Formation of metastable cubic-perovskite in high-pressure phase transformation of Ca(Mg, Fe, Al)Si2O6
Formation of metastable cubic-perovskite in high-pressure phase transformation of Ca(Mg, Fe, Al)Si2O6
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DOI：
10.2138/am.2005.1649
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发表时间：
2005-02
影响因子：
3.1
通讯作者：
Y. Asahara;E. Ohtani;T. Kondo;T. Kubo;N. Miyajima;T. Nagase;K. Fujino;T. Yagi;T. Kikegawa
中科院分区：
文献类型：
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作者：
Y. Asahara;E. Ohtani;T. Kondo;T. Kubo;N. Miyajima;T. Nagase;K. Fujino;T. Yagi;T. Kikegawa
本文对富钙富铁辉石(Ca1.03Mg0.61Fe0.23Al0.14Si2O6)的高压相变进行了原位X射线衍射实验，研究了Ca0.5(Mg，Fe，Al)0.5SiO_3钙钛矿(CM-钙钛矿)在约32 Gpa和1900℃的多元体系中的稳定性。我们观察到，当使用玻璃原料时，在1300℃左右形成立方的CM-钙钛矿，并在1800℃分解成立方钙钙钛矿、斜方镁钙钛矿和钛铁矿。在另一项以结晶辉石为原料的实验中，在转变的初始阶段同时形成了两个立方钙钛矿：钙钙钛矿和CM钙钛矿，以及正交镁钙钛矿。而立方钙钛矿在1200℃下分解为钙镁钙钛矿和针铁矿。这表明在该体系中，立方钙钙钛矿、正交镁钙钛矿和针铁矿的组装是稳定的，立方钙钛矿在32 Gpa左右、1000℃以上为亚稳定相。产物物相分析表明，镁、铁、铝优先分配成镁钙钛矿，钙钙钛矿的组成接近纯CaSiO_3。研究表明，钙钛矿型钙钛矿在Ca(Mg，Fe，Al)Si2O6辉石转变初期成核。因此，冷俯冲板块和撞击陨石是CM钙钛矿可能存在的场所。冲击球粒陨石(Tomioka和Kimura 2003)中的富钙玻璃相可能是由介稳的类钙钛矿相的玻璃化形成的。
Abstract We have carried out in-situ X-ray diffraction experiments on high-pressure transformations of a Ca- and Fe- rich pyroxene (Ca1.03Mg0.61Fe0.23Al0.14Si2O6) to investigate the stability of Ca0.5(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.