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Journal of Geophysical Research: Planets
Volume 126, Issue 1 e2020JE006589
Research Article

Evidence for a Stratified Upper Mantle Preserved Within the South Pole‐Aitken Basin

D. P. Moriarty III

Corresponding Author

NASA Goddard Space Flight Center, Greenbelt, MD, USA

University of Maryland, College Park, MD, USA

Correspondence to:

D. P. Moriarty III,

daniel.p.moriarty@nasa.gov

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R. N. Watkins

Planetary Science Institute, Tucson, AZ, USA

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S. N. Valencia

NASA Goddard Space Flight Center, Greenbelt, MD, USA

University of Maryland, College Park, MD, USA

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J. D. Kendall

NASA Goddard Space Flight Center, Greenbelt, MD, USA

University of Maryland Baltimore County, Baltimore, MD, USA

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A. J. Evans

Brown University, Providence, RI, USA

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N. Dygert

University of Tennessee, Knoxville, TN, USA

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N. E. Petro

NASA Goddard Space Flight Center, Greenbelt, MD, USA

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First published: 18 December 2020
https://doi.org/10.1029/2020JE006589

Abstract

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.

Data Availability Statement

All data and methods needed to understand, evaluate, and build upon this research are publicly available and documented through a combination of publicly accessible repositories and prior publications. Sources include:

  • Moon Mineralogy Mapper Level 2 reflectance data (available through the Planetary Data System Imaging Node: https://pds-imaging.jpl.nasa.gov/volumes/m3.html)

  • M3 parameter maps were generated from Level 2 reflectance data using the Parabolas and two‐part Linear Continuum (PLC) method (Moriarty & Pieters, 2016)

  • Spectral absorption band values for pure pyroxenes (Klima et al., 2007, 2011)

  • Lunar Prospector thorium, iron, titanium, and potassium abundance maps (https://pds-geosciences.wustl.edu/missions/lunarp/reduced_special.html)

  • Lunar Reconnaissance Orbiter Camera Global Morphological Map (Speyerer et al., 2011) and TiO2 abundances (Sato et al., 2017)

  • Lunar Orbiter Laser Altimeter topography data (https://pds-geosciences.wustl.edu/missions/lro/lola.htm.)

  • Mantle‐derived SPA ejecta model (Melosh et al., 2017)

  • LOLA Large Lunar Crater Catalog (Head et al., 2010; Kadish et al, 2011)

  • Lunar mare boundaries (Nelson et al., 2014)

  • South Pole‐Aitken Compositional Anomaly boundaries (Moriarty & Pieters, 2018)

  • Lunar sample compositional data from the Lunar Rock and Mineral Characterization Consortium (Isaacson et al., 2013)

  • Lunar Magma Ocean crystallization model compositional estimates (Elkins‐Tanton et al., 2011)

  • SELENE Lunar Magnetometer data (Tsunakawa et al., 2010)

  • Derived data products including band depths, band centers, and spectra are available through a FAIR‐enabling data repository (Moriarty et al., 2020)