Where Is the Lunar Mantle and Deep Crust at Crisium? A Perspective From the Luna 20 Samples

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 mu m) with inclusions of pyroxene, and finer-grained norites, troctolites, spinel troctolites, and gabbros (<100 mu m). 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 (Mg-suite) and lunar magma ocean products (FANs). 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. Plain Language Summary In 1972, the Union of Soviet Socialist Republics space program launched and successfully return to Earth a robotic sample return mission to the highland terrain adjacent to the Crisium impact basin. This mission returned 50 g of lunar soil. This study integrates data collected from orbit, previously collected data, and new sample data to better place the Luna 20 samples within the context of both new lunar science concepts and regional geology. Crystalline magmatic rocks from the Luna 20 site are primarily ferroan anorthosites (FANs), products of the lunar magma ocean, and Mg-suite rocks that post-dated the formation of the FANs. Both rock types represent crystallization in the intermediate to shallow lunar crust. These observation contrasts with current basin forming concepts for the Crisium basin that suggest deep crust and mantle were excavated during this event. We conclude that either the material associated with the Crisium basin is not derived from the lunar mantle, a substantial mantle component had been incorporated into the impact melt sheet associated with the Crisium basin, or that our "Earth-analog" for the upper mantle of the Moon is incorrect.