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.

中文翻译：

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俯冲的洋壳是地震作用较慢的下地幔构造的起源

地幔层析成像揭示了在地幔底部存在两个大的低剪切速度省（LLSVP）。在这里，我们检验以下假设：它们是成堆的洋壳，它们稳定地积累并加热了数十亿年。我们使用现有的全球地球动力学模型，在该模型中，致密的洋壳形成于不同的板块边界处，而俯冲则位于会聚的板块边界处。该模型套件涵盖了较低地幔条件下洋壳的预计密度范围。为了将地球动力学模型与层析成像结构进行有意义的比较，我们将其转换为地震波速模型，并明确考虑了层析成像的有限解析能力。我们的结果表明，长期回收致密的洋壳自然会导致形成具有类似于LLSVPs地震特征的热化学桩。大洋地壳对低空偏角卫星的贡献程度取决于缺乏准确数据的下地幔的密度。我们发现，LLSVPs并非仅由洋壳组成。相反，它们的底部（底部100-200 km）富含玄武岩，并且由于温度的主导作用而具有其地震信号的强度，因而向其侧面和顶部逐渐变成橄榄岩。我们得出的结论是，如果回收的地壳足够致密，则会对地幔的热化学演化产生强烈影响。大洋地壳对低空卫星速度的贡献程度取决于缺乏准确数据的下地幔的密度。我们发现，LLSVPs并非仅由洋壳组成。相反，它们的底部（底部100-200 km）富含玄武岩，并且由于温度的主导作用而具有其地震信号的强度，从而向其侧面和顶部逐渐成为橄榄岩。我们得出的结论是，如果回收的地壳足够致密，则会对地幔的热化学演化产生强烈影响。大洋地壳对低空偏角卫星的贡献程度取决于缺乏准确数据的下地幔的密度。我们发现，LLSVPs并非仅由洋壳组成。相反，它们的底部（底部100-200 km）富含玄武岩，并且由于温度的主导作用而具有其地震信号的强度，因而向其侧面和顶部逐渐变成橄榄岩。我们得出的结论是，如果回收的地壳足够致密，则会对地幔的热化学演化产生强烈影响。它们的底部（底部100-200 km）富含玄武岩，并且由于温度的主导作用而产生的地震信号强度使它们在其侧面和顶部都变成橄榄岩。我们得出的结论是，如果回收的地壳足够致密，则会对地幔的热化学演化产生强烈影响。它们的底部（底部100-200 km）富含玄武岩，并且由于温度的主导作用而产生的地震信号强度使它们在其侧面和顶部都变成橄榄岩。我们得出的结论是，如果回收的地壳足够致密，则会对地幔的热和化学演化产生强烈影响。