The nature of the Moon's surface: evidence from shock metamorphism in Apollo 11 and 12 samples

Abstract

Evidence of shock effects resulting from impact induced pressures (hundreds of kilobars range) and temperatures occurs in varying abundance in the lunar soil and microbreccias collected around the Apollo 11 site. These effects include: (1) multiple sets of planar features in silica phase(s) and plagioclase, (2) planar features (?), shock induced twin or deformation lamellae, and kink bands in clinopyroxenes, (3) thetomorphic plagioclase glass (maskelynite) in crystalline rock fragments within the breccias, (4) isotropization or partial decomposition of clinopyroxenes leading to anomalously low refractive indices with respect to initial compositions, (5) partial fusion of mineral and rock fragments; heterogeneous glasses; glass spherules (some with NiFe inclusions) of both mineral and rock compositions, (6) variable loss of Na and Ca in shocked feldspar samples, and (7) quench crystals in shock melted glasses and extensive recrystallization of these and thetomorphic glass phases. The three Apollo 12 samples thus far examined show less diversity of shock effects and the average intensity of shock damage is reduced. Planar featuers were not found and the number of glass spherules per unit volume has apparently decreased. These observations suggest that the regolith at the 12 site is younger and thus has been subjected to fewer impacts, which also accounts for its lesser apparent thickness compared with the 11 site. Microbreccias at both sites appear to derive by shock lithification of the lunar soil to produce a variably cohesive, in places glass cemented rock. Within both soil and microbreccias, rare fragments consisting mainly of anorthite-rich (An 85–95) feldspar crystals may be microanorthosites introduced from nearby sources or from more distant highlands areas. Some of these fragments appear to be microanorthosite masses granulated by shock into coherent aggregates. Several, however, seemingly consist of once loose feldspar grains which had comprised a regolithic layer originated from impact comminution of anorthosite hardrock units probably located in the lunar highlands that was then shock lithified during later impacts and ejected to the 11 and 12 sites. Implications of shock metamorphism in these lunar materials relative to the origin of the Moon's circular structures, the regolith on the maria, and the microbreccias, respectively, are considered mainly from the viewpoint of Apollo 11 data.