Anisotropic diffusion creep in postperovskite provides a new model for deformation at the core-mantle boundary
Anisotropic diffusion creep in postperovskite provides a new model for deformation at the core-mantle boundary
复制标题
后钙钛矿中的各向异性扩散蠕变为核幔边界变形提供了新模型
DOI：
10.1073/pnas.1914826116
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发表时间：
2019
期刊：
影响因子：
--
通讯作者：
Dobson D
中科院分区：
文献类型：
--
作者：
Dobson D
在地核-地幔边界以上的地幔最下端（D″）表现出强烈的地震各向异性等异常地震特征，这与主要矿物mgsio3后钙钛矿的性质有关。但是，经过十多年的研究，地震观测结果仍然不能简单地用假设钙钛矿中位错蠕变的流动模型来解释。我们通过实验和从头算模拟研究了钙钛矿和后钙钛矿相的化学扩散系数，并推导了观察到的各向异性扩散蠕变方程。在所研究的所有化学体系中，两相的实验和模拟结果非常吻合。实验和模拟结果表明，在氟化锌中，钙钛矿的单晶扩散率表现出至少3个数量级的各向异性（Da= 1000db;Db≈Dc），而在天然mgsio3体系中，单晶扩散率表现出更极端的各向异性（Da= 10000dc;Dc= 10000db）。各向异性化学扩散率导致各向异性扩散蠕变、织构生成和应变弱化流变。mgsio3后钙钛矿的结果强烈表明，在地球D″区域内，以后钙钛矿为主的区域将1)比以钙钛矿为主的区域弱得多；2)形成应变诱导的晶体优先取向，具有应变弱化流变学。这导致应变局部化，并有可能使具有显著不同纹理的区域在狭窄剪切带上通过应变接近。因此，各向异性扩散蠕变为观测到的地震各向异性的复杂性和D″地震速度的快速横向变化提供了一个有吸引力的替代解释。
The lowermost portion of Earth’s mantle (D″) above the core−mantle boundary shows anomalous seismic features, such as strong seismic anisotropy, related to the properties of the main mineral MgSiO3postperovskite. But, after over a decade of investigations, the seismic observations still cannot be explained simply by flow models which assume dislocation creep in postperovskite. We have investigated the chemical diffusivity of perovskite and postperovskite phases by experiment and ab initio simulation, and derive equations for the observed anisotropic diffusion creep. There is excellent agreement between experiments and simulations for both phases in all of the chemical systems studied. Single-crystal diffusivity in postperovskite displays at least 3 orders of magnitude of anisotropy by experiment and simulation (Da= 1,000Db;Db≈Dc) in zinc fluoride, and an even more extreme anisotropy is predicted (Da= 10,000Dc;Dc= 10,000Db) in the natural MgSiO3system. Anisotropic chemical diffusivity results in anisotropic diffusion creep, texture generation, and a strain-weakening rheology. The results for MgSiO3postperovskite strongly imply that regions within the D″ region of Earth dominated by postperovskite will 1) be substantially weaker than regions dominated by perovskite and 2) develop a strain-induced crystallographic-preferred orientation with strain-weakening rheology. This leads to strain localization and the possibility to bring regions with significantly varying textures into close proximity by strain on narrow shear zones. Anisotropic diffusion creep therefore provides an attractive alternative explanation for the complexity in observed seismic anisotropy and the rapid lateral changes in seismic velocities in D″.