Glass particles in the Luna 24 core and magma evolution at Mare Crisium

Lunar regolith returned from Mare Crisium by the unmanned Soviet lunar probe Luna 24 contains abundant homogeneous glass particles from both mare and non-mare sources. Volcanic activity and meteorite impact

are the processes most likely to have formed these glasses. Certain petrographic and chemical criteria have been used in this study to distinguish glass formed from an endogenous lunar melt. Such a glass would represent a direct sample of a lunar magma and could be very useful in deciphering the volcanic history of a particular landing site.

Major and minor element chemical analyses by electron microprobe show that most homogeneous glasses from the Luna 24 drill core can be related to either known rock types or the bulk soil. Mare glasses have high Fe0 and low Al2O3 and include a volcanic brown glass identical in composition to fine-grained ferrobasalt rock fragments in the Luna 24 sample, a green glass which matches the bulk composition of the regolith and which was probably produced by meteorite impact onto that regolith, a high-K green glass, and a high-Ti orange glass. Colorless non-mare glasses are more aluminous and were derived principally by meteorite impact onto the highlands area surrounding the Crisium basin.

The fine-grained ferrobasalt which matches the brown glass composition is the most common igneous rock type sampled by Luna 24. It is depleted in Mg relative to Fe and was probably co-saturated with olivine and plagioclase at the lunar surface. These characteristics suggest that the ferrobasalt/brown glass melt may have been derived from a more primitive Mg-rich parent magma by fractional crystallization. If so, then the

important question becomes: What was the composition of the parent mag-

ma at the Luna 24 site?

Two Mg-rich compositions are examined as possible parent magmas: the Apollo 15 green glass and the Luna 24 olivine vitrophyre. The Apollo 15 green glass has been suggested as a parent to other lunar rocks due to its very olivine normative composition. It cannot, however, be parental to the Luna 24 ferrobasalt, since it does not have the proper phase relations or rare earth element distribution. Experimental studies performed on a synthetic Luna 24 olivine vitrophyre composition yield low pressure phase relations consistent with those of the ferrobasalt. Therefore, the Luna 24 olivine vitrophyre may represent

the parent magma.