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, 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 click on photo for a magnified view Found 2008 no coordinates recorded
A single stone partially covered with remnant 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 and weighing 4,818 g was found in Northwest Africa. Upon close examination, 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 Greg Hupé noticed that the meteorite had an unusual appearance with similarities to primitive achondrites, and he purchased the meteorite in June 2008 in Er Rachidia, Morocco. A portion of the meteorite was sent to the University of Washington in Seattle (A. Irving and S. Kuehner) for analysis and classification, and NWA 5400 was determined to be a unique ungrouped achondrite. Thirty-one possibly paired stones were found by Mbarek Ait el Caid in February 2008 (NWA 5363, 2,455 g). Samples of these stones and a sample of NWA 5400 were studied by Albert Jambon (Université Pierre et Marie Curie, Paris), including O-analyses, and they were declared to constitute a pairing group. Two other stones designated NWA 6077 (1,010 g) and NWA 6292 (725 g) were found in 2008 and 2010, respectively, and they have also been demonstrated by O-isotopic analysis (Carnegie Institution in Washington D.C.) to be paired with NWA 5400.
The four recognized members of the NWA 5400 pairing group are plotted on two different 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 diagrams: (1) (courtesy of Bernd Pauli) and (2) (courtesy of Achim Raphael). The first diagram utilizes a less precise slope for the TFL (Terrestrial 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 Line) of ~0.5. Recent O-isotopic studies conducted by the Carnegie Institute, the Open University, and the University of Groningen have determined that a slope of 0.526 represents a more precise TFL slope (Rumble III et al., 2007). When constructed with updated coordinates (A. Irving), the adjusted TFL slope passes very close to the points determined for the NWA 5400 pairing group. The slope also passes through the CI chondriteRare meteorite class named after the Ivuna meteorite that fell in Tanzania in 1938. They are among the most primitive, friable (crumbly), and interesting of all meteorites, having undergone extensive aqueous alteration. They lack chondrules and CAIs as a result of this alteration, but contain up to 20% water, as Click on Term to Read More field and probably the martian field as well. The second diagram compares the NWA 5400 pairing group to the brachinites and brachinite-like meteorites Divnoe, Zag (b), and NWA 4042. It is evident that the NWA 5400 pairing group is not genetically related to either the brachinites or the brachinite-like meteorites, but probably formed by analogous processes (e.g., similar 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; Lenaz and Schmitz, 2017) on separate parent bodies. See also a linearized O-isotope plot (3) showing a slope similar to the TFL, based on data from Miller (2002).
In-depth analyses have revealed that NWA 5400 has petrographic, mineralogical, and isotopic similarities to brachinites, including a fine- to coarse-grained protogranular texture (0.1–1.5 mm) and a high 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 content (79–80.5 vol%). 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. Click on Term to Read More constitutes 16.4–19.4 vol% (approx. 1:1 opx to cpx), and it also contains minor chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More, chlorapatite, troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. Click on Term to Read More, FeNi-metal (altered), 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, and rare 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. The meteorite is considered to represent a restite from a chondritic 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 that experienced a low degree 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 and 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 melt extraction evidenced by the lack of plagioclase and lithophile elements (Burkhardt et al., 2015). As shown above, the O-isotopic composition of NWA 5400 (D. Rumble, Carnegie Institution in Washington) has values that plot on the TFL, which up until now was only true for E 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, E achondrites, CI chondrites, and the Earth–Moon 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.
Although minor terrestrial weathering has produced iron hydroxides and calcite veining, the presence of minor amounts of free 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 are indicative of little to no 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 having occurred on the parent asteroid at the time this rock was formed. The difference in Fe/Mn ratios in olivine and pyroxene in NWA 5400 compared to those in lunar and Earth samples, along with the presence of FeNi-metal in NWA 5400, eliminates both objects as the possible source parent body. That said, it was initially proposed that this unique achondrite might possibly represent a primitive terrene meteorite ejected prior to the collision which spalled the Moon. Such a petrogenetic history is based on the higher abundances of highly siderophile elements and the Os 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 composition of NWA 5400 (Shukolyukov et al., 2010). In such a scenario, NWA 5400 could have been stored in the asteroid beltBelt located between 2.12 and 3.3 AU from the Sun and located between the orbits of Mars and Jupiter containing the vast majority of asteroids. The asteroid belt is also termed the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such Click on Term to Read More until recent impacts ejected material into an Earth-crossing orbitThe elliptical path of one body around another, typically the path of a small body around a much larger body. However, depending on the mass distribution of the objects, they may rotate around an empty spot in space • The Moon orbits around the Earth. • The Earth orbits around Click on Term to Read More.
Further speculations about NWA 5400 consider an origin on ‘Theia’ itself prior to its collision with Earth 4.52 b.y. ago, consistent with the O-isotopic composition of the meteorite being indistinguishable from that of the Earth and Moon. An asteroid-sized fragment could have experienced similar storage mechanisms in the asteroid belt as described above. However, this theory is inconsistent with the ‘Icy Impactor Model’ proposed by Wolbeck and Connolly (2010). They envision the impacting planetessimal ‘Theia’ as originating near the frost line close to Jupiter, and therefore it’s composition would have been predominantly ice. The impact with proto-Earth is thought to have introduced vast quantities of steam/water which helped to stabilize the orbit of the proto-Moon and create the oceans on proto-Earth. Another possible scenario for the origin of NWA 5400 was propounded by Alan Rubin (UCLA), in which NWA 5400 could represent a unique, undifferentiated or partially differentiated E-type asteroid—as distinguished from the Shallowater, aubriteAubrites are named for the Aubres meteorite that fell in 1836 near Nyons, France. They are an evolved achondrite that is Ca-poor and composed mainly of enstatite (En100) and diopside (En50Wo50) with minor amounts of olivine (Fa0) and traces of plagioclase (An2-8). They contain large white crystals of enstatite as Click on Term to Read More, EH, and EL parent bodies—and which had a chondritic bulk composition. He considered that it may have experienced extensive metasomatic oxidationOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons, while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More processes similar to the E 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 Galim. However, this model has been refuted on the basis of the high olivine content in NWA 5400 and on the inconsistencies in its age.
An alternative model invoking moderate degrees of partial melting (13–30%) on a volatile-rich, oxidizedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons, while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More, chondritic asteroid was proposed by Day et al. (2011). Based on results of petrological, geochemical, and isotopic analyses, they suggest that a genetic link (common parent body) may exist between brachinite-like GRA 06128/9 and other brachinites. Newly acquired highly-siderophile 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 (HSE) data reflect variable extraction of metals and sulfides producing residues having a range of compositions. In light of this, they hypothesize that one complementary metal-rich residue possibly derived from the brachinite parent body might be NWA 5400. However, through studies of HSE abundances, and upon examination of potential metal-sulfide segregation processes, it was determined by Day et al. (2012) that NWA 5400 and similar brachinite-like achondrites are not likely genetically related to brachinites, but rather originated on similar volatile-rich, oxidized, chondritic precursor asteroids which experienced similar petrologic processes. The measured HSE abundances for NWA 5400 are consistent with a partially melted parent body in which heating from short-lived radionuclides (26Al) came to a halt before significant coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel. Click on Term to Read More formation could occur. The precursor material from which the brachinites (and possibly NWA 5400) derive has been determined by Gardner-Vandy et al. (2013) to be R chondrite-like (see the NWA 3151 page).
Burkhardt et al. (2015) determined the potential timing of metal–silicate segregation based on the Hf–W chronometer to be 1.2 (±0.5) m.y. after CAISub-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 formation. Burkhardt et al. (2017) determined a revised two-stage model Hf–W age of 2.2 (±0.8) 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, which they consider likely reflects the approximate timing of silicate melt extraction with the addition of W occurring shortly thereafter. Isotopic studies for NWA 5400 also indicate NWA 5400 formed very early in Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. history, as attested by its chondritic Xe similar to that of brachinites. An I–Xe analysis conducted for an olivine/apatite sample yielded a very early absolute age of 4.5689 (±0.0006) b.y. based on the Shallowater standard, coeval with CAI formation (Pravdivtseva, et al., 2015). Amelin and Irving (2011) calculated a Pb–Pb based isochron of 4.478 (±0.055) b.y., but that very young age was attributed to a terrestrial Pb contribution due to weathering. The U–Pb systematics indicate a more primitive Pb-isotopic composition than that determined for the Earth, and therefore rules out a terrestrial origin for NWA 5400. CrystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. Click on Term to Read More age studies for NWA 5400 based on Mn–Cr systematics were conducted by both Shukolyukov et al. (2010) and Sanborn et al. (2016), and they determined an age of ≤4.541 b.y. and ≤4.552 b.y., respectively. This crystallization age is somewhat younger than that determined for Brachina, but older than the proposed collision of ‘Theia’ and proto-Earth. An Ar–Ar dating study was conducted for the paired meteorite NWA 6077 by Beard et al. (2016), which shows complete resetting of this chronometer ~600 m.y. ago with another possible event ~200 m.y. ago.
The initial measurement of 54Cr abundances in NWA 5400 was found to be indistinguishable from that of both terrestrial samples and E chondrites. This lack of a Cr isotope anomaly indicates an old crystallization age, but reveals that NWA 5400 is significantly younger than Brachina (A. Irving). New Cr-isotopic analyses conducted by Sanborn et al. (2016) for NWA 5400 and paired stone NWA 6077 provided different results; the δ54Cr values are actually well resolved from terrestrial and E chondrite values (see Δ17O vs. δ54Cr coupled diagram below [top]). They also demonstrated that NWA 5400 is well resolved from other meteorite groups which are located in relatively close proximity in isotopic space, such as the anomalous eucritesMost common type of achondrite meteorite and a member of the HED group. Eucrites are basalts composed primarily of pigeonite and anorthite (An60-98). Eucrites have been placed into three subgroups based on mineralogical and chemical differences. • Non-cumulate eucrites represent the upper crust that solidified on a magma ocean after Click on Term to Read More and brachinites (see Δ17O vs. δ54Cr coupled diagram below [bottom]). The initial consideration of a potential genetic relationship between NWA 5400 and the brachinite and brachinite-like meteorites is refuted by this comparitive technique. However, utilizing this diagram does reveal a close similarity in δ54Cr values among NWA 5400, the brachinite and brachinite-like meteorites, and the anomalous eucrites, which suggests their parent bodies formed in a similar nebular reservoir. Diagram credit: Sanborn et al., 47th LPSC, #2309 (2016)
Diagram credit: Sanborn et al., 47th LPSC, #2309 (2016) Burkhardt et al. (2015, 2017 [http://dx.doi.org/10.1111/maps.12834]) have conducted in-depth petrographic, geochemical, and isotopic (O, Cr, Ca, Ti, Ni, Mo, Ru and Nd) analyses for the paired meteorites NWA 5363 and 5400. Results of their studies indicate that while NWA 5363/5400 has Δ17O values indistinguishable from Earth, other isotopic anomalies (ε54Cr, ε48Ca, ε50Ti, ε92Ni, ε92Mo, ε100Ru, ε145Nd) show significant differences from the Earth. The isotopic anomalies together with the chronological and petrographic data demonstrate that this meteorite does not represent precursor material of Earth, early impact ejectaFractured and/or molten rocky debris thrown out of a crater during a meteorite impact event, or, alternatively, material, including ash, lapilli, and bombs, erupted from a volcano. Click on Term to Read More from Earth, or remnants from ‘Theia’. Instead, Burkhardt et al. (2017) concluded that its parent body was a relatively small asteroid that originated in a unique nebulaAn immense interstellar, diffuse cloud of gas and dust from which a central star and surrounding planets and planetesimals condense and accrete. The properties of nebulae vary enormously and depend on their composition as well as the environment in which they are situated. Emission nebula are powered by young, massive Click on Term to Read More isotopic reservoir most similar to enstatiteA mineral that is composed of Mg-rich pyroxene, MgSiO3. It is the magnesium endmember of the pyroxene silicate mineral series - enstatite (MgSiO3) to ferrosilite (FeSiO3). Click on Term to Read More and ordinary chondrites, and that it experienced a petrogenetic history similar to brachinites. It is interesting to note that, with the exception of ε48Ca (no 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 data for ε100Ru), NWA 5363/5400 and angrites have values for each of these isotopic anomalies which are nearly the same or overlap within uncertainties. In another study, Amelin and Irving (2011) proposed that NWA 5400 originated from a parent body associated with one of the iron meteoriteIron meteorites consist mostly of metallic iron alloyed with typically between ~5 to ~30 wt% nickel. The main metal phases are kamacite α-(Fe, Ni) and taenite y-(Fe, Ni). Based on their group classification, they may also contain a small weight percentage of one or more of the following minerals: • Click on Term to Read More groups containing primitive Pb.
The CRE age based on 3He and 21Ne was determined to be ~29 m.y. The specimen of NWA 5400 shown above is a 5.3 g partial slice. Shown below is a composite image of both sides of the complete mass as found. Photos courtesy of Aziz Habibi