Phase stability and elastic properties of the NAL and CF phases in the NaMg2Al5SiO12 system from first principles
Phase stability and elastic properties of the NAL and CF phases in the NaMg2Al5SiO12 system from first principles
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DOI：
10.2138/am.2012.3915
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发表时间：
2012-02
影响因子：
3.1
通讯作者：
K. Kawai;T. Tsuchiya
中科院分区：
文献类型：
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作者：
K. Kawai;T. Tsuchiya
新六方铝（NAL）相和正交钙铁氧体（CF）型相被认为是下地幔压力和温度条件下洋中脊玄武岩（MORB）的主要地幔组分，它们可能含有大离子半径的碱元素。因此，NAL相和CF相的高压稳定性和弹性特性对于了解MORB俯冲的命运至关重要。本文报道了用第一性原理计算方法研究的NaMg2Al5SiO12体系的这些性质。NAL在39.6 GPa时转变为CF相，并伴有密度（+1.8%）、纵波（-0.2%）、横波（+0.9%）和体声（-1.0%）速度的不连续。利用这些结果对俯冲MORB的性质进行了评价，发现~ -5%的软锰矿和MORB的速度对比与在1100 ~ 1800 km深度的地震散射体观测到的剪切速度异常相当。然而，由于MORB内NAL相向CF相的转变对地震速度的增加不显著，因此在地震学上无法检测到。另一方面，与相变相关的各向异性变化是显著的，并且可以通过诸如剪切波分裂测量等观测来地震检测，因为CF相位与NAL相位相比具有更大的各向异性。
Abstract New hexagonal aluminous (NAL) phase and orthorhombic calcium-ferrite (CF) type phase are considered to be major mantle components of the mid-ocean ridge basalt (MORB) at pressure and temperature conditions in the lower mantle, which can potentially host alkali elements with large ionic radii. The high-pressure stability and elastic properties of both NAL and CF phases are therefore of fundamental importance for understanding the fate of subducted MORB. Here we report those properties of the NaMg2Al5SiO12 system studied by means of the first-principles computation method. NAL was found to transform to the CF phase at 39.6 GPa, accompanied by discontinuities in density (+1.8%), as well as compressional wave (-0.2%), shear wave (+0.9%), and bulk sound (-1.0%) velocities. The property of subducted MORB was evaluated using these results, and the velocity contrast between pyrolite and MORB of ~ -5% was found to be quite comparable to the shear velocity anomaly observed for seismic scatterers at depths from 1100 to 1800 km. However, since the transformation of the NAL to CF phase within MORB produces insignificant increases in the seismic velocities, it would be seismologically undetectable. On the other hand, the anisotropy change associated with the phase transition is significant and could be seismically detectable using observations such as shear wave splitting measurements since the CF phase is considerably more anisotropic compared to the NAL phase.