Composition and evolution of the lunar crust in the Descartes Highlands, Apollo 16

Abstract

Samples from the North Ray crater ejecta blanket, Apollo 16, were investigated by petrographic microscope, electron microprobe, instrumental neutron activation and Xray fluorescence analyses, and 40Ar‐39Ar and Rb‐Sr dating techniques. Nine major groups of monomict and polymict breccias were defined on the basis of microscopic texture and these were further subdivided into chemical subgroups on the basis of characteristic elements such as Al, Mg, Fe, Cr, REE, Ni, and Co. The polymict breccias — fragmental breccias, granulitic breccias, and impact melt breccias — are the result of multiple impact‐induced mechanical mixing and melting, and of thermal and impact metamorphism of rock and mineral clasts derived from primordial igneous crustal rocks. For calculations of mixing models it was found that end‐members consisting of the pristine igneous rock components present as discrete samples at the Apollo 16 site and supplemented by KREEP, dunite, and a meteoritic component yield the best fits for the composition of polymict breccias. The end‐member rocks a re: ferroan anorthosite, various magnesian gabbronorites including “sodic ferrogabbro” and “feldspathic lherzolite,” and spinei troctolite. The following model is proposed for the composition and stratigraphy of the target for North Ray crater. The lower section of the stratigraphy is composed of a megabreccia with clasts of highly feldspathic polymict breccias (KREEP‐free “Old Eastern Highland Rock Suite”) interpreted as Nectaris ejecta (Descartes formation). The top section contains KREEP‐bearing polymict breccias (KREEP‐bearing “Young Western Highland Rock Suite”) and appears to be similar to the lithologies found in the Cayley plains. This material interpreted as Cayley formation may be a distant facies of Imbrium basin ejecta deposits of the Imbrium basin. The petrographic differences between these two major selenographic units (the Descartes and the Cayley formations) in the Apollo 16 area are more distinct than the chemical differences. The petrographic and chemical composition of the primordial igneous upper crust in the regions of the Nectaris and Imbrium basins has been calculated by subtraction of the KREEP and meteoritic components from the bulk composition of the Descartes and Cayley materials. The Nectaris, crust which is better constrained, consists of 86‐87% a northosite, 4% sodic ferrogabbro, 0.5‐1.3% feldspathic lherzolite, 6‐8% regular magnesian gabbronorite, 1.8‐2.8% dunite, and 0.1% spinel troctolite. A model for the evolution of the upper lunar crust in the Descartes highlands is proposed on the basis of isotope ages and clast‐matrix relationships of polymict breccias. Essential features of this model in sequenial order are: (1) development of multiple layers of KREEP‐free “early feldspathic fragmental megabreccias” and impact melt sheets on the primordial crust in the time period from 4.4 aeons to the time of the Nectaris impact, which could have occurred as late as 3.85 aeons ago, (2) excavation of these megabreccias by the Nectaris event and d...

The lunar‐wide effects of basin ejecta distribution on the early megaregolith

“…A significant amount of our understanding of lunar history and evolution comes from what has been learned from samples brought to the Earth by the Apollo and Luna missions (Taylor 1975;Ryder and Wood 1977;James 1981;Stöffler et al 1985;Haskin and Warren 1991;McKay et al 1991;Taylor et al 1991;Papike et al 1998). But how representative of the entire lunar surface are these samples from specific landing sites?…”

Petrography and geochemistry of five new Apollo 16 mare basalts and evidence for post‐basin deposition of basaltic material at the site

“…The high-Al basalt from this study (60053,2-9) is chemically similar to Apollo 14 group 5 high-Al basalts, which are thought to be Imbrium basin ejecta deposits (Dickinson et al 1985;Shervais et al 1985). Because the Cayley Plains of the Apollo 16 site are an ejecta deposit from the Imbrium impact (Wilhelms 1987;Stˆffler 1981Stˆffler , 1985Spudis 1984), it is possible that the source region for high-Al mare basalt fragment 60053,2-9 is also as Imbrium ejecta. Because fragment 60053,2-9 has little to no discernible KREEP component, an origin within the Procellarum KREEP Terrane is not indicated, however.…”

“…Studies both of rocks (Roedder and Weiblen 1974;Nord et al 1975;James 1981;Stˆffler et al 1981Stˆffler et al , 1985Lindstrom and Salpas 1983) and of small rock fragments (Jolliff and Haskin 1995) from the regolith ejected from North Ray Crater have yielded only a single possible basalt clast (300 μm) in breccia 67915 (Roedder and Weiblen 1974). The compositions of North Ray Crater soils show none of the chemical signatures of mare basalt that is shown by the soils more distant from the craters (Korotev 1996).…”

“…Ejecta from at least three major basins, Nectaris, Serenitatis, and Imbrium, were deposited at the Apollo 16 site (Spudis 1984;Stˆffler et al 1985;Wilhelms 1987) and in some models, less than half the material in the present regolith occurred in the vicinity of the site prior to the Nectaris impact (Haskin et al 2002. Nevertheless, we think that the fragments of mare basalt in the Apollo 16 regolith are mainly from dispersal of material by smaller postbasin impacts into maria for three reasons.…”

The Puerto Lápice eucrite

“…However, regolith and fragmental breccias commonly contain clasts of different degree of shock as a result of multiple shock reworking (e.g., Stöffler et al 1991). Good examples are the lunar regolith and lunar fragmental breccias (e.g., Stöffler et al 1985;Bischoff et al 1998). When a shock stage is assigned to such a breccia, a shock pressure experienced by the whole rock is determined (Stöffler et al 1991).…”