Shock-pressure and -temperature determinations will be provided for the relatively large (>200 g), and well-studied rocks (e.g., Jackson et al. 1975; Bickel 1977; James et al. 1991) 78155, 78235, and 60025. The petrology and chemical composition of the 12 fragments of mm size from the rake soil sample 63503 were not previously investigated. Other fragments from this rake sample were previously characterized by Maurer et al. (1978) and James (1981). The petrographic, geochemical, and shock metamorphic results and the type of investigations applied are summarized in Tables 1 and 2 and Fig. 1. BSE images of each sample are shown in Figs. 2-6. Additional information on the studied samples, including optical micrographs in transmitted and polarized light are provided in Figs. S1–S5 in Appendix S1. Petrographic study of the samples enabled them to be categorized as (Tables 1-3): (1) fragments from crustal rocks (78155, 78235,139, 60025,1, and the 1–3 mm sized soil fragments 63503,11, -,14, -,15, -,16, -,17, -,20, and -,21); (2) polymict fragmental breccias (63503-,1, -,3, and -,4); and (3) impact melt rocks 63503,9 and -,13. With the exception of the Apollo 17 rocks and the soil fragment 63503,17, which are akin to the Mg-suite, all other samples are ferroan anorthosite (FAN) rocks (Fig. 1).
Fragments from the Apollo 16, 63503 Soil
The 1–3 mm rake fragments from soil sample 63503 range from single mineral phases (e.g., plagioclase, pyroxene, olivine, and ilmenite) to lithic clasts (e.g., anorthosite, impact melt, and norite).
Crustal rocks. The anorthosites 63503,11, 63503,15, 63503,16, 63503,20, 63503,21 (Figs. 3b–f) are composed dominantly of large anorthite crystals and smaller interstitial grains of pyroxene with accessory olivine. These FAN rocks show a relict coarse-grained texture that was deformed by cataclasis and subsequent thermal annealing in a few samples (Table 2). In general, the components in the fragmental breccias are angular in shape. Using transmitted light microscopy, the finely brecciated regions appear brownish while the larger plagioclase fragments appear transparent. Most plagioclase displays strong undulatory extinction and no maskelynite is observed. We could not discern whether samples 63503,11, -,15, and -,20 were affected by weak recrystallization or if they lack any recrystallization features (Table 2) because the available thick sections are thicker than the components comprising their finely cataclastized zones. However, cataclastic anorthosites lacking recrystallization features are thought to be very rare in Apollo 16 samples (Stöffler et al. 1985).
Sample 63503,11 (Figs. 3b, S1, and S2a) almost completely consists of a single plagioclase grain cross-cut by granular bands composed dominantly of plagioclase and occasionally by pyroxene fragments. A 100 μm-diameter pyroxene grain (Figs. 3b and S1) contains 2 μm diameter orthopyroxene and approximately 5 μm diameter clinopyroxene exsolution lamellae with submicron sized Ti- and Cr-rich spinel inclusions in the clinopyroxene region (phase identification by Raman spectroscopy; see Fig. S1b).
Samples 63503,15 (Figs. 3c and S2b) and -,20 (Figs. 3d and S2c) are monomict anorthositic breccias with randomly oriented plagioclase fragments set in a cataclastic matrix. No signs of recrystallization were observed. In both samples, plagioclase (An94–97) displays strong undulatory extinction and pyroxene displays weak undulatory extinction, indicating peak shock pressures of 5–20 GPa. In sample 63503,20, a 300 μm diameter plagioclase grain appears unshocked (S1c), possibly indicating that a brecciation event was the last shock event (>5 GPa) recorded in this sample. Fragment 63503,20 shows two compositionally different pyroxenes (Fig. 3d), where orthopyroxene (En61–65Fs33–37Wo2) is the host of small clinopyroxene (En42Fs13Wo45) inclusions. In sample 63503,15 (Fig. 3c), pyroxene is only present as small fragments in the matrix and not as inclusions within the large plagioclase fragments.
Sample 63503,21 (Figs. 3e, S2e, and S2f) is dominated by anorthite (An97–98) with minor olivine (Fo43–44). Pyroxene occurs as inclusions in mm-sized plagioclase grains. The cataclastic anorthosite consists of a 500 μm diameter plagioclase adjacent to a brecciated zone of randomly oriented angular plagioclase fragments. One region of the brecciated zone displays a granoblastic texture in which some fragments have 120° grain boundaries (Fig. S2f). This indicates a higher degree of thermal annealing (e.g., >500 °C Kruhl 2001 and references therein) compared with the previously described anorthosites.
Sample 63503,16 (Figs. 3f and S2d) is a granoblastic to granulitic breccia. This olivine-noritic anorthosite is composed of large plagioclase fragments hosting small inclusions of clinopyroxene. Small mafic minerals such as clinopyroxene, olivine, and rarely troilite grains are also present in the feldspathic matrix. The granoblastic texture indicates a high degree of thermal annealing and recrystallization. In addition, both plagioclase (An96–98) and olivine (Fo48–50) crystals are fractured and plagioclase displays strong undulatory extinction, indicating peak shock pressures of approximately 10–20 GPa.
In contrast to the cataclastic anorthosites, samples 63503,14 and -,17 (Figs. 4a and 4b, S3a, and S3b) represent mechanically undeformed crustal rocks. Sample 63503,14 (Figs. 4a and S3a) is an anorthosite consisting of a chemically zoned, 800 μm diameter plagioclase grain adjacent to a granoblastic textured region containing 50–100 μm diameter plagioclase crystals with well-developed 120° grain boundaries (upper left side of Fig. S3a). The chemically similar crystals developed different textures in the two regions, thus providing evidence for a secondary thermal event that resulted in either crystallization or recrystallization of the smaller plagioclase grains. No shock features are observed in either of the texturally different plagioclase crystals, indicating peak shock pressures <5 GPa. Sample 63503,17 (Figs. 4b and S3b) is an anorthositic troctolite composed of calcium-rich plagioclase (An96–98) and magnesium-rich olivine (Fo79–80) with traces of pyroxene and chromite. Olivine grains range in diameter from 10 to 400 μm and have rare orthopyroxene inclusions (En80Fs17Wo3). Micrometer-sized clinopyroxene inclusions in plagioclase were identified by Raman spectroscopy. Both olivine and plagioclase crystals have well-developed 120° grain boundaries (S3b). The equilibrium texture indicates cooling of an igneous rock in a plutonic environment or a larger impact melt pool. Using the optical microscope with crossed polarizers, all minerals display sharp extinction and no signs of shock metamorphic overprint are observed, indicating shock pressures <5 GPa.
Polymict feldspathic fragmental breccias. The three polymict feldspathic fragmental breccias 63503,1, 63503,3, and 63503,4 (Figs. 5a–c and S4a–d) contain fragments of individual minerals (e.g., olivine, pyroxene, plagioclase, chromite) and lithic clasts. The individual mineral and lithic fragments range in diameter from approximately 2 to 30 μm and 200 to 500 μm, respectively. The majority of the lithic clasts are partly to totally metamorphosed primary rocks or even breccias, and impact melt clasts. Some impact melt clasts in samples 63503,1 and 63503,3 contain μm-sized FeNi particles (Figs. 5a and 5b). The degree of thermal annealing increases from fragment 63503,1 to 63503,4 (Figs. 5a, 5 c, S4a, and S4b). Evidence for later thermal annealing was not observed in the lithic breccia 63503,1, which has a matrix consisting mainly of mineral clasts. The lithic breccia 63503,3 (Figs. 5b, S4c, and S4d) contains crystallized (devitrified) impact melt clasts and some regions of the breccia are recrystallized in situ. The granulitic texture of breccia 63503,4 documents strong thermal annealing and complete recrystallization. The relic olivine and pyroxene in this sample developed reaction rims with the neighbouring plagioclase crystals and melt.
Impact melts. Two very distinct impact melt rocks were identified amongst the particles analyzed, 63503,9 and 63503,13 (Figs. 6a, 6b, and S5a–c). The intergranular impact melt rock 63503,9 (Figs. 6a and S5c) is composed of angular to subrounded plagioclase fragments (An94–97) surrounded by interstitial and unequilibrated quenched pyroxenes (En43–48Fs19–31 Wo37–21) and a myriad of pores. The interstitial regions in 63503,9 host a large number of μm-sized FeNi particles (Ni up to 30 wt%), but no sulfur-rich particles (i.e., troilite). The intersertal impact melt rock 63503,13 (Fig. 6b, S5a, and S5b) is composed of plagioclase laths (An93–96) radially emerging from different centers and cross-cutting each other. Olivine (Fo61–63) and orthopyroxene (Opx = En63–66Fs23–30Wo3–11 and Cpx = En51–57Fs19–23Wo22–29) occur interstitially. Vesicles were observed in a few regions of the quickly quenched melt (S5b). The texture of the intersertal melt rock indicates cooling in a melt sheet thicker than 5 m to allow formation of plagioclase crystals (see Deutsch and Stöffler 1987 and references therein). This indicates a later emplacement to the Apollo 16 site. In contrast, the quickly quenched intergranular impact melt rock could be directly transported to the Apollo 16 site.