HR: 0800h
AN: MR31A-0137    [Abstracts]
TI: Synthesis of Post-perovskite Phases from Almandine-Pyrope Garnets at High Pressure
AU: * Shieh, S R
EM: sshieh@uwo.ca
AF: Department of Earth Sciences, University of Western Ontario, London, ON N6A 5B7, Canada
AU: Duffy, T S
EM: duffy@princeton.edu
AF: Department of Geosciences, Princeton University, Princeton, NJ 08544, United States
AU: Kubo, A
EM: akubo@cars.uchicago.edu
AF: GSECARS, University of Chicago, Chicago, IL 60637,
AU: Prakapenka, V
EM: prakapenka@cars.uchicago.edu
AF: GSECARS, University of Chicago, Chicago, IL 60637,
AB: The post-perovskite phase could be the dominant phase at the core-mantle boundary and may be responsible for the anomalous seismic discontinuities within and across the D" layer. However, the chemical complexity and extreme conditions near the core-mantle region may strongly affect the structure and stability field of post- perovskite phase. To understand the stability of post-perovskite phase across a wide compositional range, including Al₂O₃-rich compositions, we have synthesized and investigated the properties of post- perovskite along the pyrope-almandine, (Mg,Fe)₃Al₂Si₃O₁₂, join. Three natural samples, Py₅₈Alm₃₈Gr₃ (Alm38), Py₄₃Alm₅₄Gr₂ (Alm54), Py₂₁Alm₇₃Gr₅ (Alm73), were used in this study. The samples were mixed with gold or platinum as laser absorber and internal pressure standards. NaCl or Ar was used as a pressure medium and thermal insulators. In-situ high-pressure laser-heating experiments were carried out at 13ID-D beamline of GSECARS, Advanced Photon Source. The incident x-ray beam was focused to 4-10 μm². The high temperatures were achieved by a double-sided laser-heating technique. The two-dimensional images of x-ray diffraction patterns were collected with the exposure time in the range of 10-600 seconds. The Alm38 samples were compressed to ~145 GPa and heated at 1460-1830 K for less than an hour, both post-perovskite and perovskite phases were observed. However, when heating at temperature >1900K for longer times, the Al₂O₃ post-perovskite phase can also be observed, indicating possible disproportionation. The Alm54 samples were compressed to > 148 GPa and heated to 1780K and both post- perovskite and perovskite phases were observed. At ~155 GPa and 2080 K some new peaks were observed for this composition that may be the Al2O3 post-perovskite phase. The Alm73 samples were compressed at pressure > 150 GPa and heated at 2040-2570K, only the post-perovskite phase was observed. Our results suggest that post-perovskite phase can incorporate high iron and aluminum content but requires higher pressures and temperatures for synthesis.
DE: 1025 Composition of the mantle
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3924 High-pressure behavior
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting