Mare Tranquillitatis: Basalt emplacement history and relation to lunar samples

Galileo and Clementine multispectral data of the Mare Tranquillitatis region have been analyzed to investigate the stratigraphy of basaltic units and the effects of lateral and vertical mixing processes within the mare. The distribution of compositionally distinct mare units is observed to be correlated with previous UV/VIS ratio images, although estimates of soil titanium contents are low in some areas as a result of mixing of local basalts with nonmare feldspathic materials. Basalt units identified by their spectral properties and spectral mixture analysis are compared with groups of Apollo 11 samples deftned by previous workers on the basis of age and chemistry. Spectral studies presented here indicate that the Apollo 11 site lies at the edge of a localized western mare unit which includes the youngest and most titanium-rich basalts in Tranquillitatis (Apollo 11 high-K, high-Ti samples). In southern Tranquillitatis, these basalts have been contaminated by a large degree of mixing with nonmare feldspathic materials. Nonmare materials near the Apollo 11 site are attributed largely to crater rays from Theophilus (100 km in diameter), which is located approximately 300 km to the south. A more extensive and stratigraphically older unit exposed near Apollo 11 is related to the low-K, high-Ti Apollo 11 samples and appears to extend as a coherent surface unit as far north as the Apollo 17 site in southern Serenitatis. The distribution of this spectrally identified basalt unit supports petrologic and geochemical evidence for the grouping of the high-Ti, low-K Apollo 11 and 17 basalt samples into the same regional volcanic events. Multispectral analysis of Tranquillitatis deposits also identify low-titanium basalts in the northeastern and southeastern portions of the basin that are older than the high-Ti basalts and are believed to be unsampled by Apollo 11. Several lines of evidence suggest that the Cayley Formation along the western Tranquillitatis margin may indeed lie on top of an ancient mare deposit buried by Imbrium basin ejecta (e.g., a cryptomare deposit). The distribution of vertically excavated feldspathic premare material within the mare provides information on the depth of the mare units and the proximity of the underlying basin topography. Compositional stratigraphy observed in both sets of multispectral data supports an asymmetric premare-fill basin topography containing thicker basalts in the northwestern portion of the basin than previously predicted by crater flooding data.