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 ~17 to 20kcalmol-1 and log10(D0/a2) 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., ~660°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*) (~90-100% over 20-40Myr assuming a diffusion domain size (a) of 75μm) 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 ~200°C for 102yr duration) can also cause partial resetting of apparent 40Ar/39Ar ages. In small (a=75μm) glasses a maximum of 50-60% of 40Ar* is lost over 4Ga when buried to depths corresponding to temperatures of -15°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.
ASJC Scopus subject areas
- Geochemistry and Petrology