Subducted oceanic crust as the origin of seismically slow lower-mantle structures
Subducted oceanic crust as the origin of seismically slow lower-mantle structures
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DOI：
10.1186/s40645-020-00327-1
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发表时间：
2020-05-21
影响因子：
3.9
通讯作者：
Koelemeijer, Paula
中科院分区：
文献类型：
--
作者：
Jones, Timothy D.;Maguire, Ross R.;Koelemeijer, Paula
地幔断层扫描揭示了地幔底部存在两个大型低剪切速度省（LLSVP）。我们在这里检验这样一个假设：它们是一堆海洋地壳，经过数十亿年的不断积累和变暖。我们使用现有的全球地球动力学模型，其中致密的洋壳在发散的板块边界处形成，并在汇聚的板块边界处形成俯冲带。该模型套件涵盖了下地幔条件下洋壳的预测密度范围。为了有意义地将我们的地球动力学模型与层析成像结构进行比较，我们将它们转换为地震波速模型，并明确考虑层析成像的有限分辨率。我们的结果表明，致密洋壳的长期循环自然会导致具有与 LLSVP 相似的地震特征的热化学堆的形成。洋壳对 LLSVP 的贡献程度取决于其在下地幔中的密度，而下地幔中缺乏准确的数据。我们发现 LLSVP 不仅仅由洋壳组成。相反，它们的底部（底部 100-200 公里）富含玄武岩，并向其侧面和顶部分级为橄榄岩，其地震特征的强度源于温度的主导作用。我们得出的结论是，如果洋壳的再循环足够致密，就会对地幔的热和化学演化产生强烈影响。
Mantle tomography reveals the existence of two large low-shear-velocity provinces (LLSVPs) at the base of the mantle. We examine here the hypothesis that they are piles of oceanic crust that have steadily accumulated and warmed over billions of years. We use existing global geodynamic models in which dense oceanic crust forms at divergent plate boundaries and subducts at convergent ones. The model suite covers the predicted density range for oceanic crust over lower mantle conditions. To meaningfully compare our geodynamic models to tomographic structures, we convert them into models of seismic wavespeed and explicitly account for the limited resolving power of tomography. Our results demonstrate that long-term recycling of dense oceanic crust naturally leads to the formation of thermochemical piles with seismic characteristics similar to the LLSVPs. The extent to which oceanic crust contributes to the LLSVPs depends upon its density in the lower mantle for which accurate data is lacking. We find that the LLSVPs are not composed solely of oceanic crust. Rather, they are basalt rich at their base (bottom 100-200 km) and grade into peridotite toward their sides and top with the strength of their seismic signature arising from the dominant role of temperature. We conclude that recycling of oceanic crust, if sufficiently dense, has a strong influence on the thermal and chemical evolution of Earth's mantle.