CONSORTIUM BRECCIA 73255: MATRIX PETROGRAPHY AND EXPOSURE HISTORY; Odette B. James, U.S. Geological Survey, Reston, Va., and Kurt Marti, Univ. of Calif. at San Diego, La Jolla, Calif. Introduction: 73255 is one of six Apollo 17 light-gray breccia samples. Four were collected from Boulder 1, Station 2, at the base of South Massif; two (73215 and 73255) were taken from 1ight-mantle regolith at Station 3.-~llsix samples are genetically related [I]and appear to have been derived from the same source, a deposit of Serenitatis, ejecta atop South Massif [2]. They are interpreted as consolidated ejecta from a large lunar impact, robably the Sereni tatis event, by most petrologists who have studied them f 1,3,4]. Two papers in this volume report results of consortium studies of 73255. This paper presents a hand-specimen and petrographic description and data on exposure history; the other [5] reports Rb-Sr data. Exposure history: 73255, when collected, was sitting atop the soil on the rim of a 10-m-diameter crater. The rock has sparse micrometeorite pits on all surfaces and pit density that nowhere approaches saturation. Distribution of pits and of dust adhering to the surface suggests that the rock was tumbled at least once. 81Kr-83Kr exposure age of an interior chip (of nonvesicular dark-gray aphani te) is 149 m.y. (78~r/83~r)Sallation is quite high (0.2055+ .a01 3), suggesting that there was 1i ttle shieqdlng during the irradiation 161. (131Xe/126Xe)cosmi ray = 3.80k.12, indicating the presence of a neutron-pro- duced isotopic component; the calculated neutron flux of -0.8 n/cm2 is small, however. In sum, these data suggest that during the entire irradiation the rock was within a few centimeters of the lunar surface. Primary hand-specimen structures and lithologic characteristics: 73255 is a fist-sized oblate spheroid consisting 1argely of dense gray-to-black aphanitic rock. In the interior of the sample, the aphanite is tough and nonvesicular; it contains patches of a slightly friable light-gray mottled lithology, patches of friable white granulated clast material, and abundant 1ithic and mineral clasts. At the rock surface, there is a sharply defined rind of gray-to-black highly vesicular aphanite. Thickness of this rind ranges from 0 to 1 cm; in some laces it has been completely eroded, but in others its original surface appears to be preserved. A hani tes, The bulk of both nonvesicul ar (interior) aphani te and vesicular (+rind aphan~te is dark gray and has a dull luster; locally in both aphanites, however, color is nearly black and luster is near vitreous. The near-vi treous aphanite occurs as globules and lenses enclosed by dull-lustered aphanite, as globules and lenses within the mottled 1ithology, and as globules within and bordering granulated clast material. All types of aphanite also form distinct rinds on lithic and mineral clasts. In thin section the aphanites are seen to consist of abundant small lithic and mineral clasts set in a dark groundmass of minute grain size (<5pm). The groundmass intergrowth is texturally distinct from the clasts it encloses; its dominant constituents are plagioclase and pyroxene. Groundmass in nonvesicu- lar dull-lustered aphani te is dark gray and has microsubophi tic texture; its grain size averages -2pm (range is 1-5pm). Locally the texture is micro- variol itic or microg,*aphic. Groundmass in highly vesicular dull-lustered aphanite is coarser grained, "4pm, and shows very well developed microsub- ophi tic texture. Groundmass in near-vi treous aphani te is brown and consists 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System CONSORTIUM BRECCIA 73255 James O.B.

of a mat of mafic-mineral clasts and poor in 1i thic clasts, contain appreciable groundmass, and are not porous or friable. In other places, porous friable veinlets are aradational- into qranulated felds~athicclast material. Granulated felds~athicclast'material. This 1 ithology forms highly irregular lenses, which are as much as 2 cm long but generally <1 mm thick. It consists of friable porous aggregates of fragments, with 1 ittle or no groundmass. The fragments are mostly broken mineral grains; of these, plagioclase is dominant and olivine and pyroxene are subordinate. Lithic clasts are generally present in minor amounts, but may locally be abundant. The 1 i thi c clas ts are of: f ine-grai ned ANT-sui te rocks; f ine-grained mosaic anorthosite; deformed and partly recrystallized coarse-grained anorthosite; barely devitrified pale-brown glass; and near-vitreous aphanite. Mineral and lithic clasts. Most clasts are subangular to subrounded mineral fragments: of these, pl agiocl ase is dominant; 01 ivine and pyroxene are subordinate; and si 1 ica minerals, spinels, opaque oxides, zircon, and phos- phate minerals are present in trace amounts. Clasts of devi trified maskelyni te and recrystal 1i zed shocked plagioclase crystals are common. The remainder of the clasts are fragments of broken but ungranulated rocks. Among these, fine- grained ANT-sui te rocks are greatly dominant. Felsic ("grani tic1') fragments are present in minor amounts but are ubiquitous. Two significant types of lithic clast are present in trace amounts: pyroxene-dominant basalts having variol i tic or subophi tic textures, and Apol lo-16-type feldspathic breccias. Most clasts show no evidence of significant shock or deformation, heating, or equilibration with groundmass related to the breccia-forming event. Exceptions are: the clasts of shocked plagioclase mentioned above; sparse clasts of silica minerals that have <5um-thick pyroxene rims formed by reaction with groundmass; sparse cl as ts of recrystal 1i zed shocked pl agioclase that have rims a few microns thick in which recrystal 1 i zation produced different textures than in the cores; felsite clasts that show shock effects and/or extensive partial me1 ting ; and sparse ANT-sui te 1i thic clasts that have textures result- ing from partial melting and subsequent crystallization. Postconsol idation structures: The rock shows the effects of a very weak post- consolidation shock event. It is cut by several throughgoing fractures, and locally it has concentrations of short shock-induced tension fractures. No significant intergranular deformation was produced by this event; the only

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James O.B. deformation that may possibly be related is extremely weak extinction varia- tion, which is present in most mineral clasts. There is no evidence of significant postconsol idation heating or shear. Discussion: The textures of the aphanites indicate that they formed as fragment-laden melts, and the melts crystallized or devitrified to form the very fine grained groundmasses. The breccia-forming process was one of mechanical mixing of melt and clasts, probably during a very large impact event; the sample is not a reheated preexisting soil or breccia (reasoning leading to these conclusions was given in [I]and [7] for 73215; the same reasoning applies to 73255, which has simi 1ar characteristics). Shape of the rock, nature of its rind, and apparent absence of post- consolidation deformation .indicate that the rock has retained its original size, shape, and internal structures produced during the breccia-formi ng event. Thus, study of the structures and the interrelations of the various litholo- gies should yield insight into the processes of ejecta transport and consoli- . dation that operated in major 1unar impacts. The data now available permit some tentative speculations on the nature of some of these processes. (1) As the crater cavity grows during an impact event, impact melt and fragmented rock form a sheet lining the cavity; material in this sheet moves radially out- ward until it reaches the crater 1ip, where it is ejected [9]. The intimate mixing of clasts and me1 t to form the fragment-laden me1 ts could have occurred during radial transport across the crater floor. (2) Injection of dikes of fragment-laden melt into the basement in early stages of crater excavation, followed by later stage deformation, fragmentation, and ejection of materials consisting of mixtures of basement rocks and dikes, could have produced some of the observed structures. (3) The globular forms of some of the aphani tes and their mutual ly inclusive relationships could be the result of breaking up of the fragment-laden me1 t sheet after ejection, followed by in-flight aggregation of the particles so produced. The textures of 73255 aphanites suggest that they may have had less clast- me1 t equilibration than any of the other fragment-laden-me1 t rocks in the Apollo sample collection. 73255 shows less clast-melt equilibration and a simpler history than 73215, the other breccia currently under study by our con- sortium [I,7]. Clasts in 7321 5 aphani tes were not completely isotopically equilibrated or outgassed of Ar during the breccia-forming event [5,9,10]. Thus we anticipate that studies of 73255 clasts will permit us look back through the effects of the breccia-forming event at the nature of the preimpact source terrane. ames O.B. et al., 19/5, Proc. Lunar Sci. Conf. 6th, p. 5 S.[:idJWolfe E.W., 1975, Proc. Lunar Sci. Conf. 6th, p. 2443. tij Wood J.A., 1975, The Moon, 14, p. 505. [4] Dence M.R., Grieve R.A.F. and Plant A.G., 1976, Proc. Lunar Sci. Conf. 7th, p. 1821. [5] Compston W. , Foster J.J. and Gray C.M., this volume. [6] Marti K. and Lugmair G.W., 1971, Proc. Lunar Sci. Conf. 2nd, p. 1591. [7] James O.B., 1976, Proc. Lunar Sci. Conf. 7th, p. 2145. [8] Gault D.E., Quaide W.L. and Oberbeck V.R., 1968, in Shock Metamorphism of Natural Materials, Mono, Baltimore, p. 87. [9] MUller H.W., Plieninger T., James O.B. and Schaeffer O.A., this volume. [lo] Jessberger E.K., Kirsten T. and Staudacher T., 1976, Proc. Lunar Sci. Conf. 7th, p. 2201,

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