Lakshtanov Dmitry L, Sinogeikin Stanislav V, Litasov Konstantin D, Prakapenka Vitali B, Hellwig Holger, Wang Jingyun, Sanches-Valle Carmen, Perrillat Jean-Philippe, Chen Bin, Somayazulu Maddury, Li Jie, Ohtani Eiji, Bass Jay D
Department of Geology, University of Illinois at Urbana-Champaign, 1301 West Green Street, Urbana, IL 61801, USA.
Proc Natl Acad Sci U S A. 2007 Aug 21;104(34):13588-90. doi: 10.1073/pnas.0706113104. Epub 2007 Aug 8.
Silica is the most abundant oxide component in the Earth mantle by weight, and stishovite, the rutile-structured (P4(2)/mnm) high-pressure phase with silica in six coordination by oxygen, is one of the main constituents of the basaltic layer of subducting slabs. It may also be present as a free phase in the lower mantle and at the core-mantle boundary. Pure stishovite undergoes a displacive phase transition to the CaCl(2) structure (Pnnm) at approximately 55 GPa. Theory suggests that this transition is associated with softening of the shear modulus that could provide a significant seismic signature, but none has ever been observed in the Earth. However, stishovite in natural rocks is expected to contain up to 5 wt % Al(2)O(3) and possibly water. Here we report the acoustic velocities, densities, and Raman frequencies of aluminum- and hydrogen-bearing stishovite with a composition close to that expected in the Earth mantle at pressures up to 43.8(3) GPa [where (3) indicates an uncertainty of 0.3 GPa]. The post-stishovite phase transition occurs at 24.3(5) GPa (at 298 K), far lower than for pure silica at 50-60 GPa. Our results suggest that the rutile-CaCl(2) transition in natural stishovite (with 5 wt % Al(2)O(3)) should occur at approximately 30 GPa or approximately 1,000-km depth at mantle temperatures. The major changes in elastic properties across this transition could make it visible in seismic profiles and may be responsible for seismic reflectors observed at 1,000- to 1,400-km depth.
按重量计算，二氧化硅是地球地幔中含量最丰富的氧化物成分，而斯石英是金红石结构（P4(2)/mnm）的高压相，其中硅与六个氧原子配位，是俯冲板块玄武岩层的主要成分之一。它也可能以自由相的形式存在于下地幔和地核-地幔边界。纯斯石英在约55吉帕的压力下会发生位移型相变，转变为CaCl(2)结构（Pnnm）。理论表明，这种转变与剪切模量的软化有关，这可能会产生显著的地震信号，但在地球上从未观测到过。然而，天然岩石中的斯石英预计含有高达5重量%的Al(2)O(3)，并且可能含有水。在此，我们报告了在高达43.8(3)吉帕的压力下（其中(3)表示0.3吉帕的不确定性），接近地球地幔预期成分的含铝和含氢斯石英的声速、密度和拉曼频率。后斯石英相变发生在24.3(5)吉帕（298K时），远低于纯二氧化硅在50 - 60吉帕时的相变压力。我们的结果表明，天然斯石英（含5重量% Al(2)O(3)）中的金红石 - CaCl(2)转变应该在约30吉帕或地幔温度下约1000千米深度处发生。这种转变过程中弹性性质的主要变化可能使其在地震剖面图中可见，并且可能是在1000至1400千米深度处观测到的地震反射层的原因。
