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Structure and Petrology of a Cumulus Norite Boulder Sampled by Apollo 17 in Taurus-Littrow Valley, the Moon

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Structure and Petrology of a Cumulus Norite Boulder Sampled by Apollo 17 in Taurus-Littrow Valley, the Moon Structure and Petrology of a Cumulus Norite Boulder Sampled by Apollo 17 in Taurus-Littrow Valley, the Moon EVERETT D. JACKSON U.S. Geological Survey, Menlo Park, California 94025 ROBERT L. SUTTON U.S. Geological Survey, Flagstaff, Arizona 86001 HOWARD G. WILSHIRE U.S. Geological Survey, Menlo Park, California 94025 ABSTRACT grammatic nomenclature of Streckeisen and gabbroic composition. As yet, no system- others (Geotimes, 1973), and the cumulus atic study has been made of fragments of A glass-coated half-meter-size boulder nomenclature of Jackson (1967)1 will be rocks that have survived crushing, and in was sampled by the Apollo 17 crew at sta- used throughout. which original textures are at least partially tion 8 near the foot of the Sculptured Hills. Jackson (1971) had previously noted ex- preserved. The rock proved to be a coarse-grained amples of cumulates from among the lunar Of these, sample 15415, a thermally (0.5-cm) plagioclase-orthopyroxene cumu- rocks returned by the Apollo 11 and Apollo metamorphosed and partly shattered anor- late, and the samples are the only true nor- 12 missions (portions from samples 10084 thosite composed of more than 98 percent ites returned from the lunar surface. and 12057, respectively). The origin of the calcic plagioclase (Wilshire and others, Photographs of the boulder showed it to fine-grained Apollo 11 rock, which was 1972), was collected as a fragment in a contain at least nine structural surfaces and called a plagioclase-olivine cumulate (mi- breccia from the Apennine Mountain front. four glass veins. Orientation and inspection crogabbro) in Jackson's (1967) double This rock has an average grain size of about of three of the returned samples resulted in nomenclature systems, is not understood, 0.5 cm, and the maximum dimension of the identification of six surfaces and one although it texturally resembles a rock some plagioclase crystals is as much as 1.8 vein. One of the structural surfaces visible formed by crystal settling. The Apollo 12 cm. The texture of 15415, as viewed in in the boulder was identified as primary sample was a coarse-grained olivine- randomly cut thin sections, is neither cumulus planar lamination, which was pigeonite cumulate (olivine-gabbro) that clearly cumulus or noncumulus, in part be- folded through an angle of at least 35° be- appearedto be related to the main popula- cause of the lack of postcumulus phases tween two oriented samples, whereas frac- tion of Apollo 12 olivine and pigeonite other than plagioclase (Vernon, 1970). In ture sets representing the other surfaces phyric basalts. hand sample, the rock appeared to us to were coincident. The boulder is believed to Wilshire and Jackson (1972) pointed out have a planar lamination, but oriented slabs be a sample of the deeper highlands or that rocks called "dunites," "pyroxenites," of the sample were not cut for thin sections. submare lunar crust, derived from a depth and "anorthosites" by a number of authors A small relict of a clast in Apollo 16 sample of 8 to 30 km and somewhat shock- (Wood and others, 1970; Marvin, 1973) in 67435, with an average grain size of about metamorphosed during at least two excava- the Apollo 14 samples were simply single 0.1 cm (Prinz and others, 1973a) may also tion events. The chemical composition of recrystallized mineral grains, disaggregated be a cumulate with original textures pre- the norites, when determined, should be of from rocks that were dominantly served, and indeed, some similar rocks ap- special interest in view of the large amount feldspathic and whose grain size ranged up pear to be included among the Apollo 16 of literature concerning glass, cataclasite, to more than 1 cm. rake samples (Keil and others, 1972), espe- hornfels, and "basalt" of noritic composi- In retrospect, it would seem that no true cially some of the fragments from the LM tion returned by other Apollo missions. anorthosite (defined as a phanerocrystalline area and station 5 of that mission. At least However, the cumulus texture of the boul- rock in the sense of Holmes), no true norite, two examples of phanerocrystalline rocks der precludes its being representative of any and only one coarse-grained cumulate that appear to retain their original textures magmatic liquid composition, suggests that (olivine-gabbro, sample 12057, 20[9]) were collected by the Apollo 17 crew. Sam- the lunar crust is heterogeneously layered, closely associated with mare basalt (and ple 76535 is probably a deformed partially and that plagioclase sank, not floated, in almost undoubtedly composed of a packed recrystallized plagioclase-olivine cumulate magmatic liquids that formed the lunar collection of 0.3- to 0.4-cm-sized phenocryst with a small amount of postcumulus crust. Key words: Moon, cumulate, norite, minerals containing postcumulus plagio- clinopyroxene (Gooley and others, 1974; igneous, lunar crust, anorthosite, Apollo clase) had been returned from the moon be- Brown and others, 1974; Haskin and 17. fore the Apollo 15 mission. others, 1974; Wilshire, 1974). The rock has With the advent of Apollo 15, visits to an average grain size of about 0.5 cm and INTRODUCTION lunar highland scarps yielded an abundance would be classed as either a troctolite or an of coarse-grained rocks, metamorphosed in olivine gabbro by Streckeisen and others In the extended discussions of rocks that some degree, and most with anorthositic or (Geotimes, 1973). The rock appeared might represent samples of the primordial among the fragments of a rake sample from the North Massif (station 6), but there was lunar crust, the terms "anorthosite," "nor- 1 In this nomenclature, a cumulus crystal is a crystal ite," "cumulate," and other like words (mineral) that came into existence outside of and prior no description of the individual fragment have been used in widely different ways by to the magmatic sediment of which it now forms a part. when it was collected on the lunar surface. Postcumulus material is primary material that formed in Another relatively undamaged phanero- different authors. In this paper, the rock the places it now occupies in the magmatic sediment, as definitions of Holmes (1928), the dia- a "cement" between cumulus grains. crystalline rock was collected by the Apollo Geological Society of America Bulletin, v. 86, p. 433-442, 10 figs., April 1975, Doc. no. 50401. 433 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/4/433/3443672/i0016-7606-86-4-433.pdf by guest on 02 October 2021 434 JACKSON AND OTHERS 17 crew at station 8, at the base of the tinued, "This looks like maskelynite or at boulder, it was again photographed, this Sculptured Hills (ALGIT,2 1973a, 1973b). least blue-grey plagioclase, and a very — time by Astronaut Cernan (see Figs. 1A and It was also identified and described by the let's say light yellow-tan mineral, probably 2). Four chips were collected from the top crew, and it is the subject of this paper. orthopyroxene. It's fairly coarsely crystal- side of the norite boulder. These were as- line" (ALGIT, 1973a, p. 293). As labora- signed the LRL numbers 78235 through GEOLOGIC SETTING OF tory examination showed later, this was a 78238; however, during sample orientation THE BOULDER very accurate field description! studies (see Appendix), two of the frag- Sampling of the boulder proceeded as fol- ments were fitted together, thus eliminating Station 8, visited during the Apollo 17 lows: after climbing uphill to the rock, lo- number 78237 and changing it to 78235,2. third EVA, was located about 4 km north- cated on a 10° slope east-northeast from the The original 78235 became 78235,0. Sam- east of the LM, at the base of the Sculptured Rover, Schmitt first described the rock and ple 78238, although not fitted successfully Hills. Although the steeper (25° ±), upper then photographed it in place (NASA to 78235,0 or to 78235,2, is believed to reaches of the Sculptured Hills are among the highlands surrounding Taurus-Littrow valley, they differ from the North and South Massifs in that they appear to be more heav- ily pocked by craters, more hummocky, and more markedly lineated; they also appear to be less rocky and are a shade of gray in- termediate between the dark mantle and the massifs. Station 8 was only about 20 m above the valley floor and within the zone mapped as dark mantle in detailed pre- mission maps (Wolfe and Freeman, 1972; Lucchitta, 1972). Small cohesive clods and coherent rock fragments were common in the station area, but larger rocks and boulders were rare. With the exception of the one glass-coated norite boulder, all other rocks larger than about 20 cm examined by the crew resem- bled subfloor basalt (ALGIT, 1973a, 1973b). The crew's attention was immediately at- tracted to the norite boulder, even though it lay 50 m uphill from the Rover, because the boulder was perched on the surface, in con- trast to all other rocks larger than about 10 to 20 cm in the immediate area, which were partially buried. The fact that it was perched was interpreted by Astronaut Schmitt to mean that the rock had arrived there quite recently. In addition, the ab- sence of a boulder track and the presence of a glass coating suggested to Schmitt that the rock was probably exotic, having been thrown into the area by impact. However, the relationship of the rock to the higher part of the Sculptured Hills is not known. The morphology and color of the Sculp- tured Hills certainly suggest that the under- lying rock type may be different from the North and South Massifs, where breccias were sampled.
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