NWA 1756

LL3.10

Found October 2002
no coordinates recorded A single 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 weighing 68.2 g was purchased by a team of American collectors in Safsaf, Morocco. This ordinary chondriteWork in Progress Ordinary chondrites (OCs) are the largest meteorite clan, comprising approximately 87% of the global collection and 78% of all falls (Meteoritical Society database 2018)1. Meteorites & the Early Solar System: page 581 section 6.1 OC of type 5 or 6 with an apparent shock stage of S1, is a monomict brecciaType of breccia whose clasts are composed of a single (mono-) rock type, possibly all from a single rock unit (e.g., L6 with L6). Monomict breccias are rare on the Moon because meteoroid impacts tend to mix different kinds of rocks. The example is a terrestrial granite breccia. Image Source: that belongs to the LL chondriteOrdinary chondrites ("low Fe" / "low metal") with only 1 to 3% free metal. Their olivine is more Fe-rich than in the other ordinary chondrites (Fa27-32), implying that the LL types must have formed under more oxidizing conditions than their H or L cousins. Orthopyroxene compositions are also Fe-the rich group. It has a low 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 S1 and a weathering grade of W1/2. A thorough analysis of NWA 1756 was conducted at Northern Arizona University (T. Bunch and J. Wittke). Based on texture and TL sensitivity data, though the latter method has shortcomings in distinguishing between types 3.0 and 3.1, it was determined that NWA 1756 is consistent with a LL3.0 subtype. 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 in NWA 1756 are closely similar to those in the highly unequilibrated LL 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 Semarkona with respect to compositions, zoning profiles, and textures.

Recently, a new petrologic scheme was proposed by J. Grossman (2004), and J. Grossman and A. Brearley (2005). It is more discriminating at the lowest petrologic types, those associated with the highly unequilibrated 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 (3.0–3.2). This new classification scheme, based on a sensitive analytical technique utilizing the variation in the distribution of Cr in ferroan 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, is virtually unaffected by the processes of terrestrial weathering and aqueous alteration. The petrologic scale of the new decimal systemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with has been extended as follows:

3.00–3.05–3.10–3.15–3.2

To discriminate among subtypes below type 3.2, it has been shown that the Cr content of ferroan olivine is an excellent indicator of metamorphism. ChromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. exsolves from olivine in the incipient stages of metamorphism, initially producing heterogeneous Cr contents, and eventually homogeneously low Cr content of olivine. In a study by Chizmadia and Bendersky (2006), they determined that this sequence progresses from type 3.0, corresponding to high Cr2O3 contents of 0.3–0.4 wt%, to type 3.2, in which Cr2O3 constitutes less than 0.1 wt%. The gap between these subtypes represents type 3.1.

In addition, they have identified several other parameters, which, when used in combination, are instrumental in determining an accurate classification at the lowest petrologic grades:

At the onset of thermal metamorphism, 1) Cr is exsolved from ferroan olivine forming fine Cr-rich precipitates, which, with progressive metamorphism, become coarser within the olivine cores and form rims on the olivine surfaces; 2) very fine-grained FeS in chondruleRoughly 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 rims and in fine-grained matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. become coarser, and secondary sulfides form within chondrules; 3) Fe and Mg in olivine are homogenized 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 grains are equilibrated; 4) abundances of presolar grainsMineral grains that formed before our solar system. These tiny crystalline grains are typically found in the fine-grained matrix of chondritic (primitive) meteorites. Most grains probably formed in supernovae or the stellar outflows of red giant (AGB) stars before being incorporated in the molecular cloud from which the solar system are diminished; 5) Na and other alkalis are initially lost from the matrix and enter type-I chondrules, causing zonation, only to reverse direction with progressive metamorphism; 6) albite crystallizes from type-II chondrules causing blue CL and increased TL sensitivity.

Based on this new scheme, NWA 1756 was found to be most consistent with a 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. 3.10 with thermal metamorphism occurring at low temperature conditions of 471°C (Kimura et al., 2008). Several pristine features are present in NWA 1756, including the following: 1) the chondrule mesostasisLast material to crystallize/solidify from a melt. Mesostasis can be found in both chondrules, in the matrix around chondrules, and in achondrites as interstitial fine-grained material such as plagioclase, and/or as glass between crystalline minerals.   exhibits isotropism; 2) sub-millimeter-sized aggregates of Ni-rich metal (50–60 wt%) are present in the matrix and occur in association with sulfide and carbide; 3) sub-µm-sized silicaSilicon dioxide, SiO2. inclusions are present within the metal; 4) certain very rare minerals are present which do not exist in chondrites having even the lowest degree of metamorphism; and 5) presolar grains have been identified in cluster chondrite clasts which represent remnants of primary accretionary rocks (Metzler, 2011).

Additional methods which can be utilized in establishing the lowest petrologic subtype have been suggested by Kimura et al. (2003). These are based on distinctions in the chemical compositions of spinelMg-Al oxide, MgAl2O4, found in CAIs. group minerals, Ti-oxides, and FeNi-metal between LL3.0 and higher types. Studies of the highly volatileSubstances which have a tendency to enter the gas phase relatively easily (by evaporation, addition of heat, etc.). elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of contents and of silicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the heterogeneity are also useful.

Utilizing a different analytical procedure, Bonal et al. (2005) found that an accurate comparison could be made between the metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. grades of the CO and the ordinary chondrites using Raman spectrometry combined with petrographic analysis. Their method is based on the structural order of the chondritic organicPertaining to C-containing compounds. Organic compounds can be formed by both biological and non-biological (abiotic) processes. matter, which was initially accreted in the same proportions in both CO and ordinary chondrites. This structural order is irreversibly transformed by thermal metamorphism to a commensurate degree across chemical classes. From their data, they concluded that the CO group would span a petrologic sequence from 3.1, as represented by Colony, to a type greater than or equal to 3.8, as represented by both Warrenton and Isna.

In a further expansion of this method, Quirico et al. (2006) determined that LL3.00 Semarkona has experienced thermal metamorphism beyond the onset stage, and they proposed a new petrologic scale to provide consistency in the range as follows: Semarkona would become petrologic type (PT) 1, with PT 0 being reserved for the stage of true onset of thermal metamorphism. In a study of FeNi-metal and sulfide composition and texture by Kimura et al. (2006), the 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). (probably CO-related; Simon and Grossman, 2015) 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 Acfer 094 was determined to have experienced even less metamorphism than Semarkona and that it should be assigned a petrologic type 3.00, and perhaps a PT 0. All other meteorites analyzed to date would have a PT greater than 1, with Semarkona now considered to be petrologic type 3.01.

In subsequent studies of chromite zoning profiles along with the chromite content of individual ferroan olivine grains, Grossman (2008) was able to further resolve the petrologic type for chondrites at the lowest metamorphic stages. These two petrographic features provide a reference for a sequencial history of increasing thermal metamorphism that is consistent among olivine grains within each meteorite. For metamorphic types 3.00–3.03, chromite zoning profiles are smooth and correlate with igneous FeO zoning profiles. In addition, at this lowest metamorphic stage chromite contents account for 0.3–0.5 wt% in the chondrite groups studied. While chromite contents in type 3.05–3.10 chondrites still reflect the lowest degrees of metamorphism, chromite now exhibits igneous zoning profiles which are no longer smooth. Upon reaching a degree of metamorphism equivalent to type 3.15, chromite zoning has diminished considerably, and chromite abundance is now only 0.1–0.2 wt%. With metamorphic types of at least 3.2, no zoning is observed and chromite abundance is mostly less than 0.1 wt%.
Image credit: J. N. Grossman & A. J. Brearley
MAPS, vol. 40, #1, p. 87 (2005)
‘The onset of metamorphism in ordinary and carbonaceous chondrites’
(http://dx.doi.org/10.1111/j.1945-5100.2005.tb00366.x) Following the scheme of J. Grossman and A. Brearley (2005), the LL chondrite Semarkona, the L chondriteOrdinary chondrites low in free Ni-Fe metal (4 to 10 vol. %), containing olivine (Fa22-26) and the orthopyroxene hypersthene (Fs19-22). Average chondrule diameters (0.7 mm) are larger than those in H chondrites. The asteroid 433 Eros is suspected as a parent body, based on reflectance spectra, but most L chondrites NWA 7731, and the ungrouped (probably CO-related; Simon and Grossman, 2015) carbonaceous chondrite Acfer 094 (Kimura et al., 2006), have been assigned to the least equilibrated subtype 3.00; however, Semarkona has more recently been determined to represent a petrologic subtype of 3.01. This specific metamorphic type for Semarkona is also consistent with findings based on the FeNi-metal component, the features of which provide one of the most sensitive indicators for the onset of thermal metamorphism. The technique reveals that primary martensite decomposes to fine-grained plessiteA fine-grained intergrowth of kamacite and taenite that fills in the wedges between wide kamacite and taenite bands in octahedrites. The name derives from the Greek word for "filling." during very low degrees of thermal metamorphism in Semarkona, but which did not occurred in Acfer 094 (Kimura et al., 2008). Furthermore, they found that metal in and around Semarkona chondrules does not show a solar ratio of Co/Ni like that in Acfer 094, reflecting the greater degree of metamorphism that affected Semarkona. Moreover, low temperature aqueous alteration has occurred in Semarkona as attested by the presence of secondary alteration products such as smectite.

Kimura et al. (2008) also argue for the inclusionFragment of foreign (xeno-) material enclosed within the primary matrix of a rock or meteorite. of the carbonaceous chondrites of groups CR, CH, CB, and CM as 3.00 type specimens, notwithstanding their general designation as type 2 due to aqueous alteration features. In light of this petrologic typing paradox, they propose that a separate scale be adopted to describe aqueous alteration distinct from that which describes thermal metamorphism.

Other work has shown that the CO-group meteorite ALHA77307 is consistent with a type 3.03, while three ordinary chondrites—QUE 97008, MET 00526, and EET 90161—have been assigned the next lowest petrologic subtype of 3.05; several meteorites share the less rigorously defined 3.1 subtype. Northwest Africa 1756 is a highly unequilibrated primitive chondrite which is among the very few to have escaped significant metamorphic processes on their parent bodies, and which preserve their primordial features. In a study of unequilibrated ordinary chondrites, Metzler (2011) found cm-sized clasts in NWA 1756 and other UOCs which exhibit low matrix abundances and have features of deformation consistent with hot, plastic collisions which occurred over a short interval during the earliest chondrule formation events in the protoplanetary diskFlattened and rotating disk of dense gas and dust/solids orbiting a young star from which planets can eventually form.  . He has named these primary accretionary clasts ‘cluster chondrites’, and has demonstrated that the chondrule apparent size is inversely correlated with the degree of deformation.

Utilizing the MIT Magellan telescope in Chile and NASA’s Infrared Telescope Facility in Hawaii, a compositional analysis of the ~270 m diameter near-Earth asteroidAsteroids with orbits that bring them within 1.3 AU (195 million km) of the Sun. NEAs are a dynamically young population whose orbits evolve on 100-million-year time scales because of collisions and gravitational interactions with the Sun and the terrestrial planets. These asteroids are probably ejected from the main belt Apophis was obtained (Binzel et al., 2007). These results have been shown to be comparable to the composition measured for LL chondrites measured in the lab. The photo above shows a 1.16 g specimen of NWA 1756.