Lunar glasses of impact origin: Physical and chemical characteristics and geologic implications

Lunar glasses of impact origin are significant components of the fines and microbreccias collected by the Apollo 11 and 12 missions. Occurrences, relative abundance, optical properties, and chemical composition were studied extensively in order to assess the importance of lunar glasses with respect to the bulk composition, apparent age, and origin of the regolith. Only a small fraction of the glasses are homogeneous. They occur as well-formed particles or as angular fragments in fine fines and breccias. The homogeneous glasses show a wide range of compositions, but these compositions tend to group; it is likely that they represent essentially the composition of the parent materials. Heterogeneous impact glasses are characterized by pronounced schlieren, abundant vesicles, and inclusions of mineral fragments and small nickel-iron particles. Their chemical compositions vary but the range is restricted. They are probably derived by impact fusion of breccias or fines. Analyses of heterogeneous crater glass lining small pit-craters in a crystalline rock and in a microbreccia show that the composition of the crater glass is highly dependent on the crater size relative to the crystal or fragment size of the substrate; volatilization of major elements during fusion is negligible or minor. Mixing calculations show that the bulk chemical composition of the Apollo 11 breccias and fine fines can be obtained by combining 66–82 wt % ilmenite basalt, 17–28% plagioclase-rich glass, and 1.5–6% olivine-rich glass, and the latter two estimates represent minimum amounts of added material. Because of this large contribution of impact glasses of diverse parent materials to the breccias and fine fines, it is likely that the glasses play a critical role in the explanation of the isotopic concentrations of Rb, Sr, U, Th, and Pb that determine the apparent model age of the lunar regolith.