Petrology of Apollo 16 Poikilitic Rocks

Proceedings of the Fourth Lunar Science Conference (Supplement 4, Geochimica et Cosmochimica Acta) Vol. 1, pp. 613-632 1973LPSC....4..613S Petrology of Apollo 16 poikilitic rocks CHARLES H. SIMONDS The Lunar Science Institute, 3303 NASA Road 1, Houston, Texas 77058 JEFFREY L. WARNER and WILLIAM C. PHINNEY Geology Branch, TN6 NASA Johnson Space Center, Houston, Texas 77058 Abstract-Poikilitic rocks, virtually all with low K-KREEP composition are found in all non-mare suites of lunar samples. Most of these rocks have low calcium pyroxene oikocrysts enclosing many chadacrysts of feldspar. The poikilitic rocks are proposed to form by crystallization of an impact generated partial melt with over 70% liquid. The protolith is gas bearing polymict breccias and/or soil. The key to forming a poikilitic texture is to rapidly chill a low K-KREEP melt a few tens of degrees, so that abundant feldspar and olivine grains are nucleated. The melt then cools more slowly due to the presence of an insulating layer of material. Slow cooling through the plagioclase-olivine-pyroxene peritectic, allows few pyroxene nuclei to form. Continuous pyroxene growth is possible due to ease of silica transport through the abundant(> 50%) remaining melt. A number of observations support this hypothesis: (1) The euhedral shape of feldspar chadacrysts, not expected for subsolidus melt crystallization or solid state recrystallization. (2) Flow alignment of feldspar chadacrysts. (3) Large spherical pores, which appear to have formed in a liquid. (4) Preservation of only refractory olivine and plagioclase relics and the lack of pyroxene which had a lower melting point than olivine or plagioclase. (5) Relic feldspar is more anorthite rich, i.e., more refractory than matrix feldspar. (6) Resorbed olivine in pyroxene oikocrysts. (7) Normally zoned oikocrysts. (8) High temperature (- 1200°) partition coeffi- cients for Fe and Mg between low Ca pyroxene and augite. (9) Concentration of late crystallizing ilmenite, phosphate and high K phases between oikocrysts. (10) The compositional correlation with poikilitic texture. (11) The abundance of impact melted rocks with higher melting points than the low K-KREEP composition. INTRODUCTION POIKILITic* ROCKS, virtually all with low-K-KREEP composition, have been found at all non-mare landing sites, which indicates that they constitute a widespread and therefore important textural group of rocks. Wilshire and Jackson (1972) report poikilitic clasts in polymict breccias returned by Apollo 14. Phinney et al. (1972) reported two poikilitic fragments in 4-10 mm fines from Spur Crater on the Apennine Front. Delano et al. (1973) report that poikilitic fragments make up about 10% of 2-4 mm soil fractions from all Apollo 16 sampling localities except *Definitions of less commonly used terms: Poikilitic-the texture in which large optically continuous crystals of one mineral enclose many smaller crystals of other phases. The texture is also referred to as fishnet texture. Oikocryst-host crystal through which smaller crystals (chadacrysts) are distributed as inclusions. The net in the fishnet texture. Chadacryst-the inclusions, typically feldspar, within the oikocrysts. The holes in the fishnet. 613 © Lunar and Planetary Institute • Provided by the NASA Astrophysics Data System 614 C. H. SIMONDS et al. from the crest of North Ray (Station 11). Our work indicates that poikilitic fragments make up a similar proportion of the rake samples from Stations 1, 4, and 13. At Taurus-Littrow (Apollo 17) foliated poikilitic units up to 5 meters thick were 1973LPSC....4..613S observed in the boulders at both the North and South Massifs. In this paper the petrology of 13 poikilitic rocks returned by Apollo 16 will be discussed including 11 rake samples from Stations 1, 4, and 13 and large rocks 60315 and 65015. Also a few comments will be made about 61156 and a clast in 66035. Companion papers to this rake sample study are by Warner et al. (1973) and Gooley et al. (1973) which discuss a general classification of Apollo 16 rock types and the metallic particles in these rake samples respectively. HAND SPECIMEN PETROGRAPHY In hand specimen, the poikilitic rocks are very fine grained, dense, hard, pale- green rocks with few fractures. Some of the poikilitic rocks are mottled with rectangular patches, 0.5-3 mm long which display a slightly paler green color than observed in the region between the patches. These patches are pyroxene oikocrysts. In a few areas on several rocks, tablet shaped plagioclase up to 0.5 mm long is recognized. Other recognizable minerals include iron-nickel in smooth or hummocky textured spheres up to a millimeter across and clasts of feldspar and anorthosite, shocked to various degrees. No macroscopic foliations or lineations were noted in the Apollo 16 poikilitic rocks. All poikilitic rocks have up to 10% spherical or irregularly shaped pores 0.4-10 mm in diameter. The pore linings are nearly free of projecting crystals. MODAL MINERALOGY Modes (Table 1) were determined by counting 300 or more points on thin sections. The point counts include both relics and matrix. Chips for thin sections of the rake samples 615XX, 635XX, 645XX, and 648XX were carefully selected 2 but are small (1~20 mm ) and may not be representative of the total variability of the parent rocks, which weigh 4-20 g. Low calcium pyroxenes with more than 5 mole percent wollastonite in analyses by microprobe are classified as pigeonites, and have a higher average birefringence than pyroxenes with less than 5% wollastonite. TEXTURES The poikilitic rocks consist of crystals of pyroxene (or olivine) up to 3 mm long (the oikocrysts) enclosing abundant smaller crystals (the chadacrysts), mostly feldspar and smaller amounts of mafic silicates. The region between the oikocrysts, referred to as the inter-oikocryst region, contains crystals of the same minerals found as chadacrysts plus accessory minerals and polygonal vugs. The crystals in the inter-oikocryst region are generally coarser-grained than the chadacrysts. Scattered throughout all parts of the poikilitic rocks are vesicles and relics of plagioclase, olivine, spine!, and lithic fragments. © Lunar and Planetary Institute • Provided by the NASA Astrophysics Data System 1973LPSC....4..613S Table 1. Basic mineralogy of poikilitic rocks. @ Most Abundant t""' Oikocrysts Other Oikocrysts Mode volume % = and average Oikocryst and average "'1= composition size composition Plag+ Sample Number of Oikocryst mm of Oikocryst Mesostasis Opx Pig Aug Oliv Opaques =Q. "ti - 60315 Orthopyroxene 0.5-1.1 Augite 55 34 NF 4 6 =l'C Wo4 Enso Fs,6 Wo4, En4s Fs11 "'1 61568 Pigeonite 0.3 NF present NF present present present present '-< "ti ..... (Poikilitic ('1) Wos En10 Fs22 ....'"'1 portion only) 0 ::t."'= 0 Olivine 1.5 Pigeonite 68 NF 6 2 22 2 (IQ 61569 '-< =l'C F010 FS24 Wo9 En79 Fs,2 ,...,,0 • 63547 Pigeonite 0.4 NF 67 NF 25 NF 8 1 "ti ,:;>- "'1 W 06 En14 Fs20 0 2.. < 63556 Pigeonite 0.4 NF 68 NF 29 NF 2 1 0 ....Q. l'C W 01 En1, Fs20 Q. 63558 Orthopyroxene 0.6 Augite 56 32 NF 12 2 2 ,:;-°' C" 0 '-< Wo, Ensz Fs,, Wo,9 Enso Fs11 fr. =-l'C 64567 Olivine 0.15 NF 69 NF 10 NF 20 1 Ens, Fs,. -;:;· z '"'1 > 0 rn 64568 Pigeonite 0.5 NF 56 NF 39 1 2 2 (') > Wos En16 Fs,6 "' > 64569 Pigeonite 0.4 NF 57 NF 19 tr. 21 4 "'"'1 0 Wo6 En16 Fs,s "C 64575 Orthopyroxene 0.2 NF 47 43 NF 6 3 2 '-<=- .... W o, En1, Fs2• "'t") "' 64815 Orthopyroxene 0.15 NF 55 34 NF NF 9 2 0 Wo. Enn Fs24 a 64816 Pigeonite 0.4 NF 59 NF 34 NF 4 2 rn '-< Wo6 En10 Fs,s "'l'C 65015 Pigeonite 0.4 NF 61 NF 29 6 1 3 s W 06 Enos FS29 -°'Vt 616 C. H. SIMONDS et al. Oikocrysts enclose 70-90% of the thin section area. Of 13 poikolitic rocks studied, pigeonite was the dominant oikocryst in seven samples, orthopyroxene in four and olivine in two (Table 1). Augite oikocrysts occur in 60315 and 63359, but 1973LPSC....4..613S both samples are dominated by orthopyroxene oikocrysts. The typical low calcium pyroxene oikocryst is equant and anhedral and 0.3-0.5 mm long (Fig. la, b). A small number of the oikocrysts have well defined rectangular shapes; these tend to be somewhat longer than the equant oikocrysts. In sample 60315 the augite oikocrysts discontinuously encloses the orthopyroxene oikocrysts as if the augite were an overgrowth. Some of the low calcium pyroxene oikocrysts in all poikilitic rocks contain irregularly shaped olivine grains with smoothly curving outlines, typical of resorbtion. In all samples but one, the oikocrysts are continuous in thin section. In 61156, the feldspar grains form a continuous network enclosing many separate areas of pyroxene which extinguish together in cross-polarized light (Fig. 2). Abundant chadacrysts make up as much as 50% of the oikocrysts. The most abundant chadacrysts are euhedral feldspar tablets less than 0.04 mm long. Augite is a rare chadacryst phase as is ilmenite. The corners of the feldspar chadacrysts are slightly rounded with radii of curvature of 0.001-0.005 mm. The tablet shape and slight curvature of the comers of feldspar grains are similar to those found in feldspathic basalts such as 68415 (Warner et al., 1973). The morphology of feldspar chadacrysts varies from rock to rock (Fig. 3). 64575 like 65015 has nearly equant rectangular crystals whereas 64567 and 64569 Fig. 1. Photomicrograph of 63556. (a) Crossed polarizers, (b) uncrossed. © Lunar and Planetary Institute • Provided by the NASA Astrophysics Data System Petrology of Apollo 16 poikilitic rocks 617 1973LPSC....4..613S Fig.

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