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

Yuki Asahara^1,*, Eiji Ohtani¹, Tadashi Kondo¹, Tomoaki Kubo^1,, Nobuyoshi Miyajima², Toshiro Nagase¹, Kiyoshi Fujino³, Takehiko Yagi² and Takumi Kikegawa⁴

¹ Institute of Mineralogy, Petrology and Economic Geology, Tohoku University, Sendai, 980-8578, Japan
² The Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
³ Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10 W8, Sapporo 060-0810, Japan
⁴ Photon Factory, High Energy Accelerator Research Organization, Tsukuba, 305-0801, Japan

We have carried out in-situ X-ray diffraction experiments on high-pressure transformations of a Ca- and Fe- rich pyroxene (Ca_1.03Mg_0.61Fe_0.23Al_0.14Si₂O₆) 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)Si₂O₆ 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.

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				A. Zhang, W. Hsu, R. Wang, and M. Ding Pyroxene polymorphs in melt veins of the heavily shocked Sixiangkou L6 chondrite European Journal of Mineralogy, December 1, 2006; 18(6): 719 - 726. [Abstract] [Full Text] [PDF]