Determining the composition of Earth's lower mantle, which constitutes almost half of its total volume, has been a central goal in the Earth sciences for more than a century given the constraints it places on Earth's origin and evolution. However, whether the major element chemistry of the lower mantle, in the form of, e.g., Mg/Si ratio, is similar to or different from the upper mantle remains debated. Here we use a multidisciplinary approach to address the question of the composition of Earth's lower mantle and, in turn, that of bulk silicate Earth (crust and mantle) by considering the evidence provided by geochemistry, geophysics, mineral physics, and geodynamics. Geochemical and geodynamical evidence largely agrees, indicating a lower-mantle molar Mg/Si of ≥1.12 (≥1.15 for bulk silicate Earth), consistent with the rock record and accumulating evidence for whole-mantle stirring. However, mineral physics–informed profiles of seismic properties, based on a lower mantle made of bridgmanite and ferropericlase, point to Mg/Si ∼ 0.9–1.0 when compared with radial seismic reference models. This highlights the importance of considering the presence of additional minerals (e.g., calcium-perovskite and stishovite) and possibly suggests a lower mantle varying compositionally with depth. In closing, we discuss how we can improve our understanding of lower-mantle and bulk silicate Earth composition, including its impact on the light element budget of the core. ▪The chemical composition of Earth's lower mantle is indispensable for understanding its origin and evolution.▪Earth's lower-mantle composition is reviewed from an integrated mineral physics, geophysical, geochemical, and geodynamical perspective.▪A lower-mantle molar Mg/Si of ≥1.12 is favored but not unique.▪New experiments investigating compositional effects of bridgmanite and ferropericlase elasticity are needed to further our insight.

中文翻译：

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地球下地幔的成分


一个多世纪以来，鉴于地球起源和演化的限制，确定地球下地幔的成分（几乎占地球总体积的一半）一直是地球科学的中心目标。然而，下地幔的主要元素化学性质（例如 Mg/Si 比率）是否与上地幔相似或不同仍存在争议。在这里，我们采用多学科方法，通过考虑地球化学、地球物理学、矿物物理学和地球动力学提供的证据，来解决地球下地幔的组成问题，进而解决块状硅酸盐地球（地壳和地幔）的组成问题。地球化学和地球动力学证据基本一致，表明下地幔摩尔 Mg/Si 为 ≥1.12（块状硅酸盐地球为 ≥1.15），与岩石记录一致，并积累了全地幔搅拌的证据。然而，与径向地震参考模型相比，基于桥镁矿和铁方镁石制成的下地幔的矿物物理学地震特性剖面表明 Mg/Si ∼ 0.9-1.0。这突出了考虑其他矿物（例如钙钛矿和钙钙钛矿）存在的重要性，并可能表明下地幔的成分随深度而变化。最后，我们讨论了如何提高我们对下地幔和块状硅酸盐地球成分的理解，包括它对地核轻元素预算的影响。▪地球下地幔的化学成分对于理解其起源和演化是必不可少的。▪从矿物物理学、地球物理学、地球化学和地球动力学的综合角度回顾了地球的下地幔组成。▪≥1.12 的下地幔摩尔 Mg/Si 是有利的，但不是唯一的。▪需要新的实验来研究桥镁石和铁方镁石弹性的组成效应，以进一步深入我们的见解。