SEARCHING FOR HIGH - Al MARE BASALTS : MARE FECUNDITATIS AND LUNA 16

Introduction: Luna 16 landed on the northeastern corner of Mare Fecunditatis on September 20, 1970. The robotic mission returned to Earth with a drill core that sampled the regolith to a depth of 35cm. This core was replete with relatively high-Al mare basalts (11-20 wt% Al2O3 compared to typical mare basalts: 7-11 wt% Al2O3). The return of these samples raised two important questions: does Mare Fecunditatis actually contain high-Al mare basalts or are they ejecta from a different region of the Moon? If the basalts are from Fecunditatis, where are the flows located? Based on the positive relationship between FeO and Al2O3, and the methods first described in [1,2] and implemented in [3], we now investigate the regions where we expect to find high-Al basalts. Here we focus on Mare Fecunditatis and in a companion abstract we investigate the Imbrium basin. Methods: Clementine 5-band UV/VIS cube images were processed from the full resolution Clementine UVVIS digital image model data set using Integrated Software for Imagers and Spectrometers (ISIS). FeO concentration images of the regions were generated using the algorithm of [4] and TiO2 images using the algorithm of [5]. A two stage approach is used in our search for exposures of high-Al basalts: 1) We isolated regions of interest (ROIs) within Fecunditatis using Lunar Prospector data and the known compositions of high-Al basalts from returned samples (12-18 wt% FeO, 1.5-5 wt% TiO2, 0-4 ppm Th). 2) These identified ROIs were then examined closely u s i n g h i g h r e s o l u t i o n Clementine images, sampled at 100m/pixel. This resolution may be a bit optimistic for the data set, particularly when considering errors associated with TiO2 concentrations from Clementine [6]. However, this was taken into consideration, and we did not attempt to make any conclusions based on features less than 400m in a single dimension. Fecunditatis basin is old, pre-Nectarian [7], and filled with mare basalt averaging 500m thick [8], with estimated local thicknesses as great as 1500m (Fig. 2) [9]. Basalt fill is estimated at middleto late-Imbrian in age [7]. Dated high-Al basalts from Luna 16 indicate at least three eruptive episodes from 3.42 to 3.15 Ga, all of similar (highAl) composition [10]. The surface of Mare Fecunditatis is also very complex owing to the crossing of numerous large-scale ejecta deposits. The simplified geologic map in Fig. 3 shows the distribution of the different basaltic units in Mare Fecunditatis. This map does not distinguish every flow unit, as does [8], but instead divides the basalts based on major compositional variations and/or ages from crater counting statistics of previous workers [11]. Discussion: Material ejected by large craters that impacted in and near the basin have caused mixing sufficient to obscure the composition of the basaltic units. For example, the young, high-Ti unit(s) distinguishable in the NE and SW corners of the basin (blue patches in Fig. 1) have contaminated almost the entire mare surface. The TiO2 composition of the surface does not drop below 3 wt%, even within craters as large as 3km in diameter, although the FeO ranges from 14 to 18.5 wt%. We have two reasons to believe we can identify high-Al basalts in Mare Fecunditatis: 1) Returned samples demonstrate the presence of high-Al basalts; and 2) craters penetrating the regolith, exposing fresh basalt reveal compositions consistent with the Luna