Formation of metastable cubic-perovskite in high-pressure phase transformation of Ca(Mg, Fe, Al)Si2O6

Yuki Asahara, Eiji Ohtani, Tadashi Kondo, Tomoaki Kubo, Nobuyoshi Miyajima, Toshiro Nagase, Kiyoshi Fujino, Takehiko Yagi, Takumi Kikegawa

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Abstract

We have carried out in-situ X-ray diffraction experiments on high-pressure transformations of a Ca- and Fe- rich pyroxene (Ca1.03Mg0.61Fe0.23Al0.14Si 2O6) to investigate the stability of Ca 0.5(Mg, Fe, Al)0.5SiO3 perovskite (CM-perovskite) in a multi component system at about 32 GPa and up to 1900 °C. We observed that cubic CM-perovskite was formed at about 1300 °C and decomposed into cubic Ca-perovskites and orthorhombic Mg-perovskites and stishovite at 1800 °C when using a glass starting material. In another experiment using a crystalline pyroxene starting material, two cubic perovskites; Ca-perovskite and CM-perovskite, and orthorhombic Mg-perovskite formed simultaneously during the initial stage of the transformation. However, the cubic CM-perovskite subsequently decomposed into Mg- and Ca-perovskites and stishovite at 1200 °C. These results indicate that the assembly of cubic Ca-perovskite, orthorhombic Mg-perovskite and stishovite is stable and cubic CM-perovskite is a metastable phase at around 32 GPa and temperatures over 1000 °C in this system. Chemical analyses of product phases showed that Mg, Fe, and Al were preferentially partitioned into Mg-perovskite and the compositions of Ca-perovskite were close to pure CaSiO3. The present study shows that CM-perovskite nucleates during the initial stage of Ca(Mg, Fe, Al)Si 2O6 pyroxene transformation. Therefore, cold subducting slabs and impacted meteorites are the possible places in which CM-perovskite could exist. The Ca-rich glassy phase in a shocked chondrite (Tomioka and Kimura 2003) might have formed by vitrification of a metastable CM-perovskite-like phase.

Original languageEnglish
Pages (from-to)457-462
Number of pages6
JournalAmerican Mineralogist
Volume90
Issue number2-3
DOIs
Publication statusPublished - Feb 1 2005

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All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology

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