SOME THINGS WE CAN INFER ABOUT THE MOON FROM THE COMPOSITION OF THE APOLLO-16 REGOLITH

Characteristics of the regolith of Cayley plains as sampled at the Apo llo 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 (igneou s) and secondary (crystalline breccias) lithologic components that hav e been identified in the soil are volumetrically important and providi ng an estimate of their relative abundances, more than 3 x 10(6) combi nations of components representing nearly every lithology that has bee n observed in the Apollo 16 regolith were systematically tested to det ermine which combinations best account for the composition of the soil s. Conclusions drawn from the modeling include the following. At the s ite, mature soil from the Cayley plains consists of 64.5% +/- 2.7% com ponents 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 compon ents are highly feldspathic, with average concentrations of 31-32% Al2 O3 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% maf ic impact-melt breccias (MIMBs, i.e., ''LKFM'' and ''VHA basalts'') cr eated at the time of basin formation, 6.0% +/- 1.4% mare-derived mater ial (impact and volcanic glass, crystalline basalt) with an average Ti O2 concentration of 2.4%, and 1% postbasin meteoritic material. The MI MBs are the principal (80-90%) carrier of incompatible trace elements (rare earths, Th, etc.) and the carrier of about one-half of the sider ophile 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 me teorites). Thus, although the bulk composition of the Apollo 16 regoli th is nominally that of noritic anorthosite, the noritic part (the MIM Bs) and the anorthositic part (the prebasin components) are largely un related. There is compositional evidence that 3-4% of the soil is Th-r ich material such as that occurring at the Apollo 14 site, and one fra gment of this type was found among the small regolith particles studie d here. If regolith such as that represented by the Apollo 16 ancient regolith breccias was a protolith of the present regolith, such regoli th cannot exceed similar to -71% of the present regolith; the rest mus t be material added or redistributed since closure of the ancient rego lith breccias. The postclosure material includes the mare-derived mate rial and the Apollo-14-like component. Compositions of all mature surf ace soils from Apollo 16, even those collected 4 km apart on the Cayle y plains, are very similar, which is in stark contrast to the wide com positional range of the lithologies of which the soil is composed. Thi s 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 concentration s 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 h ave occurred during formation and deposition of the Cayley plains; suc h uniformity could not have been achieved by small postdeposition impa cts into a stratified megaregolith. Using this conclusion as one const raint, and the known distribution of Th on the lunar surface as anothe r, 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, tha t one-fourth of the Apollo 16 regolith is primary Imbrium ejecta in th e form of mafic impact-melt breccias.