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Geochemistry and Analysis of Apollo 16 Lunar Impact Glasses

Simeon S. M. Hui and Marc D. Norman

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

 

Apollo 16 Lunar Impact Spherules showing a variety of shapes, colours and sizes.

Lunar impact spherules are micron to centimetre sized 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 microbreccias and are a medium from which we can study lunar chemistry and impact history.

We have separated over 900 lunar spherules, most of them likely to be from impact origins, from the Apollo 16 fines, 66031. Using new mounting and analysis techniques we aim to obtain major and trace element compositions while preserving the maximum amount of sample for 39Ar-40Ar dating on singular particles. Preliminary tests were conducted using shards of crushed USGS standard TB-1G which represents extremes in irregularity. Using wavelength-dispersive electron microscopy techniques to obtain major element compositions of the TB-1G shards, we are able to achieve errors of less than 5% relative.

Following this success, petrographic descriptions and dimensions were obtained for 272 lunar glasses greater than 75µm in diameter along with major element compositions. There are broad positive correlations between MgO vs. FeO and negative correlations between Al2O3 and CaO vs. FeO. The majority of the impact spherules have chemistry consistent with derivation of the glasses from the local regolith.

Most impact spherules are irregular and splash-like in shape, often with a coat of adhering grains while highly spherical forms are rarer but have cleaner surfaces. We also find that irregular shapes tend to be more internally heterogeneous in major element composition than the highly spherical forms. This may indicate that highly spherical forms cooled before contacting the lunar surface suggesting a more distant origin. However, rare exotic compositions are more likely to be irregularly shaped which might be due to fragmentation of the glass.

Results of this study were presented at the 8th Australian Space Science Conference, Canberra, Australia, 29th September-1st October 2008.