| Abstract: | Abstract— Characteristics of the regolith of Cayley plains as sampled at the Apollo 16 lunar landing site are reviewed and new compositional data are presented for samples of <1 mm fines (“soils”) and 1–2 mm regolith particles. As a means of determining which of the many primary (igneous) and secondary (crystalline breccias) lithologic components that have been identified in the soil are volumetrically important and providing an estimate of their relative abundances, more than 3 × 106 combinations of components representing nearly every lithology that has been observed in the Apollo 16 regolith were systematically tested to determine which combinations best account for the composition of the soils. Conclusions drawn from the modeling include the following. At the site, mature soil from the Cayley plains consists of 64.5% ± 2.7% components representing “prebasin” materials: anorthosites, feldspathic breccias, and a small amount (2.6% ± 1.5% of total soil) of nonmare, mafic plutonic rocks, mostly gabbronorites. On average, these components are highly feldspathic, with average concentrations of 31–32% Al2O3 and 2–3% FeO and a molar Mg/(Mg + Fe) ratio of 0.68. The remaining 36% of the regolith is syn- and postbasin material: 28.8% ± 2.4% mafic impact-melt breccias (MIMBs, i.e., “LKFM” and “VHA basalts”) created at the time of basin formation, 6.0% ± 1.4% mare-derived material (impact and volcanic glass, crystalline basalt) with an average TiO2 concentration of 2.4%, and 1% postbasin meteoritic material. The MIMBs are the principal (80–90%) carrier of incompatible trace elements (rare earths, Th, etc.) and the carrier of about one-half of the siderophile elements and elements associated with mafic mineral phases (Fe, Mg, Mn, Cr, Sc). Most (71%) of the Fe in the present regolith derives from syn- and postbasin sources (MIMBs, mare-derived material, and meteorites). Thus, although the bulk composition of the Apollo 16 regolith is nominally that of noritic anorthosite, the noritic part (the MIMBs) and the anorthositic part (the prebasin components) are largely unrelated. There is compositional evidence that 3–4% of the soil is Th-rich material such as that occurring at the Apollo 14 site, and one fragment of this type was found among the small regolith particles studied here. If regolith such as that represented by the Apollo 16 ancient regolith breccias was a protolith of the present regolith, such regolith cannot exceed ～71% of the present regolith; the rest must be material added or redistributed since closure of the ancient regolith breccias. The postclosure material includes the mare-derived material and the Apollo-14-like component. Compositions of all mature surface soils from Apollo 16, even those collected 4 km apart on the Cayley plains, are very similar, which is in stark contrast to the wide compositional range of the lithologies of which the soil is composed. This uniformity indicates that the ratio of MIMBs to feldspathic prebasin components is not highly variable in the megaregolith over distances of a few kilometers, that there are no large, subsurface concentrations of “pure” mafic impact-melt breccia, and that the intimate mixing is inherent to the Cayley plains at a gross scale. Thus, the mixing of mafic impact-melt breccias and feldspathic prebasin components must have occurred during formation and deposition of the Cayley plains; such uniformity could not have been achieved by small postdeposition impacts into a stratified megaregolith. Using this conclusion as one constraint, and the known distribution of Th on the lunar surface as another, and the assumption that the Imbrium impact is primarily responsible for formation of the Cayley plains, arguments are presented that the Apollo 16 MIMBs derive from the Imbrium region, and, consequently, that one-fourth of the Apollo 16 regolith is primary Imbrium ejecta in the form of mafic impact-melt breccias. |
