COMPOSITIONAL VARIATION IN APOLLO-16 IMPACT-MELT BRECCIAS AND INFERENCES FOR THE GEOLOGY AND BOMBARDMENT HISTORY OF THE CENTRAL-HIGHLANDS OF THE MOON

High-precision data for the concentrations of a number of lithophile a nd siderophile elements were obtained on multiple subsamples from 109 impact-melt rocks and breccias (mostly crystalline) from the Apollo 16 site. Compositions of nearly all Apollo 16 melt rocks fall on one of two trends of increasing Sm concentration with increasing Sc concentra tion. The Eastern trend (lower Sm/Sc, Mg/Fe, and Sm/Yb ratios) consist s of compositional groups 3 and 4 of previous classification schemes. These melt rocks are feldspathic, poor in incompatible and siderophile elements, and appear to have provenance in the Descartes formation to the east of the site. The Western trend (higher Sm/Sc, Mg/Fe, and Sm/ Yb ratios) consists of compositional groups 1 and 2. These relatively mafic, KREEP-bearing breccias are a major component ( similar to 35%) of the Cayley plains west of the site and are unusual, compared to oth erwise similar melt breccias from other sites, in having high concentr ations of Fe-Ni metal (1-2%). The metal is the carrier of the low-Ir/A u (similar to 0.3 X chondritic) siderophile-element signature that is characteristic of the Apollo 16 site. Four compositionally distinct gr oups ( 1M, 1F, 2DB, and 2NR) of Western-trend melt breccias occur that are each represented by at least six samples. Compositional group 1 o f previous classification schemes (the ''poikilitic'' or ''LKFM'' melt breccias) can be subdivided into two groups. Group 1M(represented by six samples, including 60315) is characterized by lower Al2O3 concentr ations, higher MgO and alkali concentrations, and higher Mg/Fe and Cr/ Sc ratios than group 1F (represented by fifteen samples, including 650 15). Group 1M also has siderophile-element concentrations averaging ab out twice those of group 1F and Ir/Au and Ir/Ni ratios that are even l ower than those of other Western-trend melt rocks (Ir/Au = 0.24 +/- 0. 03, CI-normalized). At the mafic extreme of group 2 (''VHA'' melt brec cias), the melt lithology occurring as clasts in feldspathic fragmenta l breccias from North Ray crater (group 2NR) is compositionally distin ct from the melt lithology of dimict breccias from the Cayley plains ( group 2DB) in having higher concentrations of Sc, Cr, and heavy rare e arth elements and lower concentrations of siderophile elements. The di stinct siderophile-element signature (high absolute abundances, low Ir /Au ratio) suggest that the four groups of mafic melt breccia are all somehow related. Ratios of some lithophile elements also suggest that they are more closely related to each other than they are to melt brec cias from other Apollo sites. However, none of the breccia composition s can be related to any of the others by any simple process of igneous fractionation or mixing involving common lunar materials. Thus, the o rigin of the four groups of mafic melt breccia is enigmatic. If they w ere produced in only one or two impacts, then a mechanism exists for g enerating regimes of impact-melt breccia in a single impact that are s ubstantially different from each other in composition. For various rea sons, including the problem of delivering large volumes of four differ ent types of melt to the Apollo 16 site, it is unlikely that any of th ese breccias were produced in basin-forming impacts. If they were prod uced in as many as four crater-forming impacts, then the unusual sider ophile-element signature is difficult to explain. Possible explanation s are (1)the four groups of melt breccia all contain metal from a sing le, earlier impact, (2) they were each formed by related metal-rich me teoroids, or (3) some common postimpact process has resulted in metal of similar composition in each of four melt pools. Within a compositio nal group, most intrasample and intersample variation in lithophile el ement concentrations is caused by differences among samples in the pro portion of a component of normative anorthosite or noritic anorthosite . In most cases, this compositional variation probably reflects variat ion in clast abundance. For group 2DB (and probably 2NR), differences in abundance of a component of ferroan anorthosite (estimated Al2O3 ap proximate to 32%) accounts for the compositional variation. For groups 1M and 1F, the anorthositic component is more mafic (estimated Al2O3 approximate to 26%). Some group-2 samples may be related by a troctoli tic component of varying abundance.