Multiple Shallow Crustal Origins for Spinel-Bearing Lithologies on the Moon: A Perspective From the Luna 20 Mission

The Luna 20 mission returned samples from the Hilly and Furrowed Terrain of the Moon that is associated with the impact event that formed the Crisium Basin. This event potentially excavated deep crustal and upper mantle lithologies. Spinel is commonly considered to be a mineralogical indicator of rocks of high-pressure origin, and orbital data indicate the presence of spinel-bearing lithologies exterior to the basin. We have examined 166 Luna 20 particles in the 250-500 mu m size range and found 31 spinel-bearing fragments. Of these 10 are igneous plutonic Mg-suite rocks, most of which are troctolitic, and 16 are impact melt rocks. The other five are fused soil or devitrified glass fragments. The spinel-bearing lithic fragments are plagioclase-rich and do not have the high modal abundances of Mg-Al spinel previously identified in the region through remote sensing analyses. The textures, compositions, and inferred crystallization sequences of the present magmatic spinel-bearing samples are most consistent with a relatively shallow crustal (rather than a deep crustal) origin, with a petrogenesis involving assimilation of ferroan anorthosite crust by Mg-rich, mantle-derived magmas. Both relict and newly formed spinels are found in impact melt rocks and are also inferred to have formed at relatively low pressures. Thus, the presence of spinel is not an unambiguous indicator of mantle or deep crustal material. The insights gained from this study show that studies of a small, robotically collected sample can improve our understanding of regional lithologies and petrologic processes.