Evidence For A Stratified Upper Mantle Preserved Within The South Pole-Aitken Basin

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.Plain Language Summary Like the Earth, the Moon is layered into a crust and mantle. The Moon's layering was shaped by an early global melting event known as the "Lunar Magma Ocean." As the magma ocean solidified, dense minerals sank to form the mantle, while less-dense minerals floated to form the crust. Elements such as thorium are not easily incorporated into mineral structures, and remain in the liquid. Because of this, a thorium-rich dreg layer was sandwiched between the crust and mantle. These dregs are very dense and are expected to sink into the underlying mantle during or soon after crystallization. We demonstrate that the Moon's largest and oldest impact basin excavated material from this dense, thorium-rich layer before it sank. The exposed material was then diluted and obscured by four billion years of impact cratering and volcanic eruptions. However, we identify several pristine exposures created by recent craters. The impact basin also melted rocks from greater depths than the rocks it ejected. These melted rocks exhibit a much different composition. This indicates that the lunar upper mantle included two compositionally distinct layers that were exposed in different ways by this large impact event. These results have important implications for understanding the formation and evolution of the Moon.Key PointsListThe Th distribution in the South Pole-Aitken Basin is consistent with mantle-derived ejecta influenced by subsequent geologic eventsTh-bearing materials exhibit a Ti-bearing gabbronoritic composition consistent with globally distributed late lunar magma ocean cumulatesSouth Pole-Aitken impact melt is low-Ca pyroxene-rich and low-Th, suggesting stratification in the upper mantle at the time of formation