顽火辉石(Mg0.92,Fe0.08)SiO3的冲击相变和高压状态方程及其地球物理意义

 用阻抗匹配法和电探针技术在48~140 GPa冲击压力范围内对化学组分为(Mg_0.92, Fe_0.08)SiO₃、初始密度为3.06 g/cm³的天然顽火辉石进行了冲击压缩实验。根据本工作13发实验数据，结合McQueen等人的数据可以看出，(Mg_0.92, Fe_0.08)SiO₃顽火辉石在冲击压缩过程中，大约经历三个明显区域：低压相区，压力范围为0~40 GPa；混合相区，压力范围为40～67 GPa；高压相区，压力范围为68～140 GPa。在低压相区，D-u关系已由McQueen给出；而在高压相区（68～140 GPa），可由本实验数据得到。由叠加原理计算得到的混合物(Mg_0.92, Fe_0.08)O(Mw)+SiO₂(St)的D-u关系及p-ρ关系曲线明显偏离了实验数据的拟合曲线，从而排除了在高达140 GPa冲击压力下，钙钛矿结构的(Mg_0.92, Fe_0.08)SiO₃发生向氧化物化学分解相变的可能性。对高压相区的实验数据进行拟合，可以得到(Mg_0.92, Fe_0.08)SiO₃钙钛矿的Grüneisen参数γ。通过三阶Birch-Murnaghan有限应变状态方程，由冲击波实验数据得到了零压等熵体积模量K_0S=259.6(9) GPa及其对压力的一阶偏导数K′_0S=4.20(5)，其ρ₀=4.19 g/cm³。(Mg_0.92, Fe_0.08)SiO₃钙钛矿冲击压缩下的密度数据与PREM密度剖面吻合很好，支持钙钛矿为主要成分的下地幔模型。

Equation of State and Phase Transition of (Mg0.92, Fe0.08) SiO3 Enstatite under Shock Compression and Its Geophysical Implications

We performed shock wave experiments on a natural pyroxene with chemical composition close to (Mg_0.92, Fe_0.08)SiO₃ and initial density of 3.06 g/cm³ at pressures between 48 and 140 GPa, using impedance match method and electrical probe technique. Considering McQueen et. al. data, it is obvious that (Mg_0.92, Fe_0.08)SiO₃ goes through three phase regions in the procedure of shock compression: Low-pressure phase region (LPR), mixed phase region (MPR), and high-pressure phase region (HPR), corresponding to the pressure 0~40 GPa, 40~67 GPa and 68~140 GPa, respectively. In low-pressure phase region, the relationship between shock wave velocity D and particle velocity u was expressed by McQueen et. al. data. Then in high-pressure phase region (at pressures between 68 to 140 GPa), it can be described linearly from our experiment data. The calculated D-u relationship for the assemblage of (Mg_0.92, Fe_0.08)O(Mw)+SiO₂(St) is significantly different from the experimental data, excluding the possibility of chemical decomposition of perovskite to oxides during the shock compression. The Grüneisen parameter γ can be obtained by fitting the experimental data. Using the third-order Birch-Murnaghan finite strain equation of state, the shock experimental data yield a zero-pressure bulk modulus K_0S=259.6(9) GPa and its pressure derivative K′_0S=4.20(5), with ρ₀=4.19 g/cm³. A comparison of the experimental Hugoniot densities of perovskite with the PREM density profile prefers a perovskite-dominant lower mantle model.