NWA 7822

Achondrite-ungrouped
(dunite, CV-clan-related)

Purchased February 2013
no coordinates recorded A single fusion-crusted stone weighing 45.8 g was found in the Sahara Desert region and later sold to G. Hupé at the 2013 Tucson Gem 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 Show. This meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and was analyzed at the University of Washington in Seattle (A. Irving and S. Kuehner) and NWA 7822 was determined to be a dunitic lithology with 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 FeO/MnO ratios and O-isotopic values consistent with many other diverse meteorites hypothesized to belong to an extensive CV-clan (see the O-isotopic plot; K. Ziegler, UNM). In their continued analyses of this meteorite, Sanborn et al. (2015) demonstrated that NWA 7822 plots close to the CV chondritesMeteorite class named after the Vigarano meteorite that fell in Italy in 1910. They have abundant large, well-defined rimless (?) chondrules of magnesium-rich olivine (~0.7 mm diameter; 40-65 vol. %), often surrounded by iron sulfide. They also contain 7-20 vol. % CAIs. The often dark-gray matrix is dominated by Fe-rich on a Δ17O vs. ε54Cr coupled diagram (see diagram below). This meteorite is composed predominantly of pale-yellow olivine (>90 vol%) with a small component of interstitialTerm applied to ions or atoms occupying sites between lattice points. clinopyroxene and 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, along with low abundances of the typical opaque phases (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.", troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites., and chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups.).

Collisional Disruption of a Primary Planetary Body

It has been posited by some investigators that numerous large planetary bodies accreted 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, many of which fatefully suffered catastrophic collisional disruption soon thereafter. As an example, Blackburn et al. (2017) and Edwards et al. (2017) calculated the timing of the catastrophic disruption of the H- and L-chondrite parent bodies to be ~60 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. This timing is consistent with two competing dating techniques—Pb–Pb (and Hf–W) chronometry and metallographic cooling rates (Ni diffusionMovement of particles from higher chemical potential to lower chemical potential (chemical potential can in most cases of diffusion be represented by a change in concentration). Diffusion, the spontaneous spreading of matter (particles), heat, or momentum, is one type of transport phenomena. Because diffusion is thermally activated, coefficients for diffusion profiles in Fe-metal)—which record cooling associated with both an onion shell structure prior to disruption and a rubble pile after disruption, respectively. In addition, it was proposed by Irving et al. (2009) that the diverse meteorite lithologies with similar O-isotopic compositions to the HED clan of meteorites, generally considered to be derived from the asteroid 4 VestaThird largest and fourth brightest asteroid; it was discovered in 1807 by Heinrich Olbers and named for the ancient Roman goddess of the hearth. 4 Vesta has a basaltic surface composition and an average density not much less than that of Mars. Evidently lava once flowed here indicating that the, were once part of an even larger former differentiated planetary body that they named "Opis" (the mother of Vesta in Greek mythology).

Another such hypothesized collisionally-disaggregated planetary body (here named "Antaeus") was conceived by Irving et al. (2004) to have comprised many diverse lithologies, here expanded upon to include the following: a metallic 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. region composed of IIF-type iron like Del Rio (Kracher et al., 1980), IVB-type iron like Santa Clara and/or South Byron trio-type iron (Corrigan et al., 2017; Hilton et al., 2018); a core–mantleMain silicate-rich zone within a planet between the crust and metallic core. The mantle accounts for 82% of Earth's volume and is composed of silicate minerals rich in Mg. The temperature of the mantle can be as high as 3,700 °C. Heat generated in the core causes convection currents in boundary or upper mantle impact-melted zone composed of a metal+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 assemblage that corresponds to the Milton 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 (Sanborn et al., 2018), along with the NWA 176 (related to Bocaiuva; Liu, 2001) silicated iron; a dunitic mantle zone possibly represented by NWA 7822; an intensely thermally-metamorphosed stratigraphy resembling the NWA 3133 and NWA 10503 metachondrites (Irving et al., 2004; Irving et al., 2016; Sanborn et al., 2018); and a thick insulating 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 (~20 km; Davison et al., 2013), possibly involving a late accetionary stage, comprising a primitive chondrule–CAI-rich 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).   consisting of several distinct lithological zones comprising reduced Allende-like, oxidized Allende-like, and highly aqueously-altered Bali-like material. The detailed petrogenetic sequences by which each of these meteorites acquired their present form, and the question as to whether these events occurred before, during, or after a catastrophic disruption of the primary planetary body (or were associated with post-disruption daughter objects), are subjects that are still under investigation.

Importantly, the O- and Cr-isotopic signatures of Eagle Station have been utilized to establish an early formation age of 4.557 b.y., or 11 m.y. years 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 formation. According to Dauphas et al. (2005), application of the Hf–W isotopic chronometer to Eagle Station also gives a relatively late metal–silicate segregation for Eagle Station of ~10 m.y. after 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 of the HED 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. 4 Vesta (which occurred as early as 1.3 m.y. after CAI formation; Schiller et al., 2010). Since it has been calculated that melting and core–mantle differentiation due to radiogenic heating should cease after ~7–8 m.y. (Sahijpal et al., 2007), it may be inferred that heating of the Eagle Station asteroid continued until after all radiogenic 26Al and 60Fe was extinct, and that such late heating would have been generated through large impact events. In support of that reasoning, John T. Wasson (2016) presented evidence that the slow heating generated entirely by the decay of 26Al is insufficient to melt asteroids, and that an additional heat source would have been required; e.g., the rapid heating incurred from major impact events. He determined that the canonical 26Al/27Al ratio of 0.000052 is much too low to cause any significant melting, and that a minimum ratio of 0.00001 would be required to produce a 20% melt fraction on a well-insulated body having a significant concentration of 26Al. For example, the initial ratio of 0.0000004–0.0000005 calculated for the angrites Sah 99555 and D’Orbigny based on their 26Al–26Mg isochrons is too low to have generated any significant melting without an additional heat source. Therefore, impacts were a major source of heating in early solar 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 history.

Likewise, the formation scenario envisioned for the silicated irons NWA 176 and Bocaiuva is consistent with impact-heating events on a small-sized asteroid. The final mixing event was accompanied by an initial rapid-cooling stage beginning at the metal–silicate equilibriumTerm used to describe physical or chemical stasis. Physical equilibrium may be divided into two types: static and dynamic. Static equilibrium occurs when the components of forces and torques acting in one direction are balanced by components of forces and torques acting in the opposite direction. A system in static temperature of ~1100°C, and was sustained down to ~600°C. This was followed by a slow cooling stage in which a Thomson (Widmanstätten) structure was formed (Desnoyers et al., 1985). Another fast cooling stage was initiated between approximately 600°C and 300°C as indicated by the absence of tetrataenite and other petrographic features (Araujo et al., 1983). There are major structural similarities between the NWA 176 and Bocaiuva silicated irons and those silicated iron members of the IIE and IAB complex iron groups. This suggests that similar impact processes, such as a catastrophic breakup event, occurred on each of these relatively small, nonmagmatic parent bodies; however, only the IIF irons and the Eagle Station pallasites share any significant geochemical similarities with NWA 176 and Bocaiuva (Bunch et al., 1970; Curvello et al., 1983). Notably, NWA 176, Bocaiuva, and the Eagle Station pallasites, as well as other distinct meteorite lithologies, have similar O- and/or Cr-isotopic compositions to the CV 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 (Clayton and Mayeda., 1996; Liu et al, 2001; Shukolyukov and Lugmair, 2001). Taking the many similarities into account, it seems likely that these otherwise disparate meteorites originated on a common chondritic precursor parent body (Malvin et al., 1985).

In an effort to better resolve potential genetic relationships that might exist among the meteorites mentioned above associated with the hypothetical Antaeus, a Cr-isotopic analysis of olivine from the Milton pallasite was conducted by Sanborn et al. (2018). It is demonstrated on a coupled Δ17O vs. ε54Cr diagram (shown below) that Milton plots among the CV clan and plausibly shares a genetic relationship, but also that Eagle Station plots closer to the CK (or CO) 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 group. It could be inferred that both the CV and CK planetesimalsHypothetical solid celestial body that accumulated during the last stages of accretion. These bodies, from ~1-100 km in size, formed in the early solar system by accretion of dust (rock) and ice (if present) in the central plane of the solar nebula. Most planetesimals accreted to planets, but many – experienced a similar petrogenetic history in a similar isotopic reservoir of the nascent solar system. Chromium vs. OxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only IsotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: H, H (deuterium), and H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. Plot

click on image for a magnified view

Diagram credit: Sanborn et al., 49th LPSC, #1780 (2018) Notably, a formation scenario for pallasites was proposed by Asphaug et al. (2006) and Danielson et al. (2009) in which the wide variation in metal–silicate textures and bulk compositions that is observed among MG pallasite members is the result of a grazing collision between partially molten planetary embryos. They assert that such a collision resulted in the formation of a chain of smaller objects having diverse compositions. It might be more than coincidental that the O-isotopic composition of the Milton pallasite plots proximate to the trend line of the Eagle Station group pallasites (now termed the Allende Mixing line: slope = 0.94 ±0.01). Both of these rapidly-cooled pallasites contain high concentrations of the refractory 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 Ir relative to main-group (MG) pallasites (Jones et al., 2003), and they both have overlapping Fe and Ni abundances (wt%) in their 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 component; however, significant variations observed in their minor and trace 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 concentrations indicate that they each experienced different crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. processes (Hillebrand, 2004). Still, there is a good possibility that one or both of these pallasites did share a common precursor parent body with the CV clan of meteorites, at least prior to any collisional disruption event.
Diagram adapted from Korochantsev et al., 2013 With the advent of better investigative techniques, scientists have explored the possibility of a genetic relationship between IVB irons and other meteorite groups based on O-isotopic analyses. Utilizing chromite grains from IVB irons Warburton Range and Hoba, Corrigan et al. (2017) found that IVB irons share close similarities to the South Byron trio irons (Babb’s Mill [Troost’s], South Byron, and Inland Forts [ILD] 83500)–Milton pallasite grouping (MSB in diagram below). Moreover, the O-isotopic compositions of the IVB irons and the South Byron trio–Milton grouping 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 within the range of the 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 CV and CK chondrites.
Diagram credit: Corrigan et al., 48h LPSC, #2556 (2017) Subsequent to the catastrophic disruption of the primary planetary body that is envisioned here, and the sorting and re-accretion of material into a number of daughter objects, multiple impacts onto these small asteroids could have led to the formation of sub-surface melt pools tens of meters in size. Differentiation of these melt pools would have resulted in cumulus olivine sequestered above a metal layer, and an olivine residuum that had drained below this metal layer—a complex assemblage from which associated pallasitic and silicated-iron lithologies could be derived thereafter during less-energetic, rapidly-cooled impact events (Malvin et al., 1985). The anomalously-high Ir contents measured in some of the associated metal–silicate mixtures (e.g., Eagle Station grouplet, Milton) and segregated metal regions (IIF irons, South Byron trio) would be consistent with metal that crystallized at the lowest levels of the melt chamber. Such late-stage, rapidly-cooled, impact-heating events could have allowed for the retention of the original O- and Cr-isotopic composition of the primary planetary body (Humayun and Weiss, 2011 and references therein). The differences that exist in δ54Cr between chromite and olivine in the Eagle Station pallasite, but which are not observed in CV chondrites, could be the result of a distinct Cr source associated with impact projectile(s) which eventually led to the formation of the Eagle Station-type pallasites and other related lithologies (Papanastassiou and Chen (2011).

On an oxygen three-isotope diagram (see example below), the CO chondrites plot along the Allende Mixing trend line (former CCAM line), overlapping near the middle of the CV chondrite field. There is a possibility that the CO chondriteMeteorite class named after the Ornans meteorite that fell in France in 1868, are related in chemistry and composition to the CV chondrites and may, with them, represent a distinct clan of carbonaceous chondrites that formed in the same region of the early solar system. However, COs are usually blacker group, of which Isna is a marginally equilibrated example (type 3.75), is also implicated in the sequence of events that led to the formation of the diverse CV clan of meteorites as outlined above—perhaps as another of the daughter objects that accreted after a catastrophic disruption of the primary planetary body. It could be deduced that subsequent impacts involving many of these daughter objects sent fragments into storage orbits within the outer 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. Further fragmentation events (collisional cascade processes), along with the Yarkovsky effect, would have delivered samples into mean motion resonances with some fragments eventually achieving Earth-crossing orbits.
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Diagram credit: Irving et al., 79th MetSoc, #6461 (2016) The hypothesis of multiple daughter objects being formed following the catastrophic disruption of a large, partially differentiated, primary planetary body could allow for the potential inclusionFragment of foreign (xeno-) material enclosed within the primary matrix of a rock or meteorite. of several less closely-related meteorites. These may include the high-Ni irons of the South Byron trio (South Byron, ILD 83500, and Babb’s Mill), which have metallographic compositions (especially siderophile element patterns) and structures similar to the metal in Milton, including 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 spindles and associated schreibersiteNi-Fe phosphide mineral, (Fe,Ni)3P, yellowish in color and predominantly found in iron and stony-iron meteorites. Schreibersite can also be found in a variety of other meteorites including some acapulcoites, aubrites, enstatite chondrites and achondrites, lunars, ureilites, winonaites and a smattering of other meteorite types like CM, CO and CB. Schreibersite, consistent with their formation on the same parent body (Reynolds et al., 2006). These three irons and the metal component in Milton experienced a similar 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 history during formation; they each have similar depletions of easily oxidized elements as well as similar abundances of siderophiles (McCoy et al., 2008). In addition to the irons mentioned above, several other 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). ataxites may be genetically related to this high-Ni iron group, including El Qoseir, Illinois Gulch, Morradal, Nordheim, and Tucson (Kissin, 2010). However, significant differences that exist between their refractory element contents compared to those of the South Byron trio requires further work to establish a specific relationship.

The metal in each of these high-Ni iron meteorites and in Milton is consistent with early crystallization from a metallic-melt phase that experienced a low degree of 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.. Similarly, the FeNi-metal component of one member of the Eagle Station grouplet, Itzawisis, was derived from a metallic-melt source consistent with that of a differentiated, oxidized-CV source before 20% fractionation had occurred. In a like manner, the metal in Eagle Station derives from a 20% fractionated source (Humayun and Weiss, 2011), while another member of the grouplet, Cold Bay, was shown to derive from a melt source that crystallized after 40% fractionation. The most recent member of the Eagle Station grouplet to be analyzed, Karavannoe, crystallized from an even more evolved metallic-melt that had undergone >60% fractionation—though still not as evolved as the source melt from which MG pallasite metal crystallized. Karavannoe FeNi-metal has a lower Ni content than the other members of the grouplet (Korochantsev et al., 2013), and measurements show that its Ir content is intermediate between that of Eagle Station and Milton.

Further evidence for a large differentiated planetary body having CV-trends lies in the fact that CV chondrites acquired a strong unidirectional natural remanent magnetization ~9 m.y. after CAI formation, reflecting the existence of an internal core dynamo (e.g., Weiss et al., 2010; Elkins-Tanton et al., 2011; Carporzen et al., 2010, 2011; Gattacceca et al., 2013, 2016). Employing multiple investigation techniques, Shah et al. (2017) investigated the paleointensity of 19 Vigarano 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 and found values of 1.1–150 µT. The observed magnetic remanence is considered to have been acquired during brecciationThe formation of a breccia through a process by which rock fragments of of various types are recemented or fused together. events that occurred ~7 m.y. after initial parent body 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, with impact shock pressures reaching 10–20 GPa. Therefore, they reason that the original paleofield would have been ~40 µT, which is too high to be attributable to the solar windSupersonic flow of high-speed charged particles continuously blowing off a star (mostly e- and p+). When originating from stars other than the Sun, it is sometimes called a "stellar" wind. The solar wind may be viewed as an extension of the corona into interplanetary space. The solar wind emanates radially field, but is in the range of that expected for a planetary core dynamo. As research continues, further evidence for the catastrophic disruption of this former primary body could advance this hypothesis (see the Allende page for further paleomagnetic information). The photo shown above is a 1.0 g partial slice of the dunite NWA 7822, while photos of a complete slice and the original stone are shown below.

Photos courtesy of Greg Hupé—Nature’s Vault