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 o bserved to be correlated with previous UV/VIS ratio images, although e stimates of soil titanium contents are low in some areas as a result o f mixing of local basalts with nonmare feldspathic materials. Basalt u nits identified by their spectral properties and spectral mixture anal ysis are compared with groups of Apollo 11 samples defined 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 sou thern Tranquillitatis, these basalts have been contaminated by a large degree of mixing with nonmare feldspathic materials. Nonmare material s near the Apollo 11 site are attributed largely to crater rays from T heophilus (100 km in diameter), which is located approximately 300 km to the south. A more extensive and stratigraphically older unit expose d near Apollo 11 is related to the low-K, high-Ti Apollo 11 samples an d appears to extend as a coherent surface unit as far north as the Apo llo 17 site in southern Serenitatis. The distribution of this spectral ly identified basalt unit supports petrologic and geochemical evidence for the grouping of the high-Ti, low-K Apollo 11 and 17 basalt sample s into the same regional volcanic events. Multispectral analysis of Tr anquillitatis deposits also identify low-titanium basalts in the north eastern and southeastern portions of the basin that are older than the high-Ti basalts and are believed to be unsampled by Apollo 11. Severa l lines of evidence suggest that the Cayley Formation along the wester n Tranquillitatis margin may indeed lie on top of an ancient mare depo sit buried by Imbrium basin ejecta (e.g., a cryptomare deposit). The d istribution of vertically excavated feldspathic premare material withi n the mare provides information on the depth of the mare units and the proximity of the underlying basin topography. Compositional stratigra phy observed in both sets of multispectral data supports an asymmetric premare-fill basin topography containing thicker basalts in the north western portion of the basin than previously predicted by crater flood ing data.