Geochemistry of Apollo 16 Lunar Impact Glasses: Tracking Formation and Transport through Chemistry and Age Dating

Simeon Hui
Research School of Earth Sciences, Australian National Unive

*Marc Norman
Research School of Earth Sciences, Australian National University

*Fred Jourdan
Western Australian Argon Isotope Facility

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     Last modified: July 13, 2009

Abstract
Lunar Impact Spherules are micron to centimetre glass particles formed during impact events where shock induced melting of the lunar regolith and impactor produce melt splashes that can be deposited locally or be ejected far beyond the point of impact. These particles can be found within the lunar soil and in regolith breccias and are a medium from which we can study lunar chemistry and impact history.

We have separated and described 272 spherules >75µm in diameter from an Apollo 16 soil (66031,65). Using a new mounting technique we have described the petrography and collected comprehensive major and trace element chemistry using electron microprobe and laser ablation-ICPMS respectively. In addition, 30 spherules have also been dated using the 40Ar-39Ar technique. The ability to obtain this wealth of data for single particles allows us to better track the origins and processes that any individual spherule experiences.

Approximately 90% of the spherules have major and trace element chemistry that are consistent soil collected from Apollo 16. The remaining 10% have chemistry that is not consistent with Apollo 16 soils and are likely to have been transported from other regions of the Moon. This suggests that although transport of spherules on ten to hundred kilometre scales (within Apollo 16-like soils) may occur routinely, transport of spherules from distances of hundreds of kilometres is more uncommon.

Twenty-five of the spherules yielded formation ages. A third of the spherules cluster in the 0-400 m.y.; another third cluster between 400-2000 m.y. The final third cluster at the 3200-4400 m.y. The spike of 0-400 m.y. ages has been noted in spherules from Apollo 12 and 14 and may indicate a recent influx of impactors and/or destruction of older spherules. Cosmic ray exposure ages indicate that many young spherules (<500m.y. formation ages) spent most of their lifetimes in the upper 1m of the regolith while most older spherules have integrated exposure lifetimes ≤500m.y.