Lunar and Planetary Science XXX 1491.pdf

MINERALOGY AND PETROLOGY OF A PRIMITIVE SPINEL TROCTOLITE AND GABBROS FROM LUNA 20, EASTERN HIGHLANDS OF THE MOON – Gregory A. Snyder1, Lawrence A. Taylor1, Allan Patchen1, Mikhail A. Nazarov 2 & Tanya S. Semenova2, 1Planetary Geosciences Institute, U i-n versity of Tennessee, Knoxville, TN 37996 ([email protected]); 2Vernadsky Institute, Moscow, Russia, 117975 ([email protected]).

Luna 20 is the only highlands site on the eastern limb tic/troctolitic rock; 2004C = recrystallized anorthosite; of the Moon where samples have been collected; h wo - 2004D = basaltic rock. ever, the collection has received little attention in the PETROLOGY & MINERAL CHEMISTRY – west. Thus, we have studied the petrology and m n-i Spinel Troctolite 2003. This sample consists of eral-chemistry of four “new” fragments from soil e-r mostly plagioclase and small grains (<60 mm) of o i-l turned by the Luna 20 mission. These include a rare vine and spinel. This sample is unique in having x-e spinel troctolite, a troctolite, an anorthositic gabbro, tremely primitive olivine (Fo ), plagioclase (An and a gabbro. This spinel troctolite contains abundant 95-96 95- ), and spinel , sensu strictu (Al O = 66.6-68.9 wt%; Fe-Cr-poor spinels that have higher Mg/Fe than pl o- e 97 2 3 MgO = 24.9-26.1 wt%; FeO = 3.99-4.57 wt%; Cr O naste, nearly pure forsteritic olivine (Fo ), and a 2 3 95-96 = 1.81-2.25 wt%). A rar eZn-rich spinel was also rare zincian spinel. Thus, the spinel troctolite is analyzed (Al O = 55.6 wt%; FeO = 15.8 wt% ZnO; = thought to be the 2 3 most primitive piece of deep crust yet Mg Al analyzed from the Moon. INTRODUCTION – In February of 1972, the USSR landed its second successful sampling mission in the highlands just south of the Crisium ba- sin, along the east- ern limb of the Moon [1]. This robotic mission re- turned 30 g of mate- rial as a bulk scoop Si Ca sample. We have been allocated six fragments, ranging in mass from 1.25 to 7.55 mg. Sub- samples from all six fragments have been prepared for 40Ar- 39Ar analyses. However, because of the small sample sizes, only four are available for petro- logic/mineral- Fig. 1: Spinel Troctolite, 2003 chemical study. 12.6 wt%; MgO = 8.43 wt%; Cr2O3 = 4.89 wt%). The four samples were provisionally designated: 2003 FeNi metal is also present (Co = 0.67-0.72 wt%; Ni = = fine-grained ANT-suite rock; 2004B = no i-r 8.81-8.85 wt%). An x-ray map of this sample is Lunar and Planetary Science XXX 1491.pdf

PETROLOGY & MINERAL CHEMISTRY OF LUNA 20 SPINEL TROCTOLITE: G.A. Snyder et al.

shown in Fig. 1. Such a primitive composition is ex- et al. [2] discovered an assemblage in sample tremely rare in lunar rocks and is likely indicative of 73263,1,11 which included aluminous enstatite, for- a deep-seated origin. steritic olivine (Fo89-91), calcic plag (An96-97), and Troctolite 2004C. This sample is a porous trocto- pink pleonaste (with Cr/Cr+Al = 0.09 and lite consisting of mostly unzoned plagioclase (An95-97) Mg/(Mg+Fe) = 0.80). Based upon experimental and finer-grained globular olivine (Fo76-77). studies [3] which showed that the Al-enstatite-spinel Anorthositic Gabbro 2004B. This sample consists join breaks down to forsterite-cordierite in the CMAS of mostly anhedral plagioclase (100-300 mm) (An96- system at 3 kbar pressure, Bence et al. [2] estimated 97). Olivine and pyroxenes occur as interstitial grains that this assemblage would be stable at that pressure (<70 mm). Olivine varies in composition from Fo83 in the Moon, or roughly 60 km deep (near the base of to Fo73. Pyroxenes occur as two types: pigeonite the crust on the near-side). Dymek et al. [4] reported (Wo19-23En58-59Fs19-22) and augite (Wo37En48Fs14) two similar spinel cataclasite clasts in sample 72435, (Fig.2). one containing a 30 mm inclusion of cordierite. The spinels were classified as pleonastes with compo- Luna 20 Highlands sitions ranging from Mg/(Mg+Fe)=0.44-0.69 and Di Hd Cr/(Cr+Al)=0.03-0.21. Although these samples contained much less primitve spinel and olivine (Fo71-72), and no aluminous enstatite, Dymek et al. [4] postulated that these clasts could also have been excavated from deep within the lower crust. Anderson [5] described a peridotite (15445,10) 2004B 2004D Fig. 2 that contained the assemblage enstatite-plag- En Fs spinel-forsterite and estimated that this sample 2004C 2003 2004D 2004B came from 40 km depth in the crust. Snyder et Fo Fa al. [6] reported a spinel troctolite in a sample 0 20 40 60 80 100 from drive tube 68001/68002. The presence of Spinels occur as tiny grains (< 30 mm), but also as pleonaste, with Mg/(Mg+Fe)=0.65 and larger grains (up to 100 mm) and exhibit two distinct Cr/(Cr+Al)=0.077, and forsteritic olivine (Fo75-81) suggested a deep-seated origin. However, neither compositions: Mg-Al chromite (Al2O3 = 10.3-25.1 cordierite nor Al-enstatite were discovered in this wt%; FeO = 22.1-31.0 wt%; Cr2O3 = 39.5-44.3 wt%; MgO = 4.03-9.16 wt%) and more aluminous spinel sample. Roedder & Weiblen [7] reported a spinel troctolite (Al2O3 = 42.6-53.0 wt%; FeO = 19.1-20.8 wt%; fragment from the 250-500 mm fraction of Luna 20 Cr2O3 = 12.4-21.4 wt%; 11.9-14.4 wt%). FeNi metal (Co = 0.37-0.67 wt%; Ni = 1.11-6.55 wt%) is also soil. Although spinel troctolite 2003 has a similar present, especially as one large (100 mm) grain. plagioclase composition (An95-97) it is dissimilar in Gabbro 2004D. This sample is conspicuous in having higher Mg olivine (Fo95-96 vs. Fo91). The having very coarse-grained (200-400 mm) pyroxene in spinel compositions in spinel troctolite 2003 are more combination with acicular plagioclase (<200 mm in magnesian (Mg# = 91-92) than those analyzed for longest dimension). The uniform high-Ca plagioclase other Luna 20 samples (Mg# = 82; [7]) and the most Mg-enriched, chromian hercynitic spinels (Mg# < 63) composition (An91-96) is indicative of a highlands affinity. The coarse pyroxenes vary in compositions from Apollo 14 [8,9]. Thus, it appears that spinel from pigeonite (Wo En Fs ) to augite troctolite 2003 is the most primitive fragment yet 21-23 31-53 26-48 analyzed from the lower crust of the Moon. (Wo28-37En40-49Fs20-29), both zoned to more Fe-rich compositions (Fig. 2). A traverse across one of the REFERENCES: [1] Heiken, G., Vaniman, D., and largest pyroxene grains indicated zonation from a French, B.M. (1991) Lunar Sourcebook, 631; [2] Bence, ferro-augite rim (Wo38En4Fs58) to a pigeonite core A.E. et al. (1974), Proc. 5th Lunar Sci. Conf., 785-827; [3] (Wo21En49Fs30). Some finer grained pyroxenes ap- Kushiro, I. & Yoder, H.S. (1966), Jour. Petrol. 7, 337-362; [4] Dymek, R.F. et al. (1976) Proc. 7th Lunar Sci. Conf., proach hedenbergite (Wo19-25En1-5Fs70-80) in compo- sition. Minor, euhedral to subhedral olivine (50-150 2335-2378; [5] Anderson, A.T. (1973) Jour. Geol. 81, 219- mm) is associated with plagioclase and is exclusively 226; [6] Snyder, G.A. et al. (1998) LPSC XXIX; [7] Roed- fayalitic in composition (Fo ). der, E. & Weiblen, P.W. (1973) GCA 37, 1031-1052; [8] 1-2 Roedder, E. & Weiblen (1972) EPSL 15, 376-402; [9] PETROGENESIS OF SPINEL TROCTOLITE – Steele, I.M. (1972) EPSL 14, 190-194. Spinel troctolites and cataclasites are thought to repre- sent samples of the deep crust or upper mantle. Bence