50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 1033.pdf THE FORMATION CONDITIONS OF CHONDRITES: INSIGHTS FROM NORTHWEST AFRICA 10850 – AN OXIDIZED CV3 CHONDRITE. M. M. Jean1,2, A. Patchen1, M. B. Sueilem3, L. A. Taylor1, 1Planetary Ge- osciences Institute, Dept. Earth and Planetary Sciences, University of Tennessee Knoxville. 2University of Alaska- Anchorage, Dept. of Geological Sciences, Anchorage, AK 99508. 3Smara Refugee Camps of Western Sahara, Tin- douf, Algeria. ([email protected]) Introduction: For the first time, data are presented lected from Algeria, in the Tindouf province. Approx- from a detailed investigation of all components of the imately 50.3 g were recovered on August 2015, and Northwest Africa (NWA) 10850 carbonaceous chon- acquired by us shortly thereafter. Its classification as a drite. NWA 10850 is a newly classified meteorite col- carbonaceous chondrite was recently approved [1]. chondrules matrix chondrule matrix AOA chondrule CAI AOA matrix CAIs matrix AOA chondrules matrix AOA 2 mm Figure 1. Combined Mg Kα (blue) + Al Kα (green) + Ca Kα (red) X-ray maps showing chondrules, CAIs (cal- cium-aluminum inclusions), amoeboidal olivine aggregates (AOA), and other features found in NWA 10850 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 1033.pdf We have undertaken a comprehensive study to pro- Di Hd vide an overview of the petrography and geochemistry of this meteorite. Based on these results, we add NWA 10850 to the Allende-like subgroup (CV3oxA), which typically contains approximately 40 vol % matrix ma- terial (with little phyllosilicate), 40–50 vol % intact En Fs chondrules, and ~ 0.2 vol % metallic FeNi. Fo0 Fa100 Petrography: NWA 10850 consists of an ~20-30 An Ab % fine grained matrix and an assemblage of well- Figure 2. Major element compositions for NWA 10850 defined chondrules, large CAI’s (up to 2 mm), and silicates amoeboidal olivine aggregates (AOA: Fig. 1). Chon- in chondrule precursor material, or by open system drule types include barred-olivine and granular-olivine processes during chondrule formation [12]. and/or pyroxene (Fig. 1). Most chondrules have been Metal and sulfides within chondrules are small enriched in FeO along the rims and to some extent also (<10 µm) and uncommon. Metals are predominately along interior grain boundaries. The CAIs host zoned awaruite and sulfides are pentlandite (12-20 wt. % Ni spinels and aluminous clinopyroxene. Spinel zonation and 0.6-1.1 wt. % Co). Troilite and kamacite also in- is seen as Mg-rich cores with increasing Fe towards the frequently present. Minor chromite occurs in limited margins in all examples. Melilite (gehlenite) is present regions. Other metals observed include kamacite; na- in two CAIs, one is compact and spherical with a pos- tive gold and silver; argentite; indium; and carbon (as sible ‘wark-lovering’ rim; the other is more altered. graphite?). We highlight the rare occurrence of Many also contain small (<5 µm) perovskite grains, awaruite (ideally Ni Fe), which occurs in several high- and a few contain hibonite. Many CAIs in this sample 2 ly oxidized chondritic meteorites [13]. Its occurrence are highly altered, containing abundant nepheline, has previously been explained either as formed from a hedenbergite, calcite, and sodalite. Olivine and anor- melt or by the transformation of taenite at temperatures thite may also be present within some CAI. The AOA of < 500 °C [14]. generally demonstrate considerable Fe-enrichment Summary: Based on its texture, petrography, min- around each olivine grain. Fayalite content of the AOA eralogy, and phase chemical composition, NWA 10850 olivines span a similar range as the chondrules. The is a typical example of a CV chondrite. The chondrules matrix is predominately fayalitic olivine (Fa ) and 38-48 hosted within the meteorite likely formed by different hedenbergite. There has been speculation that AOAs mechanisms: 1) melting of solid precursors, including form a bridge between CAIs and chondrules [2]. Other chondrules of earlier generations and refractory inclu- phases in the matrix include pentlandite, awaruite, bar- sions, and 2) melting, evaporation, and condensation of ite, calcite, magnetite, andradite, and chromite. solids during large-scale collision between planetary- Methods: Major- and minor-element compositions size bodies. Based on the textures and mineralogy, two were determined with a Cameca SX-100 electron mi- populations of CAIs are identified: 1) very refractory, croprobe at the University of Tennessee. The micro- compact spherules composed of hibonite, grossite, Al- probe was calibrated for each session using both natu- rich pyroxene, perovskite, gehlenitic melilite, and spi- ral and synthetic standards. nel, and 2) less refractory, igneous and non-igneous Results: Olivine is zoned to varying degrees with inclusions composed of melilite, Al-Ti diopside, anor- compositions ranging from Fa up to Fa (Fig. 2). It 0.35 42 thite, and olivine. There is no evidence to indicate that is unclear if olivines in this meteotite ascribe to the all the components of NWA 10850 formed in a single nebular model [3, 4] or have asteroidal origins [5, 6, nebular reservoir as a result of a single stage process. 7]. Pyroxenes are represented by enstatite (Wo En 0.9 98 Rather, the diversity of chondrules and CAIs makes to Wo En ), diopside (Wo En to Wo En ), and 2.7 96 33 66 49 48 possible that many components of NWA 10850 have hedenbergite (Wo En to Wo En ), with rare pi- 50 45 51 49 their own individual histories. geonite (Fig. 2). Al-rich diopsides (Wo ) occur in Al- >50 References: [1] Patchen A. and Taylor L. A. (2016) Meteoriti- rich chondrules. These compositions overlap those of cal Bulletin 105; [2] Russell S.S. et al. (2017) Geochemical Journal typical CV chondrites. Plagioclase ranges from An77 to 51; [3] Nagahara H. et al. (1988) Nature 331, 516-518; [4] Nagahara H. et al. (1994) GCA 58, 1951-1963; [5] Choi B.-G. et al. (2000) An99 (Fig. 2). Glass may also be present. The Fe/Mn values of olivine and pyroxene indicate Meteoritics & Planet. Sci. 35, 1239-1248; [6] Hua X. et al. (2005) GCA 69, 1333-1348; [7] Jogo K. et al (2008) LPSC 39, 1576 (absr.); redox conditions and volatility [8, 9, 10, 11]. The [8] Miyamoto M. et al. (1993) JGR 98, 5301-5307; [9] Goodrich Fe/Mn systematics from NWA 10850 are typical of C.A and Delaney J.S (2000) GCA 64, 149-160; [10] Papike J.J et al. CV chondrites, e.g., Vigarano and Allende [11, 12]. (2003) AmMin 88, 469-472; [11] Jones R. H. (2012) Meteoritics & Differences in Fe/Mn values between different chon- Planet. Sci. 47, 1176-1190; [12] Berlin J. et al. (2011) Meteoritics & Planet. Sci. 46, 513-533; [13] Clarke R.S. et al. (1970) Smithsonian drite groups could be caused by factors such as varia- Contributions to the Earth Sciences 5, l-53; [14] Rubin A.E (1991) bility in the abundances of metal, sulfide and/or silicate AmMin 76, 1356-1362. .