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First-principles constraints on diffusion in lower-mantle minerals and a weak D' layer

机译：第一性原理限制下地幔矿物和弱D'层的扩散

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摘要

Post-perovskite MgSiO_3 is believed to be present in the D" region of the Earth's lowermost mantle. Its existence has been used to explain a number of seismic observations, such as the D" reflector and the high degree of seismic anisotropy within the D" layer. Ionic diffusion in post-perovskite controls its viscosity, which in turn controls the thermal and chemical coupling between the core and the mantle, the development of plumes and the stability of deep chemical reservoirs. Here we report the use of first-principles methods to calculate absolute diffusion rates in post-perovskite under the conditions found in the Earth's lower mantle. We find that the diffusion of Mg~(2+) and Si~(4+) in post-perovskite is extremely anisotropic, with almost eight orders of magnitude difference between the fast and slow directions. If post-perovskite in the D" layer shows significant lattice-preferred orientation, the fast diffusion direction will render post-perovskite up to four orders of magnitude weaker than perovskite. The presence of weak post-perovskite strongly increases the heat flux across the core-mantle boundary and alters the geotherm. It also provides an explanation for laterally varying viscosity in the lowermost mantle, as required by long-period geoid models. Moreover, the behaviour of very weak post-perovskite can reconcile seismic observation of a D" reflector with recent experiments showing that the width of the perovskite-to-post-perovskite transition is too wide to cause sharp reflectors. We suggest that the observed sharp D" reflector is caused by a rapid change in seismic anisotropy. Once sufficient perovskite has transformed into post-perovskite, post-perovskite becomes interconnected and strain is partitioned into this weaker phase. At this point, the weaker post-perovskite will start to deform rapidly, thereby developing a strong crystallographic texture. We show that the expected seismic contrast between the deformed perovskite-plus-post-perovskite assemblage and the overlying iso-tropic perovskite-plus-post-perovskite assemblage is consistent with seismic observations.
机译：钙钛矿后MgSiO_3被认为存在于地球最下层地幔的D“区。它的存在已被用来解释许多地震观测，例如D”反射镜和D“内高度的地震各向异性。钙钛矿后的离子扩散控制其粘度，进而控制岩心与地幔之间的热化学耦合，羽状流的发育以及深层化学储层的稳定性，在此我们报告第一原理方法的使用在地球下地幔条件下计算钙钛矿后的绝对扩散速率，我们发现钙钛矿后Mg〜（2+）和Si〜（4+）的扩散是极各向异性的，几乎有8个数量级如果在D“层中的后钙钛矿显示出明显的晶格优先取向，则快速扩散方向将使后钙钛矿达到四个数量级比钙钛矿弱。弱钙钛矿的存在会极大地增加整个岩心-地幔边界的热通量，并改变地热。它还提供了根据长期大地水准面模型的要求，在最下部地幔中横向改变粘度的解释。此外，非常弱的钙钛矿后的行为可以协调D“反射镜的地震观测，最近的实验表明钙钛矿到钙钛矿后过渡的宽度太宽，无法形成尖锐的反射镜。我们建议观察到的尖锐的D”反射器是由地震各向异性的快速变化引起的。一旦足够的钙钛矿转化为后钙钛矿，后钙钛矿将相互连接，并且应变被分配到该较弱的相中。在这一点上，较弱的钙钛矿后将开始迅速变形，从而形成很强的晶体织构。我们表明，变形钙钛矿加钙钛矿后组合与上覆各向同性钙钛矿加钙钛矿后组合之间的预期地震对比与地震观测一致。

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  • 来源
    《Nature》 |2010年第7297期|p.462-465|共4页
  • 作者

    M. W. Ammann; J. P. Brodholt; J. Wookey; D. P. Dobson;

  • 作者单位

    Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK;

    Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK;

    Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS6 6LL, UK;

    Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK;

  • 收录信息 美国《科学引文索引》(SCI);美国《工程索引》(EI);美国《生物学医学文摘》(MEDLINE);美国《化学文摘》(CA);
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  • 正文语种 eng
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