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 ~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.