TY - JOUR
T1 - Argon diffusion in Apollo 16 impact glass spherules
T2 - Implications for 40Ar/39Ar dating of lunar impact events
AU - Gombosi, David J.
AU - Baldwin, Suzanne L.
AU - Watson, E. Bruce
AU - Swindle, Timothy D.
AU - Delano, John W.
AU - Roberge, Wayne G.
N1 - Funding Information:
We would like to thank N. Zellner, J. Weirich, J. Levine, and J. Das for helpful discussions. J. Metcalf and J. Kula provided assistance in noble gas analysis. This paper has benefited from helpful reviews by W. Cassata, K. Hodges, and an anonymous reviewer. We thank G. Herzog for editorial handling and helpful comments. Funding was provided by a grant from NASA Headquarters under the NASA Earth and Space Sciences Fellowship Program (grant NNX10AP0H) and by the NASA Astrobiology Institute (grant no. NNA09DA80A to The New York Center for Astrobiology).
Publisher Copyright:
© 2014 Elsevier Ltd.
PY - 2015/1/1
Y1 - 2015/1/1
N2 - 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.
AB - 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.
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U2 - 10.1016/j.gca.2014.09.031
DO - 10.1016/j.gca.2014.09.031
M3 - Article
AN - SCOPUS:84918579439
SN - 0016-7037
VL - 148
SP - 251
EP - 268
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -