Don’t miss our talks on siderophiles in brecciated lunar meteorites ( ) and lunar cumulate impact melts ( ) on ! abstract # 2013 abstract #2061 Thursday afternoon spherules (n=3) glasses & vitrophyres (n=3) regolith nuggets (n=3) very fine-grained intergranular melt (n=2) proto-poikilitic melt (n=5) medium cumulate melt (n=8) Fig. 7 Compositions of metal particles compared to metal particles in endogenous lunar rocks. Dunite 72415 and 72417 metal particles after [ ] and [10], confidence class for pristinity (CCP) - 9 ; spinel troctolite 67435 after [13] - CCP ; “feldspathic lherzolite” (an olivine gabbro norite) 67667 [14] - CCP 7 (confidently pristine [11]); troctolite clast in 73235 [14] - CCP ; n ield for Apollo 17 basalts after [17]; metal particle in the olivine cumulate of NWA 733 after [18]; green line represents the “cosmic trend” of Ni:Co concentrations of 20:1 in chondrites; green f Shi r 161 4 (confidently pristine [after 11]) troctolite 76535 taenite after [12], CCP - 9 [11], note that this rock also contains high-Co kamacite; 8 (confidently pristine [11]) 4 (doubtful pristinity [11]) orite 78527 after [16] - CCP 4 [14]; f ields indicate compositional ranges of kamacite in equilibrated H, L and LL chondrites after [6, 19]. Orange lines encircle data from the same clasts. ş troctolite clast in 60035 after [15] - CCP 6 (pristine with caution [14]); All Shi r 161 data normalized to 100 wt%. ş LL L H Apollo 17 mare basalts feldspathic lherzolite 67667 spinel troctolite 67435 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 5 10 15 20 25 Ni (wt%) Co (wt%) 30 35 40 45 50 55 60 3.5 4.0 troctolite clast 73235,136 olivine cumulate NWA 733 troctolite 60035 norite 78527 dunite 72415 & 72417 [4-10] 100 m μ 100 m μ Acknowledgments A. J. Irving (University of Washington) for the samples, D. A. Kring (LPI Houston) for analytical support, P. Carpenter & A. Foreman (Washington University St. Louis) for EMPA support and preliminary data, R. Zeigler (Nasa-JSC Houston) for preliminary data. We appreciate funding from NASA grant NNX11AJ66G. INTRODUCTION Iron-Nickel(-Cobalt) Metal in Lunar Rocks Revisited Axel Wittmann and Randy Korotev Iron-Nickel(-Cobalt) Metal in Lunar Rocks Revisited Iron-nickel metal particles have long been used as proxies for meteoritic contamination or to support the pristine character of lunar rocks [1-4]. In Shi r 161 (Figs. 1 A-C), a lunar feldspathic regolith breccia, low bulk-rock siderophile trace element concentrations of 88 ppm Ni and 2.6 ppb Ir indicate minor meteoritic contamination [5]. Our survey of metal and sulfide particles in Shi r 161, and their petrologic contexts probes into constraints for their origins. ş ş About 60 metal and 30 sulfide particles were analyzed with the in a 550 mm thin section of Shi r 161. JEOL JXA-8200 Electron Microprobe at Washington University (Fig. 1 B-C) lunar meteorite 2 ş Care was taken to correct for the interference of the Fe k-ß X-ray peak with te Co k-a X-ray peak. SAMPLES & METHODS Sulfides and Phosphides Metal Particles Troilite is most common. Concentrations of P are typically <0.03 wt% and Ni concentrations range from <0.06 wt% to a few wt%. Three pentlandite particles have compositions comparable to lunar and terrestrial pentlandite [8-9]. Possible Fe-phosphide (P up to 9.45 wt%, Ni up to 15.4 wt%, poor totals) was only found in association with troilite and FeNi-metal in a rim on a crystallized spherule. Most metal particles are <10 m in size and are intergrown with troilite. The Ni and Co concentrations of these particles (normalized to 100 wt%) are shown in comparison with metal particle compositions in presumably endogeneous lunar rocks (Fig. 7) and in lunar impactites (Fig. 8). – – μ PETROGRAPHY 10 m μ 10 m μ A B A C SUMMARY µ µ µ µ Metal particles in Shi r 161 spherules and vitrophyres mostly resemble “meteoritic” and mare basalt compositions. Very fine- to finely crystallized melt clasts contain metal particles that have variable abundances of Ni and Co but their ratios mostly follow the chondritic trend. Cumulate (impact) melt particles tend to have very high Ni concentrations, but their compositions in single clasts can be quite variable. Comparison with metal particle compositions in some “pristine” lunar rocks suggests some of their compositions are indistinguishable from such that crystallized in larger volumes of impact melt. ş ş ş in Shi r 161 in Shi r 161 (>~100 m thick?) REFERENCES Fig. 8 Ni versus Co in Shi r 161 metal particles compared to metal particle compositions in lunar polymict rocks. green line represents the “cosmic trend” of Ni:Co concentrations of 20:1 in chondrites; range lines encircle data from the same clasts. ş Field for metal particles in granulite 79215 after [20]; “high cobalt and structures in Apollo 15 soils” after [21]; fields for Apollo 11 and Apollo 17 basalts and Apollo polymict rocks (”range of metal compositions in polymict rocks”) after [4]; “meteoritic” field after [1]; o α γ most All Shi r 161 data normalized to 100 wt%. ş “meteoritic” Apollo polymict rocks granulite 79215 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 5 10 15 20 25 Ni (wt%) Co (wt%) 30 35 40 45 50 55 60 3.5 4.0 high Ni metal particles in Apollo 15 soils pristine troctolite 76535 fine cumulate melt (n=4) Fig. 3 A B C- Note that the plane polarized light images (A) in Figs. 3-6 have the same scale. Cumulate clast #54. Well-equilibrated assemblage of subhedral olivine (Fo ) and plagioclase (An ), and intercumulus poikilitic pyroxene (En , Wo and En , Wo ). The two- pyroxene thermometer of [7] yields an equilibration temperature of 890±20°C. -PPL micrograph; and BSE images of troilite and metal particle (39.7 wt% Ni and 2.1 wt% Co). Five other metal particles in this clast have 37.5-56.4 wt% Ni and 0.5-2.1 wt% Co. 77-79 95-97 78 3 86 45 Fig. 1 A- B- C- Shi r 161 sample material. The sliced up meteorite; Thin section plane polarized light (PPL) image; Back- scattered electron (BSE) image of the thin section with outlines of clasts. ş 500 m μ 500 m μ 10 m μ 10 m μ 100 m μ 100 m μ 500 m μ 500 m μ 500 m μ 500 m μ 500 m μ 500 m μ Fig. 5 A- B- C- D- Proto-poikilitic impact melt clast #8. Optically discontinuous, proto-poikilitic olivine (Fo ) and pyroxene (An ). PPL micrograph; Cross-polarized light micrograph; BSE image ; BSE image of troilite and metal particle assemblage (33.4 wt% Ni and 1.5wt% Co). Three other metal particles in this clast have 35.4-40 wt% Ni and 1.5-1.6 wt% Co. 63-70 95-98 (En , Wo ) embed euhedral to subhedral plagioclase (rotated by ~45°) 44-61 7-36 500 m μ 500 m μ B 10 m μ 10 m μ Fig. 6 A- B- C- Intergranular-felty impact melt clast #43. Very fine-grained anhedral olivine (Fo ) and skeletal pyroxene (An ). PPL micrograph; BSE image; BSE image of troilite and metal particle (53.7 wt% Ni and 3.6wt% Co). Another metal particle in this clast has 51.7wt% Ni and 2.6 wt% Co. 78-83 95-98 (En , Wo ) are intergrown with granular plagioclase 78-81 3 500 m μ 500 m μ 500 m μ 500 m μ 500 m μ 500 m μ Fig. 4 A- B- C- D- Intergranular clast #26 shows anhedral olivine (Fo ) enclosed in poikilitic, twinned pigeonite ( ) and zoned augite (En Fs Wo ) that is interstitial to subhedral plagioclase (An ). PPL micrograph; BSE image; Cathodoluminescence false color image; BSE image of troilite and pentlandite (arrow: 33 wt% Fe, 33.7 wt% S, 28.2 wt% Ni, 5.2 wt% Co). Another pentlandite particle ( and a metal particle (44.2 wt% Ni, 2 wt% Co) were found in this clast. 46-52 39- 47 18-27 30-37 95-97 En Fs Wo to En Fs Wo 34.7 wt% Fe, 33.8 wt% S, 27.8 wt% Ni, 3.1 wt% Co) 56 38 6 47 38 15 Fig. 2 A- B- . BSE image; EMPA data for spot traverse X-Y. Note kamacite Ni (6.8±0.2 wt%) and Co (0 ) concentrations are typical for L- chondrites [6]. .82±0.07 wt% Kamacite- tetrataenite nugget 23 54 5 17 25 107 116 16 10 34 38 88 18 3 8 26 46 115 148 27 43 159 36 63 4 50 118 126 135 156 74 2 6 7 13 12 15 19 20 28 58 95 136 33 81 123 11 3 17 10 4 6 15 5 7 2 8 1 9 13 1 3 4 5 6 7 8 9 10 12 11a+b 13 30 37 44 51 52 53 59 77 102 1 31 14 39 78 105 160 161 162 polymict lithic breccia vitrophyres VLT basalt? crystallized melts monomineralic cumulates glasses hornfelses spherules Two of the particles are 60 and 40 μm nuggets . The rest are ~5–20 μm size troilite. metal-sulfide (Fig. 2 A-B) in , and mostly intergrown with Ø Host materials for the FeNi and FeS particles are clasts of cumulates (Fig. 3), finely crystallized impact melts (Figs. 4-6), and glassy impact melts. The Fes-FeNi particles occur on grain boundaries of plagioclase, pyroxene and olivine crystals, and in the glassy mesostasis. Associations with chromite or ilmenite are rare (~3 % of FeS-FeNi grains). 100 m μ 100 m μ 50 m μ 50 m μ B C D A A B C C D 5 mm A B C spherules (n=3) glasses & vitrophyres (n=3) regolith nuggets (n=3) very fine-grained intergranular melt (n=2) proto-poikilitic melt (n=5) medium cumulate melt (n=8) Dhofar 287 regolith particles [22] fine cumulate melt (n=4) 0 10 20 30 40 50 Distance [μm] 0 1 2 3 4 5 6 7 8 50 55 Ni or or [wt%] Co S Y X B 10 m μ 10 m μ X Y A Department of Earth & Planetary Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, Missouri, [email protected] Photo by G. Hup & P. 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