CR7 or Meta-CR (CR-like in MetBull 86; CR-an in MetBull DB)
Found March 2000, 20 ° 45.8′ N., 10 ° 26.5′ E.
A single mass of 3,636 g was found by a German team in the Ténéré region of the Sahara Desert in north-central Niger, specifically, at a location known as Grein. Provisionally named Te-1, it was classified by J. Otto and A. Ruh (Universitat Freiburg) as a metal-rich, coarse-grained, primitive achondriteAchondrite with an almost chondritic composition with age similar to the primordial chondrites. These should be better classified as "metachondrites".. 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 Click on Term to Read More grains are mostly 0.1–0.4 mm in size, but larger grains occur. They commonly exhibit triple-junctions, consistent with recrystallization. Large poikilitic 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 Click on Term to Read More grains are present, as well as small agglomerates of crystals, sometimes called ‘Sammelkristalle’, which usually form during melting and recrystallization processes. Unlike 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 Click on Term to Read More, these structures are composed primarily of plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite Click on Term to Read More poikilitically enclosing minor olivines and pyroxenes, and are often accompanied by FeNi-metal. Te-1 is a freshly fallen 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 Click on Term to Read More with a weathering grade of W0, and it has 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". Click on Term to Read More of S1–2.This primitive achondriteAn achondrite is a type of stony meteorite whose precursor was of chondritic origin and experienced metamorphic and igneous processes. They have a planetary or differentiated asteroidal origin where the chondritic parent body reached a sufficient size that through heating due to radioactive decay of 26Al (aluminum isotope) and gravitational Click on Term to Read More has a chemical and 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 Click on Term to Read More composition unlike that of any other meteorite. It has an O-isotopic composition distinct from any other achondrite group, plotting within the CR-field, and interestingly, very near to that of 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). Click on Term to Read More basaltic meteorite NWA 011. 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 Click on Term to Read More isotopes are similar to those of the lodraniteRare type of primitive achondrite named after the Lodran meteorite that fell in Pakistan in 1868. Initially, lodranites were grouped with the stony-iron meteorites because they contain silicates (olivine, orthopyroxene, and minor plagioclase) and Fe-Ni metal in nearly equal proportions. However, since discovery of the closely related acapulcoite group, lodranites Click on Term to Read More/acapulcoite 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. Click on Term to Read More but are not an exact match. The mineral composition and noble gasElement occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. Click on Term to Read More content of Te-1 are very similar to that of the brachinites and the brachinite-like meteorite, Divnoe; moreover, the olivine and pyroxene compositions are nearly identical to those of Brachina. Furthermore, the composition of chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More 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 Click on Term to Read More in Te-1 is also indicative of a very close relationship with Divnoe. These varied characteristics are most consistent with the grouping of Te-1 as a brachinite-like primitive achondrite. See the Tafassasset page for further information.
Te-1 has a CRE age of 45 m.y. The specimen shown above is a 1.72 g partial slice with fresh fusion crustMelted exterior of a meteorite that forms when it passes through Earth’s atmosphere. Friction with the air will raise a meteorite’s surface temperature upwards of 4800 K (8180 °F) and will melt (ablate) the surface minerals and flow backwards over the surface as shown in the Lafayette meteorite photograph below. Click on Term to Read More on one end. The photo below shows the main massLargest fragment of a meteorite, typically at the time of recovery. Meteorites are commonly cut, sliced or sometimes broken thus reducing the size of the main mass and the resulting largest specimen is called the "largest known mass". Click on Term to Read More of Te-1 with an end slice removed.
Primitive AchondriteAchondrite with an almost chondritic composition with age similar to the primordial chondrites. These should be better classified as "metachondrites"., 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). Click on Term to Read More: carbonaceous chondrite-related (CR [2000], CR-like [2002], CR-an [2006], Primitive achondriteAn achondrite is a type of stony meteorite whose precursor was of chondritic origin and experienced metamorphic and igneous processes. They have a planetary or differentiated asteroidal origin where the chondritic parent body reached a sufficient size that through heating due to radioactive decay of 26Al (aluminum isotope) and gravitational Click on Term to Read More in MetBull 105 [2017])
Found: February 14, 2000, Coordinates: 20° 45′ 48′ N. 10° 26′ 30′ E.
Twenty-six stones totaling ~110 kg, the two largest weighing ~30 kg, were found by Bernard Dejonghein in the Ténéré region of north-central Niger; all are considered to be paired. This olivine-rich 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 Click on Term to Read More was classified at the Muséum National d’Histoire Naturelle in France as the first thermally metamorphosed CR chondriteClass named for the Renazzo meteorite that fell in Italy in 1824, are similar to CMs in that they contain hydrous silicates, traces of water, and magnetite. The main difference is that CRs contain Ni-Fe metal and Fe sulfide that occurs in the black matrix and in the large chondrules Click on Term to Read More. A separate ~3.6 kg stone found independently in the same vicinity as Tafassasset was provisionally named Te-1 (previous synonym Grein 004), and it was independently analyzed at the Max-Planck-Institut für Chemie in Germany. A bulk compositional analysis of Te-1 found that it differs slightly from Tafassasset in its texture and in certain elemental abundances, but its overall similarity in texture (recrystallized with 120° triple junctions) and elemental composition to Tafassasset makes their pairing obvious. The differences observed suggest this fallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If Click on Term to Read More was composed of a heterogeneous assemblage. The meteorite NWA 5131 was found to be very similar geochemically and petrologically to Tafassasset.
Although Tafassasset is only slightly weathered to a grade of W0/1, the majority of the fusion crustMelted exterior of a meteorite that forms when it passes through Earth’s atmosphere. Friction with the air will raise a meteorite’s surface temperature upwards of 4800 K (8180 °F) and will melt (ablate) the surface minerals and flow backwards over the surface as shown in the Lafayette meteorite photograph below. Click on Term to Read More has been extensively sand-blasted away. Relict metal-bearing 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 Click on Term to Read More and 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 Click on Term to Read More rims in Tafassasset were reported by the French research team. This evidence led some to classify the meteorite as CR7 or Meta-CR. However, these features were determined by Breton et al. (2015) to be pockets of molten material containing refractory 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 Click on Term to Read More and immiscibleThe property of liquids that are mutually insoluble (won't mix together) such as oil and water or metallic and silicate melts. Click on Term to Read MoremetalElement 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 Click on Term to Read More, which upon cooling, resemble chondrule textures.
More recent research results (see below) have determined that the Tafassasset parent asteroid accreted very early, prior to the onset of chondrule formation in 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 Click on Term to Read More (CC) reservoir beyond Jupiter.
PlagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite Click on Term to Read More, chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More, and phosphates present in the matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. Click on Term to Read More of Tafassasset have been attributed to metamorphism of original fine-grained matrix material. By contrast, similar 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 Click on Term to Read More phases are found in areas that define possible relict chondrules, described as poikiloblastic aggregates by some, which have retained the textures of an earlier, pre-metamorphic stage. The abundant small troiliteBrass colored non-magnetic Fe sulfide, FeS, found in a variety of meteorites. grains present in the recrystallized olivine–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 Click on Term to Read More matrix in Tafassasset are similar to those found in CR 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 Click on Term to Read More. On an oxygen three-isotope plot, Tafassasset falls within the CR field and away from the majority of brachinites. Still, the plagioclase composition and other 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 abundances in Tafassasset are most similar to those of brachinites.
Tafassasset has similar O- and Cr-isotopic compositions to the CR chondrites, and is also similar with respect to its high abundance of siderophile elements, including its high FeNi-metal content of 8–10 vol% compared to ~7.4 vol% in CR chondrites (Nehru et al., 2010). However, in their siderophile elementLiterally, "iron-loving" element that tends to be concentrated in Fe-Ni metal rather than in silicate; these are Fe, Co, Ni, Mo, Re, Au, and PGE. These elements are relatively common in undifferentiated meteorites, and, in differentiated asteroids and planets, are found in the metal-rich cores and, consequently, extremely rare on study emphasising Hf–W systematics, Archer et al. (2019) contend that the near-zero ε183W values for metal in Tafassasset (–0.06 [±0.17] to 0.02 [±0.2]; Breton et al., 2015) distinguish it from the positive ε183W values for metal in CR chondrites (~0.4 to ~0.6; Archer et al., 2018; Budde et al., 2018, diagram), making a genetic relationship (common 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. Click on Term to Read More) doubtful.
Similar to several CR6 meteorites, Tafassasset exhibits a fractionated 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 Click on Term to Read More signature uncharacteristic for the CR group, including a depletion in refractory lithophile elements, an extremely low Zn concentration, and Al/Mg and Mn/Mg ratios that plot near more evolved achondrites. This fractionationConcentration or separation of one mineral, element, or isotope from an initially homogeneous system. Fractionation can occur as a mass-dependent or mass-independent process. Click on Term to Read More is consistent with an early stage of 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 Click on Term to Read More involving the mobilization of melts incorporating Si, P, and S, and/or perhaps a late stage of metasomatism. Classification of Tafassasset as an ungrouped primitive achondrite was suggested by the German research team (Zipfel et al., 2002) as the most plausible classification; however, the texturally evolved nature of this meteorite is not consistent with a primitive designation.
A further advancement of metamorphism along a continuum that includes the CR6 chondrites NWA 7317 (and pairings), NWA 3100, and NWA 2994 (and pairings) was invoked by Bunch et al (2008) to explain the recrystallized poikiloblastic texture in Tafassasset, and therefore the term 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. Click on Term to Read More was thought to be most appropriate for this meteorite. They also argued that the similarity in O-isotopic compositions that is observed among the non-metamorphosed CR chondrites, the metamorphosed CR6 chondrites, and Tafassasset, compared to the igneous achondrite NWA 011 (and pairings), is consistent with their derivation from a common large parent body, one which experienced internal partial melting while retaining a chondritic regolithMixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an atmosphere. Regolith is the product of "gardening" by repeated meteorite impacts, and thermal processes (such as repeated heating and cooling cycles). Click on Term to Read More.
Tafassasset is a recrystallized meteorite that is petrographically consistent with a low-degree partial melt with a retained metal component that was derived from Renazzo-like precursor source material. It subsequently experienced equilibration processes through an extended period of thermal metamorphism. Tafassasset is considered to be closely related to the brachinites and other FeO-rich primitive achondrites, and the meteorite has been characterized by Nehru et al. (2010) as an unusual brachinite derived from a CR-like precursor body through partial differentiationA process by which a generally homogeneous chondritic body containing mostly metal, silicates and sulfides will melt and form distinct (differentiated) layers of different densities. When the melting process continues for a long enough period of time, the once chondritic body will re-partition into layers of different composition including Click on Term to Read More. A Fa vs. Fs plot demonstrates this genetic relationship, as well as a relationship with the more primitive anomalous achondrites Divnoe and RBT 04239 (Gardner et al., 2007). However, a genetic relationship between Tafassasset and the CR group could be excluded based on differences in elemental compositions, noble gasElement occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. Click on Term to Read More ratios, and solar gas abundances. The CRE age of Tafassasset is also much higher (76.1 ±15.2 m.y.) than that of any CR 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 Click on Term to Read More (<10 m.y.). Still, it has been suggested by some investigators that all of the differences between Tafassasset and CR chondrites may be the result of an increased degree of metamorphism and/or metasomatism experienced by Tafassasset.
A study in which Tafassasset was compared with the brachinites was undertaken by Nehru et al. (2003). They determined that the texture, modal abundances, and mineral compositions of Tafassasset were very similar to Brachina, although differences were found to exist for Tafassasset with respect to its equilibration temperature, O-isotopic composition, and high abundance of metal. In a similar comparison made by Patzer et al. (2003), it was found that the level of radiogenic 129Xe measured in Tafassasset is similar to that of some brachinites. They also found that the trapped 132Xe component of Tafassasset was lower than that of CR chondrites, and that the 36Ar/132Xe ratio is at least 10× lower than it is in CR chondrites.
As with brachinites, Tafassasset was determined to have an ancient Pb–Pb age of ~4.563 b.y. (Göpel et al., 2009, 2015). It was also determined that its Cr systematics are the same as those for Renazzo, and that its 54Cr excess is the first such occurrence in a carbonaceous achondrite (Göpel and Birck, 2010). The carbonaceous achondrites NWA 011/2976, NWA 6704/6693, and NWA 2994/6901 have since been determined to have similar positive ε54Cr and ε50Ti values (Sanborn et al., 2018). In a study of the Mn–Cr systematics for Tafassasset, Göpel et al. (2015) ascertained an absolute age of 4.56351 (+0.00025/–0.00026) b.y. anchored to the D’Orbigny angriteType of evolved achondrite meteorite that represent some of the earliest stages of asteroidal differentiation and magmatism in our solar system. Angrites are named for the Angra dos Reis meteorite, which fell in Rio de Janeiro, Brazil, in early 1869. They are basaltic (mafic) rocks, often containing porous areas, and Click on Term to Read More. Based on Al–Mg systematics, Dunlap et al. (2015) calculated an upper limit of <4.5677 b.y. ago for the timing of Al/Mg fractionation during differentiation on the Tafassasset parent body.
In their study, Breton et al. (2015) ascertained a ‘most reliable’ metal phase Hf–W age for Tafassasset of 2.9 (±0.9) m.y. after CAIsSub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they Click on Term to Read More, corresponding to the timing of metal-silicate segregation; this corresponds to an absolute age of 4.5644 b.y. By comparison, the Hf–W age of CR chondrites was determined by Budde et al. (2018) to be somewhat younger at 3.63 (±0.62) m.y. after CAIs. It is also noteworthy that chondrule formation for CR chondrites was also calculated employing Al–Mg and Pb–Pb chronometry by Schrader et al. (2017) and Amelin et al. (2002), respectively. Their studies provided corrected ages of 3.75 (±0.24) and 3.66 (±0.63) m.y. after CAIs, respectively. Moreover, a temporally similar accretionAccumulation of smaller objects into progressively larger bodies in the solar nebula leading to the eventual formation of asteroids, planetesimals and planets. The earliest accretion of the smallest particles was due to Van der Waals and electromagnetic forces. Further accretion continued by relatively low-velocity collisions of smaller bodies in the Click on Term to Read More age of 3.5 (±0.5) m.y. after CAIs was determined for CR chondrites by Sugiura and Fujiya (2014). These dates are inconsistent with a common parent body for CR chondrites and Tafassasset.
In a petrographic analysis and O-isotope study conducted by Gardner-Vandy et al. (2012), it was found that samples of Tafassasset have O-isotopic ratios that plot within the CR-chondrite field, and that it was equilibrated at an oxygen fugacityUsed to express the idealized partial pressure of a gas, in this case oxygen, in a nonideal mixture. Oxygen fugacity (ƒO2) is a measure of the partial pressure of gaseous oxygen that is available to react in a particular environment (e.g. protoplanetary disk, Earth's magma, an asteroid's regolith, etc.) and Click on Term to Read More of ~IW–1. They determined that this meteorite experienced a low degree of partial melting on a small parent body without reaching isotopic homogeneity. Overall, Tafassasset was found to be most similar to the ungrouped achondrites LEW 88763 and Divnoe, as well as to the brachinites. The study concluded that Tafassasset is not consistent with partial melting of CR chondrites, although each meteorite appears to have formed within the same 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 Click on Term to Read More reservoir. In their comprehensive study of Tafassasset, Breton et al. (2015) used thermal modeling to derive a size for the Tafassasset parent body of 30–50 km in diameter, an early timing of accretion at 0.8–1.2 m.y. after CAIs, a partial melting degree of 20–25% due to radiogenic 26Al, and a formation depth for the Tafassasset lithology of 7.3–7.7 km. They also inferred that this relatively small asteroid experienced a high cooling rate of ~300–400 K/m.y. near the closure temperature for the Hf–W chronometer.
In a contrary scenario presented by Nehru et al. (2012, 2014), Tafassasset (and LEW 88763) may represent the residua of a low-degree partial melting event that occurred at some depth within a late-accreted chondritic veneer on a large (~400 km diameter) CR-like differentiated parent body. Subsequent impact excavation of the crustOutermost layer of a differentiated planet, asteroid or moon, usually consisting of silicate rock and extending no more than 10s of km from the surface. The term is also applied to icy bodies, in which case it is composed of ices, frozen gases, and accumulated meteoritic material. On Earth, the Click on Term to Read More would have exposed the underlying Tafassasset and brachinite lithologies. For more information pertaining to the latter scenario, see the LPSC abstract ‘Primitive’ and igneous achondrites related to the large and differentiated CR parent body by Bunch et al. (2005), and the MetSoc abstract Tafassasset and Primitive Achondrites: Records of Planetary Differentiation by Nehru et al. (2014).
Efforts to better resolve the relationship that exists between Tafassasset and other anomalous meteorites continues. As provided by Sanborn et al. (2014), a coupled Δ17O vs. ε54Cr diagram is one of the best diagnostic tools for determining genetic relationships between meteorites. Moreover, Sanborn et al. (2015) demonstrated that ε54Cr values are not affected by aqueous alteration. The diagrams below include Tafassasset, and it is apparent that it plots within the CR chondrite field. The specimen of Tafassasset pictured above is a 4.45 g partial slice with an edge of preserved fusionProcess in which two lighter atomic nuclei combine to form a heavier atomic nucleus. Very high temperatures are normally required in order for atomic nuclei to collide with sufficient energy to overcome the Coulomb barrier (their mutual electrostatic repulsions). Fusion that occurs under high-temperature conditions is called thermonuclear fusion. Fusion Click on Term to Read More crust.
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014) <!–
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014) –>
17O vs. ε54Cr and ε50Ti for CR Carbonaceous Achondrites
click on image for a magnified view
Diagrams credit: Sanborn et al., GCA, vol. 245, pp. 577–596 (2019)
‘Carbonaceous Achondrites Northwest Africa 6704/6693: Milestones for Early Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. Chronology and Genealogy’
(https://doi.org/10.1016/j.gca.2018.10.004)
Meta-CV or -ung ‘CX’ trend (AchondriteAn achondrite is a type of stony meteorite whose precursor was of chondritic origin and experienced metamorphic and igneous processes. They have a planetary or differentiated asteroidal origin where the chondritic parent body reached a sufficient size that through heating due to radioactive decay of 26Al (aluminum isotope) and gravitational Click on Term to Read MoreungroupedModifying 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). Click on Term to Read More in MetBull 105) Purchased May 2015 no coordinates recorded A single 407 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 Click on Term to Read More was found in the Sahara Desert, possibly near Tan Tan, Morocco, and subsequently purchased by F. Kuntz in Zagora, Morocco on behalf of Planetary Studies Foundation in Galena, Illinois. An analysis was conducted at the University of Washington in Seattle (A. Irving and S. Kuehner), and NWA 10503 was determined to be a texturally-evolved meteorite exhibiting ~120° triple junctions 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 Click on Term to Read More visible. The elevated 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 FeO/MnO ratios are higher than those for ordinary 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 Click on Term to Read More and are consistent with a 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 Click on Term to Read More classification. In May 2016, a lot of smaller stones having a combined weight of 215 g was purchased by B. Hoefnagels. This group of stones was designated NWA 10859, and results of petrographic and isotopic analyses (A. Irving and S. Kuehner, UWS; K. Ziegler, UNM) led to the determination that they are paired with NWA 10503.
On an 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 Click on Term to Read More three-isotope diagram the values plot away from all other analyzed meteorites and along an extension of the trend line for the ungrouped pallasiteOne of two main classes of stony-iron meteorite, the other being mesosiderites. Pallasites are igneous in nature and characterized by crystals of olivine, sometimes peridot (green gem quality clear olivine crystals), embedded in a matrix of Fe-Ni metal. The type specimen, weighing 680 kg, was found in the mountains near Click on Term to Read More Milton (K. Ziegler, UNM; see diagram below). NWA 10503 was initially classified as an ungrouped carbonaceous 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. Click on Term to Read More that might be related to the Milton pallasite. click on image for a magnified view
Diagram credit: Irving et al., 79th MetSoc, #6461 (2016) In an effort to better resolve potential genetic relationships that might exist among these meteorites, a Cr-isotopic analysis was conducted by Sanborn et al. (2018) for NWA 10503 as well as for 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 Click on Term to Read More from the Milton pallasite. It is demonstrated on a coupled Δ17O vs. ε54Cr diagram (shown below) that both meteorites plot among the CV clan and plausibly share a genetic relationship. Chromium vs. Oxygen 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. Click on Term to Read More Plot click on diagram for a magnified view
Diagram credit: Sanborn et al., 49th LPSC, #1780 (2018) In an effort to further resolve differences between the CV and CK chondriteClass of carbonaceous chondrite named for the Karoonda meteorite that fell in Australia in 1930. They are more oxidized than all other carbonaceous chondrites and genetically distinct from CV chondrites. CK chondrites appear dark-gray or black due to a high percentage of Cr-rich magnetite dispersed in a matrix of dark Click on Term to Read More groups, Yin and Sanborn (2019) analyzed Cr isotopes in a significant number and broad range of meteorites. Their study included samples from each of the three CV subgroups (oxA, oxB, Red), anomalous CV3 chondrites, a C3-ungrouped, several CK members, and other potential CV-related meteorites including NWA 10503 and Milton (see top diagram below). It is demonstrated that the CV and CK meteorites are clearly resolved into two distinct isotopic reservoirs. In addition, it is shown by the ε54Cr values that NWA 10503 plots among the CV-related meteorites. A coupled Δ17O vs. ε54Cr diagram plotting all of the meteorites in the study is shown in the bottom diagram below. Cr Isotope Weighted Average For CV and CK Chondrites click on photo for a magnified view
O–Cr Diagram For CV and CK Chondrites CK: orange shades; CV: green shades; Achondrites: open click on photo for a magnified view
Diagrams credit: Yin and Sanborn et al., 50th LPSC, #3023 (2019) In a study of two newly recovered ungrouped carbonaceous meteorites, the unequilibrated chondriteA chondrite with heterogeneous mineral compositions (e.g., olivine grains with differing FeO/(FeO+MgO) ratios. NWA 11961 and the dunitic brecciaWork in Progress ... A rock that is a mechanical mixture of different minerals and/or rock fragments (clasts). A breccia may also be distinguished by the origin of its clasts: (monomict breccia: monogenetic or monolithologic, and polymict breccia: polygenetic or polylithologic). The proportions of these fragments within the unbrecciated material Click on Term to Read More NWA 12264, Irving et al. (2019) further populated the O-isotopic trend line previously defined by NWA 10503 and the Milton pallasite; they termed this the ‘CX’ trend. However, Cr isotope data obtained for all of these meteorites have resolved both NWA 11961 and NWA 12264 as potential new carbonaceous parent bodies distinct from that of NWA 10503 and Milton, the latter previously considered possible members of the CV-clan (see diagrams below). ‘CX’ Oxygen Isotope Trend Line click on photo for a magnified view
O–Cr Diagram for ‘CX’ Trend Meteorites click on photo for a magnified view
Diagrams credit: Irving et al., 50th LPSC, #2542 (2019) Another ungrouped metachondrite with affinities to carbonaceous chondrites is NWA 2788. This meteorite has a metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. Click on Term to Read More texture exhibiting ~120° triple junctions, elevated Fe/Mn and Ca/Na ratios, and an O-isotopic composition that plots very near to the TFL. Bunch et al. (2006) suggest that if a chondrule-bearing representative of this 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. Click on Term to Read More is found and identified in the future it should be termed a ‘CT chondrite’ (see NWA 2788 photos, abstracts [1, 2], and isotopic plot).
Northwest Africa 10503 is a somewhat friable meteorite with features indicating a low degree of terrestrial weathering and 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". Click on Term to Read More. Two views of a 3.79 g fragment of NWA 10503 are shown above. In the top photo below are some of the larger stones representing the paired NWA 10859, while two stones of NWA 10503 with green-colored fusion crustMelted exterior of a meteorite that forms when it passes through Earth’s atmosphere. Friction with the air will raise a meteorite’s surface temperature upwards of 4800 K (8180 °F) and will melt (ablate) the surface minerals and flow backwards over the surface as shown in the Lafayette meteorite photograph below. Click on Term to Read More are shown in the bottom photo. Photos courtesy of Ben Hoefnagels
Photos courtesy of Fabien Kuntz
Meta-LL <!–(Achondrite-ung submitted to MetBull) –> click on photo for a magnified view Found 2011 no coordinates recorded A single fusionProcess in which two lighter atomic nuclei combine to form a heavier atomic nucleus. Very high temperatures are normally required in order for atomic nuclei to collide with sufficient energy to overcome the Coulomb barrier (their mutual electrostatic repulsions). Fusion that occurs under high-temperature conditions is called thermonuclear fusion. Fusion Click on Term to Read More crusted stone weighing 224 g was found in Northwest Africa and subsequently sold to 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 Click on Term to Read More dealer G. Hupé in Zagora, Morocco. A sample was submitted for analysis and classification to the University of Washington in Seattle (A. Irving and S. Kuehner), and NWA 7030 was determined 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. Click on Term to Read More with similarities 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 Click on Term to Read More group.
Northwest Africa 7030 is a texturally evolved meteorite that lacks any 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 Click on Term to Read More, and it exhibits an overall texture akin to a fine-grained igneous cumulateIgneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber. Click on Term to Read More. The predominant lithology was described as consisting of 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 Click on Term to Read More, clinopyroxene, sodic plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite Click on Term to Read More, chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More, troiliteBrass colored non-magnetic Fe sulfide, FeS, found in a variety of meteorites. and taeniteLess common than kamacite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Taenite, γ-(Fe,Ni), has 27-65 wt% Ni, and forms small crystals that appear as highly reflecting thin ribbons on the etched surface of a meteorite; the name derives from the Greek word for "ribbon." Click on Term to Read More. A less prevalent greenish-colored, igneous-textured lithology is also present. Its composition is different from that of the primary lithology in that it contains interstitialTerm applied to ions or atoms occupying sites between lattice points. Click on Term to Read More potassic-feldspar (sanidine) along with albitic plagioclase, and it contains much less troilite. Another difference is the presence of orthopyroxeneOrthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These end-members form an almost complete solid solution where Mg2+ substitutes for Fe2+ up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca2+ contents occur Click on Term to Read More and rare kamaciteMore common than taenite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Kamacite, α-(Fe,Ni), contains 4-7.5 wt% Ni, and forms large body-centered cubic crystals that appear like broad bands or beam-like structures on the etched surface of a meteorite; its name is derived from the Greek word Click on Term to Read More in the minor lithology (Fernandes et al., 2014).
The O-isotopic composition of NWA 7030 was analyzed at Okayama University (R. Tanaka) and it was shown to plot within the 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 Click on Term to Read More field. Bulk densityMass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.). Some substances (like gases) are easily compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible gases and is much Click on Term to Read More measurements for NWA 7030 were conducted at the University of Central Florida (D. Britt [UCF]; B. Macke and G. Consolmagno [Vatican]) employing both pycnometer and glass bead methods (results forthcoming).
Although NWA 7030 was shown to have affinities to the LL-chondrite group, it has many anomalous features that make it unique, and therefore, could be considered as 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). Click on Term to Read More. The K–Ar age was determined for the minor lithology, and the results indicate the occurrence of two separate impact heating/degassing events that have disturbed this isotopic chronometer: one at ~4.12 b.y. and the other at ~2.39 b.y. (Fernandes et al., 2014).
The specimen of NWA 7030 shown above is a 0.77 g end section. The top photo below shows the fresh fusion-crusted mass, while the bottom photo shows the interior of a 15.62 g slice with both lithologies—photos courtesy of Greg Hupé.
Photos courtesy of Greg Hupé—Nature’s Vault
Meta-L (L7 in MetBull 102 [submitted as Achondrite-ung]) Found 2010 no coordinates recorded A stone weighing 134 g was found in the Sahara and sold to Chladni’s Heirs. A sample was submitted for analysis and classification to the University of Washington in Seattle (A. Irving), and NWA 6348 was determined 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. Click on Term to Read More (Irving et al., 2005) with affinities to 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 Click on Term to Read More group. The 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 Click on Term to Read More has 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". Click on Term to Read More of S3 and a weathering grade of W2.
Northwest Africa 6348 is a texturally evolved meteorite with grains of variable sizes, but which do not appear to constitute separate clasts. Although no 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 Click on Term to Read More were identified, some fine-grained areas might represent relict chondrules. 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 Click on Term to Read More constituents include 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 Click on Term to Read More, orthopyroxeneOrthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These end-members form an almost complete solid solution where Mg2+ substitutes for Fe2+ up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca2+ contents occur Click on Term to Read More, clinopyroxene, sodic plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite Click on Term to Read More, and opaque phases.
Northwest Africa 6348 has been classified by A. Irving as the first metachondrite from the L 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 Click on Term to Read Moreparent 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. Click on Term to Read More. The term metachondrite was proposed to describe those achondrites which are texturally evolved 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 Click on Term to Read More with completely recrystallized 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 Click on Term to Read More. They lack chondrules and have elemental abundance ratios and O-isotopic compositions that show affinities to existing chondrite groups. Meteorites described as metachondrites have been found representing the CV, CR, H, L, and LL groups, while unique metachondrite NWA 2788 is considered by Bunch et al. (2006) to be associated with an unknown 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 Click on Term to Read More parent body; 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 and abstract #P51E-1246). In addition, some groups of achondrites having chondrule-bearing members might also be more appropriately described as metachondrites, including the acapulcoitesPrimitive achondrite that belongs to a small group named after the Acapulco meteorite that was observed to fall in Mexico in 1976. Acapulcoites are made mostly of fine-grained olivine (Fo3-14), orthopyroxene(En86-97), Ca-rich pyroxene (En51Wo44), plagioclase (An12-31), Ni-Fe metal, and troilite. They are transitional between primordial chondritic matter and more differentiated Click on Term to Read More and winonaites as demonstrated by Monument Draw and NWA 725, respectively. These chondrule-bearing members have been referred to as ‘AC chondrites’ or ‘W chondrites’ for those associated with acapulcoites and winonaites, respectively. The photo shown above is a 2.9 g specimen of NWA 6348.
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CR7 or Meta-CR (CR-like in MetBull 86; CR-an in MetBull DB)
