NWA 1839

Meta-CO or -CV
(L7 in MetBull 89)

Found 2003
no coordinates recorded A 121.8 g meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and was found in the Sahara Desert and later purchased by American collector N. Oakes in Rissani, Morocco in June 2003. This meteorite was analyzed at Northern Arizona University (T. Bunch and J. Wittke) and was initially determined to be a highly recrystallized L7 chondriteChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such. Notwithstanding this L7 classification, a subsequent oxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only isotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: ¹H, ²H (deuterium), and ³H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. study (D. Rumble III, CIW) showed that NWA 1839 plots on the Carbonaceous ChondriteCarbonaceous chondrites represent the most primitive rock samples of our solar system. This rare (less than 5% of all meteorite falls) class of meteorites are a time capsule from the earliest days in the formation of our solar system. They are divided into the following compositional groups that, other than Anhydrous MineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous (CCAM) line within the field of CO chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such.

The similarly classified meteorite NWA 3133/2643 was found to have FeO/MnO ratios in olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion and pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification category. which are higher than those for ordinary chondrites, and was found to contain other elemental ratios most consistent with a CV carbonaceous chondrite composition (Schoenbeck et al., 2006). These factors led to a tentative revised classification of NWA 3133/2643 by NAU scientists as an ungroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). primitive achondriteAchondrite with an almost chondritic composition with age similar to the primordial chondrites. These should be better classified as "metachondrites".. Subsequent to this revision, a change in the nomenclature previously used to describe those primitive achondrites which are texturally evolved chondrites was proposed by Irving et al. (2005). The term ‘metachondrite’ was suggested to describe those chondrule-free stony meteorites which are actually texturally-evolved chondrites with completely recrystallized and highly equilibrated textures resulting from high degrees of metamorphism or partial meltingAn igneous process whereby rocks melt and the resulting magma is comprised of the remaining partially melted rock (sometimes called restite) and a liquid whose composition differs from the original rock. Partial melting occurs because nearly all rocks are made up of different minerals, each of which has a different melting, and which have elemental ratios and O-isotopic compositions demonstrating affinities to existing chondrite groups (e.g., CV, H, L, and LL). The unique meteorite NWA 2788 has a metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. texture exhibiting ~120° triple junctions, elevated Fe/Mn and Ca/Na ratios, and an O-isotopic composition that plots very near to the TFL; this meteorite is considered by Bunch et al. (2006) to be a metachondriteTerm used to describe a metamorphosed chondrite. Also referred to as a type 7 chondrite. Metachondrites are texturally evolved rocks derived from chondritic precursors and some have been classified as primitive achondrites. associated with an unknown carbonaceous chondrite parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. (see diagram below). If a chondrule-bearing representative of this parent body is found and identified in the future, it was suggested that it be termed a ‘CT chondrite’ (see NWA 2788 photos, abstracts [1, 2], and isotopic plot).

While NWA 1839 and NWA 3133/2643 are both metachondrites with carbonaceous chondrite affinities, and both are purported to have originated from the same nomad collector source, initial O-isotopic studies showed that they are not paired. Further O-isotopic analyses conducted at The Open University, UK, plot within the CO group overlapping the highly extended CV field. In addition, the bulk elemental composition (e.g., Mn/Mg and Al/Mg ratios) shows a close similarilty exists with the CO chondrites (Sanborn et al., 2015). In their continued analyses of this meteorite, Sanborn et al. (2015) demonstrated that NWA 1839 plots very near to the CV chondritesMeteorite class named after the Vigarano meteorite that fell in Italy in 1910. They have abundant large, well-defined rimless (?) chondrules of magnesium-rich olivine (~0.7 mm diameter; 40-65 vol. %), often surrounded by iron sulfide. They also contain 7-20 vol. % CAIs. The often dark-gray matrix is dominated by Fe-rich on a coupled Δ17O vs. ε54Cr diagram (see diagram below). Therefore, in consideration of all the data established thus far, NWA 1839 is considered to be genetically related to the carbonaceous chondrites, and could possibly represent the first CO metachondrite.
click on image for a magnified view

Diagram credit: Irving et al., 79th MetSoc, #6461 (2016)

Chromium vs. Oxygen Isotope Plot

click on diagram for a magnified view

Diagram credit: Sanborn et al., 49th LPSC, #1780 (2018) Northwest Africa 1839 is well preserved and exhibits only moderate weathering. It has an equigranular texture with no relict chondrulesRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at  ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine visible. The low-Ca pyroxenes contain an abundance of CaO (Wo2.3), which by definition (see below) surpasses the maximum amount found in type 6 chondrites (ave. 1.3–1.8 mol%). Curiously, the high-Ca pyroxenes in NWA 1839 do not follow the defined CaO abundances of a type 7 chondrite—that is, a lower abundance for a higher petrologic typeMeasure of the degree of aqueous alteration (Types 1 and 2) and thermal metamorphism (Types 3-6) experienced by a chondritic meteorite. Type 3 chondrites are further subdivided into 3.0 through 3.9 subtypes.. Instead, in contrast to type 6 chondrites which have an average value of ~Wo45, NWA 1839 contains clinopyroxene with Wo47 (Mittlefehldt and Lindstrom, 2001). Northwest Africa 1839 exhibits a shock stageA petrographic assessment, using features observed in minerals grains, of the degree to which a meteorite has undergone shock metamorphism. The highest stage observed in 25% of the indicator grains is used to determine the stage. Also called "shock level".               of only S1, perhaps indicating a long period of annealing following peak shock effects.

Type 7 ordinary chondrites were originally defined by Dodd et al. (1975) according to specific petrographic characteristics. They listed three metamorphic criteria to distinguish between petrographic types 6 and 7:

1. the presence of poorly defined chondrules in type 6, but only relict chondrules in type 7
2. low-Ca pyroxenes in type 6 contain no more than 1.0 wt% CaO (1.0 wt% = ~1.9 mol% Wo), but more than 1.0 wt% in type 7; conversely, the CaO content of high-Ca pyroxenes decreases from type 6 to type 7
3. feldsparAn alumino-silicate mineral containing a solid solution of calcium, sodium and potassium. Over half the Earth’s crust is composed of feldspars and due to their abundance, feldspars are used in the classification of igneous rocks. A more complete explanation can be found on the feldspar group page. grains gradually coarsen to reach a size of at least 0.1 mm in type 7

For those meteorites that experienced metamorphic temperatures high enough for metal–sulfide melting to occur, which most commonly occurs as a result of impact events, an igneous texture would be produced (Mittlefehldt and Lindstrom, 2001). In these cases the use of the Van Schmus–Wood classification scheme is no longer valid, and these meteorites could instead be referred to as primitive achondrites or metachondrites, or they might be impact melts.

Two views of a 1.43 g slice of NWA 1839 are shown above. The photo below shows a thin sectionThin slice or rock, usually 30 µm thick. Thin sections are used to study rocks with a petrographic microscope. viewed in partially polarized light in which the equigranular texture and absence of chondrules and metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn is evident.

Photo courtesy of T. Bunch—Northern Arizona University