Evidence for a Stratified Upper Mantle Preserved within the South Pole - Aitken Basin

• Published in: Journal of geophysical research. Planets 2021-01, Vol.126 (1), p.n/a
• Main Authors: D P Moriarty, III, Watkins, R N, Valencia, S N, Kendall, J D, Evans, A J, Dygert, N, Petro, N E
• Format: Article
• Language: English
• Subjects: Bearing materials; Composition; Crystallization; Earth crust; Ejecta; Evolution; Exposure; Ilmenite; Impact melts; Lunar And Planetary Science And Exploration; Lunar craters; lunar dichotomy; Lunar evolution; lunar magma ocean; Lunar mantle; lunar prospector; Lunar surface; Magma; Mineralogy; Minerals; Moon; moon mineralogy mapper; Ocean models; Oceans; Resurfacing; Rocks; South Pole; South Pole‐Aitken Basin; Stratigraphy; Surfacing; Thorium; Upper mantle; Volcanic craters; Volcanic eruption effects; Volcanic eruptions
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2020JE006589

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 latestage 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.