Mineralogical and chemical characterization of lunar highland soils: Insights into the space weathering of soils on airless bodies

With reflectance spectroscopy, one is measuring only properties of the fine-grained regolith most affected by space weathering. The Lunar Soil Characterization Consortium has undertaken the task of coordinated characterization of lunar soils, with respect to their mineralogical and chemical makeup. It is these lunar soils that are being used as ground truth for all airless bodies. Modal abundances and chemistries of minerals and glasses in the finest size fractions (20-45, 10-20, and < 10 mu m) of four Apollo 14 and six Apollo 16 highland soils have been determined, as well as their bulk chemistry and IS/FeO values. Bidirectional reflectance measurements (0.3-2.6 mu m) of all samples were performed in the Reflectance Experiment Laboratory. A significant fraction of nanophase Fe-0 (np-Fe-0) appears to reside in agglutinitic glasses. However, as grain size of a soil decreases, the percentage of total iron present as np-Fe-0 increases significantly, whereas the agglutinitic glass content rises only slightly; this is evidence for a large contribution to the IS/FeO values from the surface-correlated nanophase Fe-0, particularly in the < 10 mu m size fraction. The compositions of the agglutinitic glasses in these fine fractions of the highland soils are different from the bulk chemistry of that size; however, compositional trends of the glasses are not the same as those observed for mare soils. It is apparent that the glasses in the highland soils contain chemical components from outside their terrains. It is proposed that the Apollo 16 soils have been adulterated by the addition of impact-transported soil components from surrounding maria.