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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Predicted elastic properties of the hydrous D phase at mantle pressures: Implications for the anisotropy of subducted slabs near 670-km discontinuity and in the lower mantle
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Predicted elastic properties of the hydrous D phase at mantle pressures: Implications for the anisotropy of subducted slabs near 670-km discontinuity and in the lower mantle

机译：地幔压力下含水D相的预测弹性：对距地表670 km处和下地幔附近俯冲板的各向异性的影响

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The dense hydrous magnesium silicate called the D phase is the most likely candidate to recycle hydrogen into the lower mantle in subduction zones. As seismology represents the preferred method to detect this mineral in subduction zones, the single crystal elastic tensor has been calculated using first principle methods. A triple cell (P(3)over bar>ml) was used to account for an ordered ideal composition of the D phase to lower mantle pressures. The elastic tensor of the mineral at ambient conditions is predicted to be C-11,,=387.7 C-33=287.7, C,,=100.4, C-12=108.0, C,,=51.1, C-14=-14.6 in GPa with bulk modulus at zero pressure K-0= 163 GPa and a statistical error of about 4.0 GPa for all constants. The pressure derivatives of the elastic constants have been determined to pressure of 80 GPa. Our tensor correctly predicts the published experimental measurements of linear compressibility along the a- and c-axes, the logarithmic pressure derivative of the c/a axial ratio and bulk modulus at zero pressure. It contradicts the experimentally measured elastic tensor reported by (Liu, L.-G., Okamoto, K., Yang, Y.J, Chen, C.-C., Lin, C.-C., 2004. Elasticity of single-crystal phase D (a dense hydrous magnesium silicate) by Brillouin spectroscopy. Solid State Commun. 132, 517-520), which fails to reproduce the experimental compressibility data. Using the elastic constants and density predicted to 85 GPa, the seismic anisotropy of P- and S-waves has been calculated as a function of pressure. The single crystal has high P-wave velocities and shear wave splitting in the basal plane below 20 GPa. At high pressures, the maximum P-wave velocities have threefold symmetry normal to a-axes in the basal plane, whereas the maximum shear wave splitting is parallel to a-axes with twofold symmetry. The D phase has high Vp and Vs anisotropy at ambient pressure, 17.6% for Vp and 19.9% for Vs. The Vp anisotropy decreases with increasing pressure to about 30 GPa, while it remains constant at 8% from 30 to 85 GPa. The Vs anisotropy decreases to 17% at 20 GPa and then increases to 22% at 85 GPa. The isotropic velocity ratios of S to P (Rs/p), bulk sound to S (R phi/s) and density to S (Rp/s) are calculated as a function of pressure. The Rs/p shows very little variation with pressure with low value of 0.6, whereas both R phi/s and Rp/s increase with pressure to peak values of 2.8 and 2.0 respectively at about 60 GPa. The presence of the D phase in subducted horizontal 'stagnant' plates in the circum Pacific could contribute to a number of seismic observations, such as the observed velocity heterogeneity at depths between 500 and 1000 km and the fast S-waves travelling horizontally with horizontal polarization in this depth range. Further the lower density of the D phase compared with anhydrous minerals could influence buoyancy of the hydrated 'stagnant' slabs. The transformation of the D phase to perovskite plus ferripericlase during thermal annealing may be responsible for horizontal slab seismogenesis. (c) 2007 Published by Elsevier B.V.
机译：被称为D相的致密含水硅酸镁是最有可能将氢循环到俯冲带下地幔中的候选人。由于地震学是探测俯冲带中这种矿物的首选方法，因此已经使用第一原理方法计算了单晶弹性张量。使用三重池（P（3）over bar> ml）来说明D相的有序理想组成，以降低地幔压力。矿物在环境条件下的弹性张量预计为C-11，= 387.7 C-33 = 287.7，C，= 100.4，C-12 = 108.0，C，= 51.1，C-14 = -14.6在零压力下K-0 = 163 GPa时的体积模量为GPa，所有常数的统计误差约为4.0 GPa。弹性常数的压力导数已确定为80 GPa。我们的张量正确预测了已发布的沿a轴和c轴的线性可压缩性的实验测量结果，c / a轴向比的对数压力导数和零压力下的体积模量。它与（Liu，L.-G.，Okamoto，K.，Yang，YJ，Chen，C.-C.，Lin，C.-C.，2004.报告的实验测量的弹性张量相矛盾。单晶的弹性布里渊光谱法测定的D相（致密的含水硅酸镁），Solid State Commun。132，517-520），无法再现实验的可压缩性数据。使用预测为85 GPa的弹性常数和密度，已计算出P波和S波的地震各向异性是压力的函数。单晶在20 GPa以下具有较高的P波速度和在基面上的剪切波分裂。在高压下，最大P波速度具有垂直于基面中a轴的三重对称性，而最大剪切波分裂平行于具有双重对称性的a轴。 D相在环境压力下具有较高的Vp和Vs各向异性，Vp为17.6％，Vs为19.9％。 Vp各向异性随压力增加到约30 GPa而减小，而从30 GPa到85 GPa则保持恒定在8％。 Vs各向异性在20 GPa时降低到17％，然后在85 GPa时增加到22％。根据压力计算出S与P的各向同性速度比（Rs / p），S的体声比（R phi / s）和S的密度比（Rp / s）。 Rs / p在低值为0.6的压力下显示很小的变化，而R phi / s和Rp / s随压力在大约60 GPa时分别增加到2.8和2.0的峰值。在环太平洋俯冲的水平“停滞”板中存在D相，可能有助于许多地震观测，例如在500至1000 km深度处观测到的速度非均质性以及水平极化在水平方向传播的快速S波在这个深度范围内。此外，与无水矿物相比，D相的较低密度可能会影响水合“停滞”板的浮力。在热退火过程中，D相转变为钙钛矿加铁硬脂酸酶可能是水平平板地震发生的原因。 （c）2007年由Elsevier B.V.

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