^1,2David J. Gombosi,^1,2Suzanne L. Baldwin,^2,3E. Bruce Watson,^4,2Timothy D. Swindle,^5,2John W. Delano,^6,2Wayne G. Roberge
¹Department of Earth Sciences, Syracuse University, Syracuse, NY 13244
²New York Center for Astrobiology, Rensselaer Polytechnic Institute, Troy, NY 12180
³Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180
⁴Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ 85721
⁵Department of Atmospheric and Environmental Sciences, University at Albany (SUNY), Albany, NY 12222
⁶Department of Physics, Applied Physics & Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180

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 20 kcal mol-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 20-40 Myr assuming a diffusion domain size (a) of 75 μm) 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 102 yr duration) can also cause partial resetting of apparent 40Ar/39Ar ages. In small (a=75 μm) glasses a maximum of 50-60% of 40Ar∗ is lost over 4 Ga 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.

Reference
Gombosi DJ,Baldwin SL,Watson EB,Swindle TD,Delano JW, Roberge WG (2014) Argon diffusion in apollo 16 impact glass spherules: Implications for 40Ar/39Ar dating of lunar impact Events. Geochimica et Cosmochimical Acta (in Press)
Link to Article [DOI: 10.1016/j.gca.2014.09.031]

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