Stability of (Mg,Fe)SiO3-Perovskite at Lower Mantle Pressure and Temperature Conditions

Shock recovery experiments of (Mg0.92,Fe0.08)SiO3 enstatite were conducted at pressure range from 60 to 110 GPa (the corresponding temperature is estimated as 2 300~4 800 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 SiO2 and (Mg0.92,Fe0.08)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 (Mg0.92,Fe0.08)SiO3 to oxides SiO2 and (Mg0.92,Fe0.08)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.