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 deposition of a “Nectarian feldspathic fragmental megabreccia” in the Descartes area, (3) introduction of KREEP‐basaltic material into the upper crust, mainly west and northwest of the Descartes site and formation of KREEPy impact melt sheets (most probably from 4.05 to 3.80 aeons ago), (4) Imbrium impact at 3.8 aeons ago and deposition of KREEP‐bearing “Imbrian feldspathic fragmental breccias” at the Descartes site which form the Cayley formation, and (5) redeposition , mixing, and melting of all preexisting breccias yielding KREEP‐bearing and KREEP‐poor post‐Cayley fragmental breccias and melt ejecta (“glass bombs”) as the result of small impacts (craters < 1.5 km in diameter) in the Cayley plains and the Descartes mountains, respectively.