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0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System PETROLOGY of RAKE SAMPLES

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0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System PETROLOGY of RAKE SAMPLES PETROLOGY OF RAKE SAMPLES FROM APOLLO 16 STATIONS 1, 4 AND 13. J. Warner, W.C. Phinney, NASA/MSC, Houston, TX 77058; C.H. Simonds, LSI, Houston, TX 77058; R. Gooley and G.E. Lofgren, NAsA/MsC, Houston, TX 77058. The petrogenesis of rake samples from Apollo 16 Stations 1 (33 fragments), 4 (43 fragments), and 13 (39 fragments) was investigated by studying thin sec- tions of 48 smples, perfoming partial analyses by electron microprobe of plagioclase, olivine, pyroxene, and mesostasis, and performing crude broad beam analyses by electron microprobe. Companion studies report on the petrology of the poikilitic rocks (1)and the metal-spherules(2). We classified the rocks into 7 main lithic types: cataclastic anorthosite, breccia, glass, devitrified glass, mesostasis-rich diabase, diabase, and poiki- litic. Many samples contain more than one lithic type. In some cases the various parts of the rocks are clearly related as being allochthonous and auto- chthonous; e.g. , breccias contain clasts of cataclastic anorthosite. However, in other cases distinction.between host and guest is not clear. Table 1 illus- trates the range of combinations of lithic types observed. Note that there are no samples that contain combinations of glass, devitrified glass, mesostasis- rich diabase, or diabase. Cataclastic Anorthosite. Cataclastic anorthosite (61549, 61558, 61575, 64535,' 64537, 64546, 64559, 64579, 64818, 64819, 63566) is the same as the type. - I1 rocks described by the pol lo 16 PET (3) and-sample 15415. Breccia. Breccias are polymict rocks; i.e., they contain clasts of several different lithic and mineral types. The breccias have been subdivided on the basis of their matrix mineralogy and texture. Glassy matrix breccias (61525, 61535, 64587, 64588, 64826, 64827, 63575, 63589) are texturally similar to the low grade breccias described by Warner (4). Feldspathic matrix breccias (61516, 64548) are texturally similar to 14063 and 14082 described by Warner (4). Meta- morphosed breccias (64576, 63578, 63598) are texturally similar to the inter- mediate to high grade breccias described by Warner (4). In several breccias (61548, 64565, 63538) the matrix consists in part of mesostasis and skeletal plagioclase laths which range in size by a factor of 5 in different parts of each thin section. These latter rocks are considered as melted matrix breccias. -Glass. Several samples (61535, 63575) consist dominantly of flow-banded glass. They contain large clasts of glassy matrix breccia, smooth-walled spher- lcal vesicles, and numerous tiny (2~)metal blebs, A few equant plagioclase crystals ($25~across) occur in 61535 suggesting the start of the devitrifica- tion described below. Devitrified Glass. The devitrified glasses are holocrystalline and contain smooth-walled spherical vesicles. They consist of plagioclase (h9I,,~ 7), oli- vine (F06~-79),and mesostasis. The texture of the plagioclase allows the following subdivisions: dendritic with equant crystals (61556), dendritic (64577, 63538, 63566), dendritic to spherulitic (64579) and spherulitic (61588). A few relict, subrounded plagioclases (50 to lOOw across) are scattered through- out each sample. Mesostasis--Rich Diabase. The mesostasis-rich diabase samples (64535, 64537, 64559, 64585, 64586, 64818, 63557, and clasts in 64546, 64548) are intermediate both texturally and mineralogically between devitrif ied glass and diabase. The mesostasis-rich diabase samples are all holocrystalline, contain rough-walled spherical vesicles. and have a dark chocolate brown color in thin section. 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System PETROLOGY OF RAKE SAMPLES Warner, J. et 8.1. Mineralogically they consist of "H" shaped plagioclase laths (An9 1-9 6 ) with interstitial olivine (Fo71-e4), mesostasis, and colorless to red spinel. Relict, rounded to subrounded, unshocked plagioclase (75-400ym across) is scattered throughout each sample. Diabase. Diabase is used here to indicate "igneous" looking, subophitic, . Relict rounded to each sam le. At least one diabase (61549) has a recrystallized texture. Sample 6E 576 has a diabasic rather than a subophitic texture as it contains skeletal to euhedral olivine crystals not cut by plagioclase laths. Poikilitic. The poikilitic samples are similar to rocks such as 61156. They contain irregular vugs and compositionally zoned mafic silicate oiko- crysts that range in size from 200 to 2000pm across and in shape from prismatic to irregular ovoids. These rocks may be subdivided by the mineral0 y of the dominant oikocryst : olivine (61569, 64567) ; orthopyroxene (64568, 6f569, 64575, 64815, 64816, 63547, 63558) i and pigeonite (61568, 63556). The oikocrysts enclose plagioclase and auglte or olivine. The outlines of the oikicrysts are marked by a concentration of lathy ilmeRite, clinopyroxene, and Ca-phosphate. Relict plagioclase (75-600 microns across, An96) and olivine (50-100 microns across, Fo79 ) are scattered throughout each sample. The boundaries of these groups are somewhat arbitrary; some samples repre- sent gradations between two groups. For example: 64577 and 63557 are between devitrified glass and mesostasis-rich diabase; 64559 is between mesostasis-rich diabase and diabase. 64585 and 63557 are between mesostasis-rich diabase and poikilitic; and 64567 is between diabase and poikilitic. The sequence of tex- tures, especially the gradational boundaries from glass to devitrified glass to mesostasis-rich diabase to diabase to poikilitic suggests a genetic sequence for these rocks. Bulk Fe0-A1203-Ca0 in mol percent for 35 samples (no anorthosites) is shown in Figures 2 and 3. Most samples group about the Reid et al. (5) high- land basalt or anorthositic gabbro composition. The two most Fe-poor samples are a feldspathic matrix breccia, and a feldspathic diabase. As is shown in Figure 3, the poikilitic rocks appear to be more Fe-rich than the remaining lithic types. Other broad beam data suggest that the poikilitic rocks are also enriched in K, Ti, and P relative to the other lithic types. Some of the dia- basic samples (those that contain augite) are less Fe-rich than the bulk of the other samples. The approximately constant bulk composition of these rocks, especially the series from breccia and glass through to diabase supports the suggestion that these rocks with diverse texture and mineralogy are genetically related. We suggest that the glasses through poikilitic rocks represent a sequence of cooling histories of partially melted material. Since the Ni content of similar Apollo 16 rocks is between 100-260 ppm (3); i.e., in the range of lunar soils and breccias and a factor of 2 to 4 higher than primary lunar igneous rocks (6,7), we further suggest that the source material was similar to the feldspathic and glassy matrix breccias we have described. The melted matrix breccias represent source material that has been only incipiently melted. 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System .
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