Mineral-Chemical and Isotopic Variations in Apollo 16 Impact-Melt Breccias

Abstract

Lunar impact-melt breccias were formed by intense bombardment of the Moon's surface by meteorites. The compositions of these rocks may record the composition of the underlying lunar crust, but also may contain components that were transported from much larger basin-forming impacts several hundred kilometers away. In this study, a suite of nine impact-melt breccia samples were analyzed for their texture and mineral compositions and a subset of these samples were analyzed for Sr and Nd-isotopic compositions. These new data were integrated into accumulated databases on the mineral-chemical and geochemical compositions of Apollo 16 impact-melt breccias to develop a better understanding of the formation of these rocks and the nature of the materials from which they were derived. Although not pronounced, there are some distinct mineralogical features characteristic of the various breccia groups and subgroups defined in McKinley et al. (1984) and Korotev (1994). Rocks in the various subgroups have isotopic characteristics that are correlated with the geochemical features used to classify them. Mineralogical and chemical features of Group 3 and Group 4 breccias (feldspathic) are clearly inherited from their anorthositic source. The large compositional variation in Group 3 and Group 4 breccias (mafic) is more complicated, requiring a significantly more mafic and incompatible trace element–rich source than anorthosite. Although rocks of the magnesian and alkali suites have these compositional characteristics, the amount of added material required to shift the compositions of feldspathic breccias to mafic breccias is prohibitive. The mafic impact-melt breccias are interpreted to have been emplaced as a result of the Imbrium impact, and thus contain a record of the composition of the middle to lower crust in the Imbrium region. Compositional similarities between Group 1 breccias and Apollo 15 KREEP basalts suggest that these two rocks may ultimately have the same source region.