Lithologic distribution and geologic history of the Apollo 17 site: The record in soils and small rock particles from the highland massifs

Abstract

— Through analysis by instrumental neutron activation (INAA) of 789 individual lithic fragments from the 2 mm–4 mm grain‐size fractions of five Apollo 17 soil samples (72443, 72503, 73243, 76283, and 76503) and petrographic examination of a subset, we have determined the diversity and proportions of rock types recorded within soils from the highland massifs. The distribution of rock types at the site, as recorded by lithic fragments in the soils, is an alternative to the distribution inferred from the limited number of large rock samples. The compositions and proportions of 2 mm–4 mm fragments provide a bridge between compositions of <1 mm fines, and types and proportions of rocks observed in large collected breccias and their clasts. The 2 mm–4 mm fraction of soil from South Massif, represented by an unbiased set of lithic fragments from station‐2 samples 72443 and 72503, consists of 71% noritic impact‐melt breccia, 7% incompatible‐trace‐element‐(ITE)‐poor highland rock types (mainly granulitic breccias), 19% agglutinates and regolith breccias, 1% high‐Ti mare basalt, and 2% others (very‐low‐Ti (VLT) basalt, monzogabbro breccia, and metal). In contrast, the 2 mm–4 mm fraction of a soil from the North Massif, represented by an unbiased set of lithic fragments from station‐6 sample 76503, has a greater proportion of ITE‐poor highland rock types and mare‐basalt fragments: it consists of 29% ITE‐poor highland rock types (mainly granulitic breccias and troctolitic anorthosite), 25% impact‐melt breccia, 13% high‐Ti mare basalt, 31% agglutinates and regolith breccias, 1% orange glass and related breccia, and 1% others. Based on a comparison of mass‐weighted mean compositions of the lithic fragments with compositions of soil fines from all Apollo 17 highland stations, differences between the station‐2 and station‐6 samples are representative of differences between available samples from the two massifs. From the distribution of different rock types and their compositions, we conclude the following: (1) North‐Massif and South‐Massif soil samples differ significantly in types and proportions of ITE‐poor highland components and ITE‐rich impact‐melt‐breccia components. These differences reflect crudely layered massifs and known local geology. The greater percentage of impact‐melt breccia in the South‐Massif light‐mantle soil stems from derivation of the light mantle from the top of the massif, which apparently is richer in noritic impact‐melt breccia than are lower parts of the massifs. (2) At station 2, the 2 mm–4 mm grain‐size fraction is enriched in impact‐melt breccias compared to the <1 mm fraction, suggesting that the <1 mm fraction within the light mantle has a greater proportion of lithologies such as granulitic breccias which are more prevalent lower in the massifs and which we infer to be older (pre‐basin) highland components. (3) Soil from station 6, North Massif, contains magnesian troctolitic anorthosite, which is a component that is rare in station‐2 South‐Massif soils. (4) Compositional differences be...

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

“…5a and 5b) and ferromagnesian trace-element compositions (e.g., Cr, Sc) (Fig. 6b) of 62243,10-22 are similar to VLT basalt fragments found at the Apollo 17 site (Wentworth et al 1979;Jolliff et al 1996). Furthermore, concentrations of incompatible trace elements (e.g., Sm, Ba) (Figs.…”

“…We analyzed the 506 lithic fragments for concentrations of 26 elements by instrumental neutron activation analysis (INAA) as we have done in similar studies in the past (Jolliff et al 1991(Jolliff et al , 1996Jolliff and Haskin 1995). We prepared each particle for analysis by ultrasonic rinsing in a bath of acetone to remove any dust coating, examination under a binocular microscope to make a preliminary lithologic classification, weighing, and sealing in a high-purity silica tube for neutron irradiation.…”

“…We ground the remaining material of each fragment using an alumina mortar and pestle, fused the powder with a molybdenum strip resistance heater in an argon atmosphere, and mounted the resulting glass bead in epoxy. We then polished the fused beads and analyzed them with the electron microprobe using 15 kV accelerating voltage, 20 nA beam current, and 30-50 μm spot size to obtain major-element concentrations (for a more complete description, see Jolliff et al 1996). Majorelement concentrations of Apollo 16 mare basalts are listed in Table 1.…”

Some things we can infer about the Moon from the composition of the Apollo 16 regolith

“…Unlike the soils from the North Massif of Apollo 17 (Korotev and Kremser, 1992;Jolliff et al, 1996), nonmare mafk plutonic rocks are not required in excess of -3% as discrete components to account for the composition of the soil. Overall, the average composition of the nonmare mafic components is not highly magnesian (Mg' = 63.5, Table 7).…”

“…In Model 1, the fraction of MIMB component is 29% for the average mature soil (Table 6) and the modified model shows that this value, in fact, represents a nanow range, from 25% at stations 1 and 2 to 32% at stations 5 and 6. By comparison, at the Apollo 17 site where units of mafic melt breccia appear to be concentrated at the tops of the massifs (Rhodes et al, 1974;Jolliff et al, 1996), the abundance of the MIMB component is more variable among soil samples. For example, among nonmare components of the massif soils, the South Massif soils contain 42-52% MIMB component (range among light-mantle stations 2, 2% and 3), whereas the soils of the North Massif across the valley contain 27-31% (stations 6, 7, and 8;Korotev and Kremser, 1992).…”