Argon diffusion in Apollo 16 impact glass spherules: Implications for 40Ar/39Ar dating of lunar impact events

The 40Ar/39Ar technique applied to impact glass has been used to date both terrestrial and lunar impact events. The ability to utilize the 40Ar/39Ar technique rests on the assumption that impact glasses are closed to the loss of daughter product, 40Ar*, after formation. Diffusion experiments were performed on three Apollo 16 lunar impact glasses and yielded activation energies for 39Ar of similar to 17 to 20 kcal mol(-1) and log(10)(D-0/a(2)) values of -5.2 to -6.0 s(-1). The resulting diffusion coefficients are interpreted as minimum values and the Apollo 16 glass is probably some of the least retentive of lunar glasses, as the degree of non-bridging oxygen is at one end of the range in lunar glasses. At temperatures below the glass transition temperature (i.e., similar to 660 degrees C), the data can be explained by volume diffusion from a single diffusion domain. Modeling shows that Apollo 16 composition glass could lose significant quantities of radiogenic argon (40Ar*) (similar to 90-100% over 20-40 Myr assuming a diffusion domain size (a) of 75 mu m) due to diurnal temperature variations on the lunar surface, although 40Ar* loss is highly sensitive to exposure duration and effective diffusion domain size. Modeling shows that loss from transient thermal events (e.g., heating to similar to 200 degrees C for 10(2) yr duration) can also cause partial resetting of apparent 40Ar/39Ar ages. In small (a = 75 mu m) glasses a maximum of 50-60% of 40 Ar* is lost over 4 Ga when buried to depths corresponding to temperatures of -15 degrees C. Results indicate that caution should be exercised in interpreting lunar impact glass 40Ar/39Ar ages, as the assumption of closed system behavior may have been violated, particularly in glasses with low fractions of non-bridging oxygen. (C) 2014 Elsevier Ltd. All rights reserved.