Where is the Lunar Mantle and Deep Crust at Crisium? A Perspective from the Luna 20 Samples

1,2,3C.K. Shearer,4,5,6D.P. Moriarty,1S.B. Simon,4N. Petro,1,2J.J. Papike
Journal Geophysical Research (Planets) Link to Article [https://doi.org/10.1029/2022JE007409]
1Institute of Meteoritics, University of New Mexico, Albuquerque, NM, 87131 United States
2Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM, 87131 United States
3Lunar and Planetary Institute, Houston, TX, 77058 United States
4NASA Goddard Space Flight Center, Greenbelt, MD, 20771 United States
5University of Maryland, College Park, MD, 20742 United States
6Center for Research and Exploration in Space Science and Technology, College Park, MD, 20742 United States
Published by arrangement with Jon Wiley & Sons

Remote sensing observations have been interpreted to indicate that the Crisium basin-forming event excavated deep crust and upper mantle. Samples from the highlands adjacent to the Crisium basin returned by Luna 20 (L-20) bring a unique perspective for evaluating this concept. The magmatic lithologies returned from the noritic Hilly and Furrowed Terrain (nHFT) by L-20 are coarse-grained feldspar (>300 µm) with inclusions of pyroxene, and a finer-grained norites, troctolites, spinel troctolites, and gabbros (<100 µm). These two suites represent ferroan anorthosites (FANs) and the Mg-suite, respectively. There is limited evidence for mantle or deep crustal material within the nHFT samples. Ultramafic rocks such as dunites and orthopyroxenites are absent, and Mg-rich olivine- and orthopyroxene-bearing-assemblages are derived from magmatic rocks emplaced in the shallow crust. These lithic fragments represent pre-Crisium episodes of magmatism and lunar magma ocean products. The lack of deep lithologies at the L-20 site seems contradictory to excavation models for Crisium. Mineralogical-chemical differences suggest a higher FAN component in the rim and that this represents FANs excavated from the deep lunar crust. If it exists, the Mg-rich olivine previously identified within the Crisium rim is most likely related to deep, complementary versions of the Mg-suite rocks from L-20. The material associated with the Crisium basin is not derived from the lunar mantle but represents crustal lithologies from the shallow to deep crust, a substantial mantle component may have been incorporated into the Crisium basin impact melt sheet, or that our “Earth-analog” for the lunar upper mantle is incorrect.


Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s