The evolution and compositional structure of the lunar mantle has been extensively modeled but insufficiently constrained by observations. Here, we identify and characterize mantle materials exposed by the Moon's largest impact basin to better understand the composition, stratigraphy, and evolution of the upper mantle. The vast South Pole‐Aitken Basin (SPA) exhibits a broad, crescent‐shaped thorium and potassium distribution. These incompatible elements are predicted to be concentrated in the dregs of the lunar magma ocean during end‐stage crystallization. Through consideration of basin formation models convolved with subsequent geologic evolution, we demonstrate that the distribution and implied stratigraphy of Th‐ and K‐bearing materials across SPA are consistent with an upper mantle ejecta origin. The most pristine exposures of these materials are confined to northwest SPA and also exhibit elevated Ti and Fe (relative to the farside highlands) in association with a gabbronoritic mineralogy. This is consistent with late‐stage magma ocean assemblages predicted by petrologic models. In contrast, SPA impact melt derived from greater depths is associated with a low‐Ca pyroxene‐dominated assemblage. Together, these compositional patterns are evidence for a stratified ancient upper mantle. Importantly, the incompatible‐element‐enriched, ilmenite‐bearing ferroan gabbronoritic cumulates evidently had not participated in gravitational overturn at the time of SPA formation. Contrary to recent hypotheses invoking nearside sequestration of incompatible elements to explain hemispherical differences in crustal building and volcanic resurfacing, it follows that incompatible elements were globally distributed in the magma ocean at the time of SPA formation.

中文翻译：

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在南极-艾特肯盆地内保留的分层上地幔的证据

月球地幔的演化和组成结构已被广泛地模拟，但是没有被观测结果所限制。在这里，我们确定并描述了月球最大撞击盆地暴露的地幔物质，以更好地了解上地幔的组成，地层和演化。广阔的南极-艾特肯盆地（SPA）表现出宽广的新月形shaped和钾分布。预计这些不相容元素将在末期结晶过程中集中在月岩浆海洋的残渣中。通过考虑盆地形成模型以及随后的地质演化，我们证明了整个SPA中含Th和K的物质的分布和隐含地层与上地幔顶射起源一致。这些材料最原始的暴露仅限于西北SPA，并且与辉长岩矿物学有关，还显示出较高的Ti和Fe（相对于远处的高地）。这与岩石学模型预测的晚期岩浆海洋组合是一致的。相反，源自更大深度的SPA冲击熔体与低钙辉石为主的组件有关。这些组合模式一起证明了古老的上地幔分层。重要的是，SPA形成时，不相容元素丰富的，含钛铁矿的铁锰铁累积体显然没有参与重力翻转。与最近的假说相反，这些假说利用不相容元素的近侧隔离来解释地壳构造和火山铺面中的半球形差异，