Untitled Document
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.