The composition and geologic setting of mare deposits on the far side of the moon
Gillis, Jeffrey John
Anderson, John B.
Doctor of Philosophy
The Clementine Mission acquired the first global compositional data for the Moon. Five discrete wavelengths (415, 750, 900, 950, 1000 nm) span the ultraviolet-visible spectrum. Using this data, I have performed a systematic study of the mare deposits on the far side of the Moon, between 80' east and 80' west longitude. The major study areas are the Australe, Marginis, Moscoviense, Orientale, Smythii, and South Pole-Aitken basins. Mare basalt deposits within these basins or regions around them are characterized by their volume, age, chemistry and geologic setting. The integration of this information yields information concerning the thermal history, lateral and vertical heterogeniety of the mantle, and conditions at the time of eruption for far side mare deposits. Furthermore, this information provides constraints for models that discribe lunar volcanism and the composition of the mantle. Volcanism on the Moon's far side was not as protracted or as voluminous as on the near side. However, multiple episodes of volcanism are recognized in each of the major basins studied. Brief periods of volcanism occurred in degraded basins such as Campbell, Compton, Freundlich-Sharnov and Mendel-Rydberg and the isolated deposit Kohlshutter. The average iron and titanium composition of far side basalt deposits is lower relative to basalt deposits on the near side. Impact mixing has more efficiently contaminated the lower volume mare deposits with highlands material. Very high-Ti basalts, like those at Apollo 11 and 17 landing sites, are absent. Late stage moderately high-Ti units (4-6 wt.% $\rm TiO\sb2)$ are located in the Moscoviense, South Pole-Aitken and Australe basins. My interpretation of these data is that the proposed thicker far side crust has prevented very high-Ti basalts from erupting onto the surface of the far side of the Moon. A simple stratified mantle with high-Ti late cumulates situated above low-Ti early cumulates does not explain the observed distribution of late stage, moderately high-Ti basalt units on the far side.