Notes and Discussions H. G. WILSHIRE \ G G SCHABER ] Geological Survey, Menlo Parkj California 94025 L. T. SILVER California Institute of Technology, Pasadena, California 91109 W. C. PHINNEY National Aeronautics and Space Administration, Manned Spacecraft Center, Houston, Texas 77058 E. D. JACKSON U.S. Geological Survey, Menlo Par\, California 94025

Geologic Setting and Petrology of

Apollo 15 Anorthosite (15415)

ABSTRACT Apennine Mountains, the major ring of moun- tains bounding the Imbrium basin (Fig. 1; 15 sample number 15415, popularly Carr and others, 1971; Apollo Lunar Geology called the "Genesis Rock," is coarse-grained Investigation Team, 1971), with the view of anorthosite composed largely of calcic plagi- obtaining samples of pre-Imbrian lunar crust. oclase with small amounts of three pyroxene Sample number 15415 was collected by astro- phases. The rock was found as a clast in a piece nauts David R. Scott and James B. Irwin from of friable soil breccia on the lip of crater, the Apennine front on the lower slopes of a small young crater on the lower slopes of the Hadley Delta during their second excursion on Apennine Mountains. The mode of occurrence the lunar surface. Small samples of anorthositic of sample 15415 indicates that it has undergone rock obtained during earlier Apollo missions at least two, and possibly three or more, frag- appeared to be exotic to the mare regions, and mentation events. These events are reflected in suggested the possible presence in the highlands the texture of the rock by shattered and of an early anorthositic crust formed by mag- granulated minerals. An earlier thermal meta- matic accumulation of plagioclase (Wood and morphic event is represented by irregular bands others, 1970; Keil and others, 1970). The lunar- of coarsely recrystallized plagioclase and minor surface identification of a large, coarse-grained pyroxene that cross larger plagioclase grains. rock composed mainly of plagioclase was there- Preliminary observations of textural relations fore recorded enthusiastically by Scott and of the large plagioclase grains are consistent Irwin. either with accumulation of plagioclase followed by overgrowth of cumulus grains and post- GEOLOGIC SETTING cumulus crystallization of minor interstitial Spur crater, from which the largest collection pyroxene, or with metamorphic recrystalliza- of samples (including sample 15415) from the tion that eradicated original textures. Any of Apennine front was obtained, lies on the lower the events in the complex history of this rock slopes of Hadley Delta about 60 m above the may have affected apparent radiometric ages. mare surface (Fig. 1). The crater apparently Comparative abundance of similar, though penetrated a regolith composed of a mixture of smaller, pieces of anorthositic rock in the area debris for the most part eroded from the and dominance of originally coarse-grained Apennine Mountains and in smaller part gabbroic-anorthositic clasts in breccia at Spur ejected from South Cluster on the nearby mare crater suggest that sample 15415 is the least- surface (Fig. 1). Rare fragments of mare basalt, deformed member of a suite of similar rocks characterized by an abundance of reddish- that were ejected from beneath the regolith brown pyroxene, are present in the soil at Spur at Spur crater. crater but are lacking in the much more preva- INTRODUCTION lent breccia fragments presumably excavated from beneath the soil. The breccia contains The principal objective of the A folio 15 many fragments of "anorthosite" and plagi- mission was to sample part of the front of the oclase-rich "gabbroic" rocks, but most frag-

Geological Society of America Bulletin, v. 83, p. 1083-1092, 14 figs., April 1972 1083

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Spur crater -O Sample^ I km 15415

Figure 1. Locality map of the Apennine-Hadley region of the Moon.

merits are so severely granulated that they do While rock 15415 was lifted easily off the clod not superficially resemble sample 15415. We with the collecting tongs and was therefore believe, however, that they form a related weakly attached to it, it appeared to group of rocks of which sample 15415 represents Scott (August, 1971, oral commun.) to have the least-deformed member. been an inclusion in the underlying material. Sample 15415 rested conspicuously (Figs. 2, Furthermore, two small fragments 1 to 2 cm 3) on a clod (sample 15435) of darker material in diameter of similar plagioclase-rich rock were and, together with the clod, formed one of the embedded in the clod (sample 15435). These largest (17 cm) fragments in the immediate observations strongly suggest that rock 15415 vicinity. The texture and albedo of sample was a clast in the weakly bonded material on 15415 were described by the crew as unique which it was found. Debris has accumulated at in the area, perhaps because of the large size of the base of the clod, burying it from 1/8 to 1/4 the reflecting plagioclase cleavage surfaces. its height, suggesting that both the clod and There are, however, a large number of rock rock 15415 were in their pre-sampled position fragments with similar albedos scattered around for a long period of time. The weakly bonded the north rim of Spur crater (Fig. 4) as seen soil breccia is glassy and composed of mineral, in NASA photographs AS15-90-12227 and lithic, and glass debris. 12228 (a fact later confirmed by Scott and The mode of occurrence of sample 15415 Irwin). Similar samples, not yet studied in indicates that it has undergone at least two detail, were found in the rake sample and as fragmentation events: (1) separation of rock clasts in breccia collected at Spur crater. 15415 from its parent mass of anorthositic rock

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Figure 2. Sample 15415 in place on the lunar sur- is exposed at the top of a fragment of friable soil breccia. face. The white anorthosite fragment, about 9 cm long,

and incorporation into a breccia; and (2) granulated about 1 cm across and separation of the soil breccia containing sample perhaps 4 to 5 mm thick on one edge of the rock 15415 from its source (or, alternatively, and less suggests the presence of other fracture sets likely in our opinion, local weak shock lithifica- along which the rock has broken. Coarse tion of soil around the clast in question). Since individual feldspar grains are shattered and most other anorthositic rocks sampled or cut by thin granulated zones in which the photographed at Spur crater are clasts in more plagioclase appears chalky white in contrast to coherent breccia, a third fragmentation event, the translucent light-gray color in the unbroken which isolated sample 15415 from a more parts. No preferred orientation of either grains coherent breccia, may have occurred between or twin planes was found, but the conditions of events (1) and (2). examination were far from ideal. In thin section the rock is composed of three PETROGRAPHY OF SAMPLE 15415 principal size groups of plagioclase: (1) very Sample 15415 is a blocky subangular rock small angular fragments and fine interlocking whose size is approximately 9x6x4 cm. It aggregates along the crushed zones; (2) is composed of 99 percent calcic plagioclase regularly shaped (polygonal) grains in the size (about Ang?; see Stewart and others, 1972) in range 0.1 to about 1.3 mm (Fig. 5); and (3) grains as much as 18 mm in maximum dimen- more irregular grains that range from 1.3 to sion and 1 percent or less of very light brownish more than 6 mm (Fig. 5) and are as large as 18 pyroxene grains typically less than 100 p across. mm in hand specimen. The comparatively small The rock is cut by a set of widely spaced (1 cm) thin sections do not reveal the true coarseness parallel fractures marked by thin zones of of the rock as seen under low-power binocular intensely granulated feldspar. A small area of magnification. Pyroxene occurs along bound-

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Explanation

rocks fragments > approximately 0.5 cm

completely buried rock or clod ,-tr & sharp crater with raised rim

intermediate crater with raised rim

subdued crater

shallow trough or lineament

s h ad ow

base of small ridge or mound

.5m Figure 3. Sketch map showing sample 15415 and its surroundings.

aries of the plagioclase, in places molded on The intense shattering and granulation ob- irregularities of the plagioclase, and as in- served macroscopically are readily evident in clusions in the coarse plagioclase. The pyroxene thin section. Small displacements of twin planes enclosed in the plagioclase is augite, while that in large feldspar grains along the fractures are along grain boundaries is in some cases augite common, as are displacements of one set of and in other cases large grains of pigeonite. The crush zones by another. Very irregular zones augite is commonly twinned, very pale brown, along some fractures contain plagioclase that and free of inclusions, whereas the pigeonite is extinguishes at the same position as does one set slightly darker and contains minute oriented of polysynthetic twins in coarse plagioclase cut inclusions that have not been identified. The by the fractures. Patchy distribution of pigeonite is generally coarser (to 0.2 mm) than twinned plagioclase within single grains also the associated augitic pyroxene (to 0.1 mm), appears to be a result of shattering. These and some pigeonite grains appear to have thin patches are not the result of physical or lattice rims of augite. A single grain of orthopyroxene rotation, because faint traces of twins parallel containing clinopyroxene exsolution lamellae to those in surrounding plagioclase cross locally and enclosed by a clinopyroxene jacket was into the essentially untwinned areas. Thin seen. Two grains are tentatively identified as crushed zones appear in plane-polarized light olivine and one as apatite. as faintly pinkish brown and have an average

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60

0-40

20 rT,."H- 1.0 2.0 4.0 8O '/is 1/8 1/4 1/2 I 2 Millimeters Figure 5. Size-frequency histograms of large, irregular plagioclase grains (left) and of smaller polyg- onal plagioclase grains (right). Number on histograms Figure 4. Sketch map showing the distribution of is the number of grains measured. light (high-albedo) rock fragments (irregular back areas) in the vicinity of sample 15415. Lined areas are shadows of the . to the younger shatter fractures. Boundaries that appear to be unmodified commonly have a refractive index less than that of the host coarse sawtooth pattern (Fig. 10) or are very plagioclase. The size of the crushed material is irregular on a small scale. One junction was so small that the presence or absence of glass observed (Fig. 11) in which large plagioclase could not be definitely ascertained, and for the grains are joined at a triple point similar to most part we were unable to determine whether those of the smaller polygonal plagioclase, and or not the plagioclase in these zones is recry- one section has such extensive development of stallized. The largest crushed zone (Fig. 6) coarse polygonal plagioclase that distinction of seen in thin section is composed of angular the two size-textural groups is very difficult. plagioclase debris with a little colorless glass. However, original crystal boundaries, un- Pyroxene grains appear to have reacted with modified by polygonization, may occur at the fine glassy material to form dark grains with places where pyroxene is molded on several diffuse extinction. plagioclase grains. Small rounded cores (about Polygonal plagioclase in the size range 0.1 to 1/3-grain diameter) of some of the irregular 1.3 mm occurs in irregular patches (Fig. 7) and plagioclase grains differ slightly in extinction bands (Fig. 8) that cross single grains of coarse position from the rims; these may indicate plagioclase (see Lunar Sample Preliminary compositional zoning or deformation. Examination Team, 1971a). Although original fractures are completely healed, the offset of HISTORY OF SAMPLE 15415 twins (Figs. 8, 12) in the host plagioclase along Textural features observed macroscopically such bands indicates that the polygonal and in thin section clearly show that sample plagioclase formed along microfaults. Polygonal 15415 has had a complex history. The youngest grains are commonly joined at triple points events affecting the rock resulted in shattering with interfacial angles near 120° (Figs. 7, 9). and granulation, and such structures cross both Pyroxene grains that occur at some triple junc- polygonal and large irregular plagioclase (Fig. tions of polygonal plagioclase form three 12). The intersection and offset of fracture additional triple points (Fig. 9). Other zones probably reflect the history of multiple pyroxenes are found along binary grain bound- fragmentation events, perhaps three or more, aries of plagioclase or enclosed within plagi- which may be inferred from the mode of occur- oclase grains. rence of the rock. The character of the shatter- Boundaries between large plagioclase grains ing closely resembles shock fracturing of are commonly modified either by development plagioclase in basalt from Lonar crater, India, of polygonal grains or by microfaults belonging and for the most part appears to indicate a low

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Figure 7. Photomicrograph showing patch of polygonally re- crystallized plagioclase near bound- ary of two large plagioclase grains.

order of shock, perhaps below 100 kb (Chao, size compared to a thin section, by the poly- 1968). The presence of glass in one crushed gonally recrystallized plagioclase, and by the zone, however, suggests pressures above ap- shock fracturing. Boundaries between plagi- proximately 300 kb (Chao, 1968), if the glass oclase grains that are unmodified by recrystal- is shock-formed. lization or by shock fracturing could have The polygonal plagioclase predates granula- formed as a result of accumulation of plagioclase tion and was formed in fractures crosscutting followed by postcumulus overgrowth along the large irregular plagioclase grains. The with minor space-filling pyroxene growth (see textural relations so closely resemble those of Jackson, 1967). The texture of the coarse terrestrial high-grade thermally metamor- irregular grains could also have resulted from phosed rocks that it seems clear that the polyg- extensive thermal metamorphism of the rock, onal plagioclase reflects a thermal event. The and present data available to us are not coarseness of the polygonal plagioclase and its adequate to unequivocally distinguish these lack of zoning suggest long-term annealing. The processes. In this connection, it would be of partial recrystallization affected both plagi- particular interest to know the composition and oclase and pyroxene. exsolution relations of pyroxene in all three of Textural relations of the coarse, irregular its textural settings: (1) inclusions in large plagioclase grains are obscured by their sheer plagioclase grains; (2) directly associated with

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Figure 8. Photomicrograph showing irregular bands of poly- gonal plagioclase, with a little pyroxene, cutting a single large plagioclase grain.

Figure 9. Photomicrograph showing small pyroxene grain at a triple point of polygonally re- crystallized plagioclase grains. The pyroxene forms three additional triple points.

polygonally recrystallized plagioclase, either interpretations are based. The early crystalliza- along grain boundaries or enclosed in the tion and recrystallization episodes tend to ob- polygonal grains; and (3) along boundaries scure the event that initiated any given radio- between large plagioclase grains. Other impor- genic system. The later shock metamorphic tant information bearing on the question of events are probably also capable of modifying, igneous or metamorphic origin of the textural if not' 'resetting," some of the isotopic systems. relations among the large plagioclase grains includes study of the rock fabric. The grain RELATION OF SAMPLE 15415 TO size is so coarse that this would require slabbing ITS SURROUNDINGS the specimen and examination with a collimated The principal objective of Apollo 15 was to light source. We hope that these data will be sample crustal material from the Apennine obtained in other more detailed studies of the Mountains. The mode of occurrence of sample rock. 15415 does not in itself allow us to say whether The age and initial conditions during the or not it represents the material sought in the original formation of this rock are of great mission. Past experience has shown that far- interest. Several events recorded by the com- travelled material is present in the regolith at plex textures we have observed could have all sites so far visited on the Moon (see Shoe- influenced radiogenic systems upon which age maker and others, 1971). When collected,

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Figure 11. Photomicrograph showing a large plagioclase grain (left) with a triple point junction similar to those of polygonal plagioclase junctions.

sample 15415 was a clast in a friable soil breccia signs of polygonal recrystallization (Fig. 14). and hence may have been derived from sources The pyroxene content of these rocks ranges outside the Hadley Rille area. However, there from less than 1 percent to about 35 percent. are several more broadly based reasons for Hence, the relative abundance of rocks like supposing that the rock is locally derived: rock 15415 and the great abundance of other rocks 15415 is not unique in the area; similar though that are texturally and compositionally akin to smaller rocks occur in the soil breccia on which it suggest that rock 15415 is the least-deformed rock 15415 rested, in the rake sample from Spur member of a suite of coarse-grained gabbroic- crater, on the surface around sample 15415 anorthositic rocks. The abundance of these (reported by Scott and Irwin), and as clasts in rocks along the sampled part of the Apennine coherent breccias obtained along the Apennine front suggests that they are not exotic. front. Large breccia blocks at and near Spur A regolith-free surface on the Apennine crater consist principally of cataclastically front would presumably be composed of breccias deformed light-colored rocks set in a black dominated by clasts of originally coarse-grained aphanitic matrix. These clasts, like rock 15415, feldspathic rocks. The origin of the clasts in are characterized by very pale-colored pyroxene in these breccias is, of course, not known. They in dotninantly feldspathic rocks. As seen in thin may have been thrown there by early impact section, the one such rock so far examined was events elsewhere on the Moon, then exposed by originally coarse grained (Fig. 13) and retains formation of the Imbrium basin, or they may

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Figure 12. Photomicrograph showing polygonal plagioclase grains along a microfault (shown by offset of twin lamellae) in a single plagioclase grain. Note ir- regular bands of finely crushed plagioclase that cross polygonal plagioclase as well as its host coarse plagioclase.

Figure 13. Photomicrograph showing relict coarse plagioclase in a cataclastically deformed norite from Spur crater.

represent brecciation as a result of the Imbrium event. In any case, the signs of multiple breccia- tion so well displayed in the rocks (Lunar Sample Preliminary Examination Team, 1971b), are lacking in the Apollo 15 breccias. It appears, therefore, that the rocks obtained on the Apennine front, including sample 15415, generally represent lower levels of the lunar crust that are less reworked than levels sampled by Apollo 14. ACKNOWLEDGMENTS We are greatly indebted to David R. Scott and fames B. Irwin for discussions of the field relations of sample 15415. Open exchange of information on the mineralogy and petrology of the sample with David B. Stewart was very Figure 14. Photomicrograph showing relict polyg- beneficial. We are grateful to Ray Sabala for his onally recrystallized plagioclase in norite of Figure 13.

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excellent drafting of the illustrations, to C. G. petrology: New York, Pergamon Press, p. Utter for his expert photographic work, and 561-598. K. A. Howard and D. C. Ross, U.S. Geological Lunar Sample Preliminary Examination Team, Survey, for critical review of the manuscript. 1971a, Preliminary examination of Apollo 15 lunar samples: Science (in press). This work was done under NASA contracts 1971b, Preliminary examination of lunar T-4738A and T-65253-G. samples from Apollo 14: Science, v. 173, no. REFERENCES CITED 3998, p. 681-693. Shoemaker, E. M., Jackson, E. D., and Hait, Apollo Lunar Geology Investigation Team, 1971, M. H., 1971, Surficial and bedrock stratigraphy Preliminary report on the geology and field of the landing site [abs.]: Geol. Soc. petrology at the Apollo 15 landing site: U.S. America, Abs. with Programs (Ann. Mtg.) v. Geol. Survey open-file rept., 30 p. 3, no. 7, p. 703. Carr, M. H., Howard, K. H., and El-Baz, F., 1971, Stewart, D. B., Ross, M., Morgan, B. A., Apple- Geologic maps of the Apennine-Hadley region man, D. E., Huebner, J. S., and Commeau, of the Moon: U.S. Geol. Survey Map 1-723. R. F., 1972, Mineralogy and petrology of Chao, E.C.T., 1968, Pressure and temperature lunar anorthosite 15415, in Proceedings of the histories of impact metamorphosed rocks-based Third Lunar Science Conference, January on petrographic evidence, in French, B. M., 1972, Houston, Texas (in press). and Short, N. M., eds., Shock metamorphism Wood, J. A., Dickey, J. S., Jr., Marvin, U. B., and of natural materials: Baltimore, Mono Book Powell, B. N., 1970, Lunar anorthosites: Corp., p. 135-168. Science, v. 167, p. 602-604. Jackson, E. D., 1967, Ultramafic cumulates in the Stillwater, Great Dyke, and Bushveld in- trusions, in Wiley, P. J., ed., Ultramafic and MANUSCRIPT RECEIVED BY THE SOCIETY OCTOBER related rocks: New York, John Wiley & Sons, 28, 1971 p. 19-38. PUBLICATION AUTHORIZED BY THE DIRECTOR, U.S. Keil, K., Bunch, T. E., and Prinz, M., 1970, GEOLOGICAL SURVEY Mineralogy and composition of CALIFORNIA INSTITUTE OF TECHNOLOGY, PASA- lunar samples, in Proceedings of the Apollo 11 DENA, CALIFORNIA, CONTRIBUTION No. 2121 lunar science conference, v. 1, Mineralogy and (SILVER)

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