as stability of both MgSiO₃-rich ppv + Al₂O₃-rich ppv phases. If such phase relations are also applicable to mantle relevant compositions, sharp seismic velocity discontinuity at D'' may not be explained by post-perovskite phase transition due to gradual change of mixing ratio of pv + ppv within a certain pressure interval. The ppv phase disappeared between 60 and 40 GPa during decompression. By comparison with the other published experimental data, we found that volume of ppv phases along the MgSiO₃ - Al₂O₃ join are generally comparable with those of any other Mg-rich and Al-free ppv phases synthesized from MgSiO₃ and iron bearing system, but are significantly smaller than ppv synthesized in basaltic composition.
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3900 MINERAL PHYSICS
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005