  Lunar impact glasses, quenched melts produced during cratering events on the
Moon, have the potential to provide not only compositional information about
both the local and regional geology of the Moon but also information about the
impact flux over time. We present in this paper the results of 73 new 40Ar/39Ar
analyses of well-characterized, inclusion-free lunar impact glasses and
demonstrate that size, shape, chemical composition, fraction of radiogenic 40Ar
retained, and cosmic ray exposure (CRE) ages are important for 40Ar/39Ar
investigations of these samples. Specifically, analyses of lunar impact glasses
from the Apollo 14, 16, and 17 landing sites indicate that retention of
radiogenic 40Ar is a strong function of post-formation thermal history in the
lunar regolith, size, and chemical composition. Based on the relationships
presented in this paper, lunar impact glasses with compositions and sizes
sufficient to have retained 90% of their radiogenic Ar during 750 Ma of cosmic
ray exposure at time-integrated temperatures of up to 290 K have been
identified and are likely to have yielded reliable 40Ar/39Ar ages of formation.
Additionally, ~50% of the identified impact glass spheres have formation ages
of <500 Ma, while ~75% of the identified lunar impact glass shards and spheres
have ages of formation <2000 Ma. The observed age-frequency distribution of
lunar impact glasses may reflect two processes: (i) diminished preservation due
to spontaneous shattering with age; and (ii) preservation of a remnant
population of impact glasses from the tail end of the terminal lunar
bombardment having 40Ar/39Ar ages up to 3800 Ma. A protocol is described for
selecting and analyzing lunar impact glasses.
