Trace element identification of three chemically distinct very low titanium (VLT) basalt glasses from apollo 17
Lunar basaltic samples of VLT (Very Low Titanium) composition occur primarily as small glass fragments in the regolith at the Apollo 17 landing site. Electron microprobe analyses of glasses from the double drive tube 79001/2 (90-150 μm size fraction of soils from mean depths of 0.8-16.8 cm) indicate that 34 of 90 (38%) are of VLT composition. A subset of twenty glasses was removed from the thin sections using a micro-coring device and analyzed using special micro-INAA techniques to obtain major and trace element abundances. On the basis of the results, we delineate three distinct groups which have been named for their relative amounts of Co: HICo, MECo, and LOCo Apollo 17 VLTs. The most magnesian HICo basalt glasses (4 of 34, or about 12% of the VLT glasses) are probably pristine pyroclastics. Soil breccias 79135 and 70295 apparently contain only this type of VLT glass. The medium-Co MECo glasses (6 of 34, 18%) strongly resemble the largest sample of VLT composition, the olivine vitrophyre 78526, as well as all three of the Apollo 17 VLT lithic fragments that have been analyzed for trace elements. The LOCo VLT glasses (23 of 34, 68%) are more enigmatic. Trace element concentrations require that these three VLT compositions cannot be related by simple igneous processes. The distribution of the three types provides important information on the sequence of events that produced the regolith at the Apollo 17 site.