下地幔温压下(Mg,Fe)SiO_3钙钛矿的相稳定性——对冲击回收样品的微观分析

Stability of (Mg,Fe)SiO_3 -Perovskite at Lower Mantle Pressure and Temperature Conditions

在60～110GPa冲击压力(估算温度为2300～4800K)范围内进行了5发原始样品为(Mg0.92,Fe0.08)SiO3顽火辉石的冲击压缩回收实验,对回收样品进行的X射线衍射(XRD)和红外吸收光谱(IR)分析结果表明:(1)回收样品的主相均是单链状结构硅酸盐,而非钙钛矿结构;(2)回收样品中均未观察到氧化物SiO2和(Mg0.92,Fe0.08)O的XRD和IR特征谱线;(3)回收样品的XRD、IR特征谱线变得简略,并发现了与原始样品有某些不同的特征谱线,随冲击压力增加,这种变化趋于明显。(4)通过对比冲击压力在85GPa以下和97GPa以上回收样品的XRD、IR特征谱线,没有观察到明显的新谱线特征出现。结合先前的冲击Hugoniot状态方程实验数据分析,可以认为:在冲击压缩过程中样品处于钙钛矿结构,在冲击卸载过程中样品发生了由钙钛矿结构向单链状结构的逆转相变;特别是,在实验的温度压力范围内,不可能发生由(Mg0.92,Fe0.08)SiO3向SiO2和(Mg0.92,Fe0.08)O的化学分解相变,顽火辉石的高压相———钙钛矿结构是稳定的。回收样品和原始样品的谱线差异可能对应于高压加载或卸载过程引起的某种晶格畸变,而高压加载导致钙钛矿型顽火辉石晶格畸变的可能性更大。这一结果将对下地幔矿物学模型的建立和下地幔地震波探测结果的解释提供基础物理依据。

Shock recovery experiments of (Mg_(0.92),Fe_(0.08))SiO_3 enstatite were conducted at pressure range from 60 to 110 GPa (the corresponding temperature is estimated as 2300～4800 K).X-ray diffraction (XRD) and infrared absorption spectra (IR) of the recovered samples indicate that:(1) The main phase of recovered samples is single-chain structure enstatite,not the perovskite structure;(2) There is no evidence for the existence of oxides SiO_2 and (Mg_(0.92),Fe_(0.08))O in the recovered samples;(3) After shock compression,some peaks different from original sample were found and these differences are getting more obvious with the increasing of pressure;(4) No obviously different peaks were found between the recovered samples below 85 GPa and those above 97 GPa,from the comparison of XRD and IR spectra among these samples.Combined with the former analysis of experiments of Hugoniot equation of state of enstatite,it was considered that enstatite transformed into perovskite structure under shock compression.The microanalysis requires retrogressive transformation of shocked samples during the process of pressure release.Especially,there is no possibility for the chemical decomposition reaction of (Mg_(0.92),Fe_(0.08))SiO_3 to oxides SiO_2 and (Mg_(0.92),Fe_(0.08))O occurring under shock compression,so the high-pressure phase of enstatite-perovskite structure remains stable at the experimental conditions.Meanwhile,we attribute the spectra difference between original sample and recovered samples to a crystal distortion under shock compression or release process and the loading process more probably resulted in such distortion.The results of this investigation provide basic physical evidence for the minerals composition model and the interpretation of seismic wave data for the lower mantle.