Geophysical analysis of the Earth's lower mantle has revealed the presence of two superstructures characterized by low shear wave velocities on the core-mantle boundary. These Large Low Shear Velocity Provinces (LLSVPs) play a crucial role in the dynamics of the lower mantle and act as the source region for deep-seated mantle plumes. However, their origin, and the characteristics of the surrounding deep mantle, remain enigmatic. Mantle plumes located above the margins of the LLSVPs display evidence for the presence of this deep-seated, thermally and/or chemically heterogeneous mantle material ascending into the melting region. As a result, analysis of the spatial geochemical heterogeneity in ocean island basalts provides constraints on the structure of the Earth's lower mantle and the origin of the LLSVPs. In this study, we focus on the Galápagos Archipelago in the eastern Pacific, where bilateral asymmetry in the radiogenic isotopic composition of erupted basalts has been linked to the presence of LLSVP material in the underlying plume. We show, using spatial variations in the major element contents of high-MgO basalts, that the isotopically enriched south-western region of the Galápagos mantle—assigned to melting of LLSVP material—displays no evidence for lithological heterogeneity in the mantle source. As such, it is unlikely that the Pacific LLSVP represents a pile of subducted oceanic crust. Clear evidence for a lithologically heterogeneous mantle source is, however, found in the north-central Galápagos, indicating that a recycled crustal component is present near the eastern margin of the Pacific LLSVP, consistent with seismic observations.

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地球深部地幔结构的地球化学约束和 LLSVPs 的起源

对地球下地幔的地球物理分析揭示了在地核-地幔边界上存在两个以低剪切波速度为特征的上层结构。这些大的低剪切速度省（LLSVPs）在下地幔的动力学中起着至关重要的作用，并作为深部地幔柱的源区。然而，它们的起源以及周围深部地幔的特征仍然是个谜。位于 LLSVP 边缘上方的地幔柱显示出存在这种深层、热和/或化学异质地幔物质上升到熔化区的证据。因此，对海岛玄武岩空间地球化学异质性的分析提供了对地球下地幔结构和 LLSVPs 起源的限制。在这项研究中，我们关注东太平洋的加拉帕戈斯群岛，那里喷发玄武岩放射性同位素组成的双边不对称性与底层羽流中 LLSVP 物质的存在有关。我们利用高 MgO 玄武岩的主要元素含量的空间变化表明，加拉帕戈斯地幔的同位素富集西南地区（指定为 LLSVP 材料的熔化）没有显示地幔源岩性非均质性的证据。因此，太平洋 LLSVP 不太可能代表一堆俯冲洋壳。然而，在加拉帕戈斯中北部发现了岩性异质地幔源的明确证据，表明在太平洋 LLSVP 东缘附近存在一个再循环的地壳成分，这与地震观测结果一致。