Krymarys Ewa, Murakami Motohiko, Saha Pinku, Liebske Christian
Department of Earth and Planetary Sciences, Institute of Geochemistry and Petrology, ETH Zürich, Zürich, Switzerland.
Commun Earth Environ. 2025;6(1):713. doi: 10.1038/s43247-025-02700-8. Epub 2025 Aug 28.
Large Low Shear Velocity Provinces (LLSVPs) near the core-mantle boundary (CMB) are key yet enigmatic structures. Their origin is often linked to the accumulation of subducted mid-ocean ridge basalt (MORB), but computational models question MORB as the sole source due to its predicted high shear wave velocity compared to normal mantle. This uncertainty is compounded by the lack of direct sound velocity measurements at CMB pressures. Here we address this gap through ultrahigh-pressure shear wave velocity measurements on CaCl- and α-PbO-type SiO, major phases in MORB, at pressures exceeding those of the CMB. Our results show shear velocities in dense SiO phases are ~ 7-14% lower than previous predictions under these conditions. Incorporating these values into MORB models suggests that the typical seismic anomaly of -1.5% (δln ) observed in LLSVPs can be explained by ~ 23-33 vol.% oceanic crust along a cold slab geotherm, without invoking extreme thermal anomalies (+1500 K). Considering a subduction history exceeding 2 billion years, this scenario supports long-term MORB accumulation at the lowermost mantle. These findings provide new constraints on LLSVP composition and offer critical insights into deep mantle dynamics and the evolution of Earth's interior.
核幔边界（CMB）附近的大型低速剪切波速区（LLSVPs）是关键但神秘的结构。它们的起源通常与俯冲的洋中脊玄武岩（MORB）的堆积有关，但计算模型对MORB作为唯一来源提出质疑，因为与正常地幔相比，其预测的剪切波速度较高。由于在CMB压力下缺乏直接的声速测量，这种不确定性更加复杂。在这里，我们通过对MORB中的主要相CaCl型和α-PbO型SiO₂在超过CMB压力的条件下进行超高压剪切波速度测量来填补这一空白。我们的结果表明，在这些条件下，致密SiO₂相中的剪切速度比先前的预测低约7%-14%。将这些值纳入MORB模型表明，在LLSVPs中观察到的典型-1.5%（δln ）地震异常可以通过沿着冷板地热的约23%-33%体积的洋壳来解释，而无需引入极端热异常（+1500 K）。考虑到超过20亿年的俯冲历史，这种情况支持了MORB在最下地幔的长期堆积。这些发现为LLSVP组成提供了新的限制，并为深部地幔动力学和地球内部演化提供了关键见解。
