Ultrahigh-Pressure Phase Transformations and the Constitution of the Deep Mantle

  1. Murli H. Manghnani and
  2. Yasuhiko Syono
  1. Eiji Ito and
  2. Eiichi Takahashi

Published Online: 21 MAR 2013

DOI: 10.1029/GM039p0221

High-Pressure Research in Mineral Physics: A Volume in Honor of Syun-iti Akimoto

High-Pressure Research in Mineral Physics: A Volume in Honor of Syun-iti Akimoto

How to Cite

Ito, E. and Takahashi, E. (1987) Ultrahigh-Pressure Phase Transformations and the Constitution of the Deep Mantle, in High-Pressure Research in Mineral Physics: A Volume in Honor of Syun-iti Akimoto (eds M. H. Manghnani and Y. Syono), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM039p0221

Author Information

  1. Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori-Ken 682-02, Japan

Publication History

  1. Published Online: 21 MAR 2013
  2. Published Print: 1 JAN 1987

ISBN Information

Print ISBN: 9780875900667

Online ISBN: 9781118664124

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Keywords:

  • Mineralogy and Crystal Chemistry;
  • Phase transformations;
  • High Pressure-High Temperature Research

Summary

In order to clarify the nature of the 670-km discontinuity and the state of the lower mantle, phase equilibria in the systems MgO-FeO-SiO2 and CaSiO3-MgSiO3-Al2O3 were investigated in the pressure range of 10–27 GPa and at temperatures up to 1600°C, using a uniaxial split-sphere apparatus (USSA-5000). We found that the dissociation of the spinel phase into the assemblage of perovskite and magnesiowiistite could be responsible for the 670-km discontinuity. The dissociation was completed in a narrow pressure interval (less than 1 GPa), thereby making the discontinuity very sharp. By considering the detailed phase relations and the estimated physical properties of the lower mantle, we determined possible compositions of the lower mantle in the system MgO-FeO-SiO2. In the system CaSiO3-MgSiO3-Al2O3, the stability field of majorite; was observed to expand rapidly towards the CaSiO3-MgSiO3 join in the pressure range of 10–18 GPa, and to retrograde towards the grossular-pyrope join at pressures higher than 23 GPa, dissociating MgSiO3-rich perovskite and an unquenchable “CaO-rich phase” with diopsidic composition. Therefore majorite is expected to be an important constituent in the transition zone, and the dissociation of majorite; could contribute to the 670-km discontinuity. The complete dissociation of majorite, however, requires a fairly large pressure interval (∼3 GPa), which would produce MgSiO3-rich perovskite, a “CaO-rich phase” with a trace amount of stishovite, and an “Al2O3 rich phase” as the lower mantle constituents. Based on the above results and assuming a peridotitic mantle composition, we describe the mineralogical constitution of the deep mantle down to the lower mantle.