Luna 20 spinel-bearing lithic fragments

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 µm 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 the 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.月球20号（Luna 20）任务采集了月球丘陵沟壑地形的样本，该地形与形成澄海盆地（Crisium Basin）的撞击事件密切相关。此次撞击事件可能挖掘出了深部地壳与上地幔的岩性物质。尖晶石（spinel）通常被认为是高压成因岩石的矿物学指示标志，轨道探测数据显示该盆地外部存在含尖晶石的岩性物质。本研究对166颗粒径介于250~500微米的月球20号样本颗粒开展了分析，共发现31颗含尖晶石的碎屑。其中10颗为火成深成镁质岩套（Mg-suite rocks）岩石，多数为橄长岩质（troctolitic）岩石，另有16颗为撞击熔融岩。剩余5颗为熔融土壤或脱玻化玻璃碎屑。这些含尖晶石的岩屑以富斜长石为主要特征，并不具备此前通过遥感分析在该区域识别出的高模态含量镁铝尖晶石（Mg-Al spinel）。本次研究中含尖晶石的岩浆成因样本的结构、成分以及推断的结晶序列，与相对浅部地壳（而非深部地壳）的成因最为吻合，其岩石成因涉及富镁地幔源岩浆对铁斜长岩（ferroan anorthosite）地壳的同化作用。在撞击熔融岩中同时发现了残余尖晶石与新生尖晶石，且推断二者均形成于相对低压的环境中。由此可见，尖晶石的存在并非地幔或深部地壳物质的明确指示标志。本研究获得的认识表明，针对小型机器人采集样本的分析，能够增进我们对区域岩性与岩石学过程的理解。