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. (mmjean@alaska.edu)

Introduction: For the first time, data are presented

from a detailed investigation of all components of the Northwest Africa (NWA) 10850 carbonaceous chon-drite. NWA 10850 is a newly classified meteorite col-

lected from Algeria, in the Tindouf province. Approx-imately 50.3 g were recovered on August 2015, and acquired by us shortly thereafter. Its classification as a carbonaceous chondrite was recently approved [1].

2 mm

CAIs

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matrix

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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

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We have undertaken a comprehensive study to pro-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 chondrules, and ~ 0.2 vol % metallic FeNi.

Petrography: NWA 10850 consists of an ~20-30 % fine grained matrix and an assemblage of well-defined chondrules, large CAI’s (up to 2 mm), and amoeboidal olivine aggregates (AOA: Fig. 1). Chon-drule types include barred-olivine and granular-olivine and/or pyroxene (Fig. 1). Most chondrules have been enriched in FeO along the rims and to some extent also along interior grain boundaries. The CAIs host zoned spinels and aluminous clinopyroxene. Spinel zonation is seen as Mg-rich cores with increasing Fe towards the margins in all examples. Melilite (gehlenite) is present in two CAIs, one is compact and spherical with a pos-sible ‘wark-lovering’ rim; the other is more altered. Many also contain small (<5 µm) perovskite grains, and a few contain hibonite. Many CAIs in this sample are highly altered, containing abundant nepheline, hedenbergite, calcite, and sodalite. Olivine and anor-thite may also be present within some CAI. The AOA generally demonstrate considerable Fe-enrichment around each olivine grain. Fayalite content of the AOA olivines span a similar range as the chondrules. The matrix is predominately fayalitic olivine (Fa38-48) and hedenbergite. There has been speculation that AOAs form a bridge between CAIs and chondrules [2]. Other phases in the matrix include pentlandite, awaruite, bar-ite, calcite, magnetite, andradite, and chromite.

Methods: Major- and minor-element compositions were determined with a Cameca SX-100 electron mi-croprobe at the University of Tennessee. The micro-probe was calibrated for each session using both natu-ral and synthetic standards.

Results: Olivine is zoned to varying degrees with compositions ranging from Fa0.35 up to Fa42 (Fig. 2). It is unclear if olivines in this meteotite ascribe to the nebular model [3, 4] or have asteroidal origins [5, 6, 7]. Pyroxenes are represented by enstatite (Wo0.9En98 to Wo2.7En96), diopside (Wo33En66 to Wo49En48), and hedenbergite (Wo50En45 to Wo51En49), with rare pi-geonite (Fig. 2). Al-rich diopsides (Wo>50) occur in Al-rich chondrules. These compositions overlap those of typical CV chondrites. Plagioclase ranges from An77 to An99 (Fig. 2). Glass may also be present.

The Fe/Mn values of olivine and pyroxene indicate redox conditions and volatility [8, 9, 10, 11]. The Fe/Mn systematics from NWA 10850 are typical of CV chondrites, e.g., Vigarano and Allende [11, 12]. Differences in Fe/Mn values between different chon-drite groups could be caused by factors such as varia-bility in the abundances of metal, sulfide and/or silicate

in chondrule precursor material, or by open system processes during chondrule formation [12].

Metal and sulfides within chondrules are small (<10 µm) and uncommon. Metals are predominately awaruite and sulfides are pentlandite (12-20 wt. % Ni and 0.6-1.1 wt. % Co). Troilite and kamacite also in-frequently present. Minor chromite occurs in limited regions. Other metals observed include kamacite; na-tive gold and silver; argentite; indium; and carbon (as graphite?). We highlight the rare occurrence of awaruite (ideally Ni2Fe), which occurs in several high-ly oxidized chondritic meteorites [13]. Its occurrence has previously been explained either as formed from a melt or by the transformation of taenite at temperatures of < 500 °C [14].

Summary: Based on its texture, petrography, min-eralogy, and phase chemical composition, NWA 10850 is a typical example of a CV chondrite. The chondrules hosted within the meteorite likely formed by different mechanisms: 1) melting of solid precursors, including chondrules of earlier generations and refractory inclu-sions, and 2) melting, evaporation, and condensation of solids during large-scale collision between planetary-size bodies. Based on the textures and mineralogy, two populations of CAIs are identified: 1) very refractory, compact spherules composed of hibonite, grossite, Al-rich pyroxene, perovskite, gehlenitic melilite, and spi-nel, and 2) less refractory, igneous and non-igneous inclusions composed of melilite, Al-Ti diopside, anor-thite, and olivine. There is no evidence to indicate that all the components of NWA 10850 formed in a single nebular reservoir as a result of a single stage process. Rather, the diversity of chondrules and CAIs makes possible that many components of NWA 10850 have their own individual histories.

References: [1] Patchen A. and Taylor L. A. (2016) Meteoriti-cal Bulletin 105; [2] Russell S.S. et al. (2017) Geochemical Journal 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) 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.); [8] Miyamoto M. et al. (1993) JGR 98, 5301-5307; [9] Goodrich C.A and Delaney J.S (2000) GCA 64, 149-160; [10] Papike J.J et al. (2003) AmMin 88, 469-472; [11] Jones R. H. (2012) Meteoritics & 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 Contributions to the Earth Sciences 5, l-53; [14] Rubin A.E (1991) AmMin 76, 1356-1362.

En Fs

Di Hd

Fa100

Fo0

An Ab Figure 2. Major element compositions for NWA 10850

silicates

1033.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)