DaG 319

Ureilite
Polymict
Deep 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 fragmental 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

Found 1997
27° 01.68′ N., 16° 21.52′ E. Three fragments totaling 740 g were found in the Libyan Desert. Two other stones that were recovered later, DaG 665 (363 g) and DaG 874 (64.6 g), are possibly paired. Dar al Gani 319 is one of only a handful of polymict members among a mostly monomict ureilite population. This particular 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 brecciated structure and contains solar type rare gases, suggesting a surface regolith residence on the ureilite 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. Diamonds and many types of inclusions have been found in this meteorite. The diverse clasts have been categorized into seven general groups (Ikeda et al., 2000):

1. coarse-grained maficOne of the two broad categories of silicate minerals, the other being felsic, based on its magnesium (Mg) and/or iron (Fe) content. Mafic indicates silicate minerals that are predominantly comprised of Mg and/or Fe.The term is derived from those major constituents: Magnesium + Ferrum (Latin for iron) + ic (having Click on Term to Read More lithic clasts
2. fine-grained mafic lithic clasts
3. felsicOne of the two broad categories of silicate minerals, the other being mafic, based on the magnesium (Mg) and/or iron (Fe) content. Felsic indicates silicate minerals that are not predominantly comprised of Mg and/or Fe. The term is derived from feldspar + ic (having the character or form of). The Click on Term to Read More lithic clasts and gabbroic clasts
4. dark clasts
5. sulfide- and metal-rich lithic clasts
6. 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 and 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 fragments
7. isolated 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 clasts, mostly single crystals

The main clastA mineral or rock fragment embedded in another rock. Click on Term to Read More type present in polymict ureilites is the common Type I monomict ureilite, containing mainly 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, pigeoniteLow-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, found as a major mineral in eucrites and shergottites. In order to be considered pigeonite, the clinopyroxene must contain 5 to 20 mol % of calcium (Wo5 - 20). Chondrites of petrologic types 4 and below contain significant low-Ca clinopyroxene. During metamorphism to higher temperatures, all existing, and carbonElement commonly found in meteorites, it occurs in several structural forms (polymorphs). All polymorphs are shown to the left with * indicating that it been found in meteorites and impact structures: a. diamond*; b. graphite*; c. lonsdalite*; d. buckminsterfullerene* (C60); e. C540; f. C70; g. amorphous carbon; h. carbon nanotube*. Click on Term to Read More. Type I ureilites crystallized from the residues that remained after the extraction of a partial melt component, and they probably represent 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 Click on Term to Read More material. Type II ureilite clasts, which contain mainly olivine, augiteHigh-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, that occurs in many igneous rocks, particularly those of basaltic composition. In order to be considered augite, the clinopyroxene must contain 20 to 45 mol % of calcium (Wo20 - 45). An important and unique Martian meteorite is NWA 8159, that has been classified as an augite basalt. Click on Term to Read More, and 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, along with minor sulfide 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, are present in a much lower abundance. Magmatic inclusions, usually containing a feldspathic glass component, are present in olivine and orthopyroxene grains of the Type II clasts. Type II ureilites are magmatic cumulates formed by fractional crystallizationA crystallization process in which minerals crystallizing from a magma are isolated from contact with the liquid. It is a key process in the formation of igneous rocks during the process of magmatic differentiation. Also known as crystal fractionation. Click on Term to Read More of basaltic magmas derived from alkali-rich chondritic precursor material. They crystallized within shallow magmaMolten silicate (rock) beneath the surface of a planetary body or moon. When it reaches the surface, magma is called lava. Click on Term to Read More chambers at high cooling rates.

The abundant felsic clasts present in polymict ureilites are mainly porphyritic clasts composed 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 and pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification category. Click on Term to Read More phenocrysts in an alkali-rich groundmass. These felsic clasts have an O-isotopic composition consistent with the missing basaltic component of the ureilite parent body, and are associated with three distinct igneous lithologies—in order of increasing formation depth: albitic (~An0–32), labradoritic (~An33–70), and olivine–augite feldspathic clasts (Cohen et al., 2004; Goodrich et al., 2016, 2017). In addition, minor abundances of magnesian anorthite-rich (~An89–90, ~Fo93) and ferroan anorthite-rich (>An95, ~Fo50–70) clasts were identified by Goodrich and Wilson (2014). These feldspathic clasts were formed by ~20% 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, primarily fractional melting along with some 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 Click on Term to Read More batch melting (representing clasts with incompatible 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 depletion and enrichment, respectively) of the chondritic precursor material early in ureilite parent body (UPB) history (Kita et al., 2004; Cohen et al., 2004; Goodrich et al., 2017). This melting was followed by rapid fractional 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 of the felsic magma and loss through explosive volcanism, or alternatively, through loss of peripheral layers following an oblique collision with a smaller planetesimal (Downes et al., 2008).

The crystallization age of one of the albitic feldspathic clasts, determined by both Mn–Cr and Al–Mg systematics (Kita et al., 2007), was found to be a very old 4.562 b.y. This concordance in chronometers might reflect the impact-disruption thought to have occurred on the UPB, and the age attests to the formation of the UPB within ~10 m.y. of the collapse of the solar nebulaThe primitive gas and dust cloud around the Sun from which planetary materials formed.. Other chronometric systems such as the short-lived Hf–W chronometer indicate 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 of the UPB occurred within ~1 m.y. of Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. history (Budde et al., 2014).

Two plagioclase feldspar-enriched (~70 vol%) trachyandesitic clasts, designated MS-MU-011 and MS-MU-035, were found as part of the Almahata Sitta polymict ureilite 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. It is considered that these clasts likely sample the UPB 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, or possibly an alkali- and water-rich localized melt pocket, and they are similar to several albitic clasts identified in polymict ureilites by Goodrich et al. (2017). These two trachyandesitic clasts have an Ar–Ar isochron age for glass-bearing pyroxene of 4.556 (±0.015) b.y., which is ~1 m.y. younger than other felsic clasts found in DaG 319 or any other polymict ureilites (Bischoff et al., 2014). This age constrains the timing of the UPB disruption event to ≥6.5 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. Also part of the Almahata Sitta fall is MS-MU-012, which is the first known plagioclase-bearing olivine–augite ureilite lithology, with an O-isotopic composition consistent with a primary origin on the UPB (Bischoff et al., 2015; Goodrich et al., 2016).

In a similar manner, the unbrecciated ureilite EET 96001 has been found to contain a consequential component (at least 32 vol%) of compositionally diverse K-rich feldspars, probably former crustal basaltBasalt is the most common extrusive igneous rock on the terrestrial planets. For example, more than 90% of all volcanic rock on Earth is basalt. The term basalt is applied to most low viscosity dark silicate lavas, regardless of composition. Basalt is a mafic, extrusive and fine grained igneous rock Click on Term to Read More material, located within FeS-rich veins (Warren et al., 2006). It is considered plausible that this assemblage of a shallow-formed feldsparAn alumino-silicate mineral containing a solid solution of calcium, sodium and potassium. Over half the Earth’s crust is composed of feldspars and due to their abundance, feldspars are used in the classification of igneous rocks. A more complete explanation can be found on the feldspar group page. Click on Term to Read More and a more deeply occurring FeS-rich vein was then gently mixed with the ureilite groundmass at a time when the jumbling of diverse materials could have occurred; i.e., at the time of the catastrophic disruption and reassembly of the proto-ureilite parent body. During this period, rapid smelting reactions would have produced large volumes of CO gas having forces responsible for mixing diverse materials and causing explosive volcanism accompanied by loss of most of the basaltic melt. Certain elemental ratios which establish distinct correlations at Fo~82–85 constrain the timing of this disruption event to a period when parent body melting had started to produce relatively magnesian melts (Downes et al., 2007). These Mg-rich melts re-accreted to form inclusions within polymict ureilites such as DaG 319.

The dark carbonaceous clasts found in DaG 319 have a CI-like texture and may reflect late-stage accretion onto the UPB (Ikeda et al., 2003). Some of these dark clasts contain anhydrous minerals that have O-isotopes which are consistent with a ureilite origin. Some dark clasts are Fa-free and show evidence within phyllosilicate-rich nodules, veins, and 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 having experienced complete hydrationReaction of a substance with water. Click on Term to Read More by water highly depleted in 16O. Other dark clasts are Fa-bearing and are similar to fayalites found in 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 Click on Term to Read More. These Fa-bearing clasts contain exotic rock fragments of basaltic and peridotitic textures with forsteritic olivines, and they experienced only mild hydration concurrent with the host dark clast. These Fa-bearing clasts may have been indigenous to the regolith zone.

Rare sulfide-rich and metal-rich clasts contain a silicate-rich matrix similar to that found in the Allende-like CV3 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 (Ikeda et al., 2003). The silicate-rich matrix of the sulfide-rich clasts has 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 isotopic composition that plots along the CCAM mixing line near to the other ureilitic clasts, and it may represent the precursor material of the unbrecciated ureilites at a moderate depth. Consistent with this scenario, the reductionOxidation 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 of FeO in the silicate-rich matrix material, coupled with the subsequent removal of residual Fe within the melt, would have produced a final residue similar to the low Fe content of unbrecciated ureilites. In a like manner, the somewhat lighter oxygen composition of the silicate-rich matrix in the reducedOxidation 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 metal-rich clasts suggests that they may represent the precursor material of unbrecciated ureilites at a deeper location on the ureilite parent body.

The O-isotopic compositions of chondrule fragments identified in DaG 319 show similarities to those of both known and unknown ordinary chondrites, likely representing impactors onto the ureilite parent body. In addition, both incompletely equilibrated and equilibrated chondriteChondrite with minerals of uniform composition (e.g., all of the olivine grains have the same composition) due to diffusion during thermal metamorphism. Such chondrites would be petrologic type 4 (some thermal metamorphism) to type 7 (complete thermal metamorphism of chondritic material). Type 7 chondrites are also referred to as metachondrites. Click on Term to Read More fragments are present which are likely R chondrite material, possibly unique from R chondrites in our collections (Goodrich et al., 2016). The olivines in these equilibrated chondrite fragments have many characteristics in common with R chondrites, including a low Mg#, low Ca and Cr contents, low metal content, and high Ni content, as well as the presence of chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More and pyrrhotiteIron sulfide group of minerals whose composition ranges widely between its end members pyrrhotite (Fe7S8) whose crystal structure is monoclinic, and troilite (FeS) whose crystal structure is hexagonal. Its general formula is Fe1−xS (where x = 0 to 0.17). The troilite phase is found mainly in meteorites and in the Click on Term to Read More phases (Downes and Mittlefehldt, 2006). It has also been reported that other 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 group members contain similar R chondrite-like clasts. Isolated PO 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 of type 3.1–3.2 and type 4 have been identified in DaG 319 having OC or RC compositions (Goodrich et al., 2016). A complex L/LL chondrite breccia found in DaG 319 was characterized by Goodrich and Gross (2015).

Other exotic clasts identified in DaG 319 and other polymict ureilites include those representing material from E chondrites or aubritesAubrites 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, and clasts derived from objects not yet represented in our collections (Downes et al., 2008). Notably, certain anorthite–fassaite-bearing clasts (>An95) have chemical and O-isotopic compositions that are similar to angrites, and these may have originated on the 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 parent body (Goodrich and Wilson, 2014). The first representative sampling of a quenched 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 clast found in DaG 319 was described by Goodrich et al. (2015). Components in this small (~1 × 1.5 mm) angrite clast show some similarities to those in the basaltic/quenched angrites A-881371 and LEW 87051. In a contrasting view, Cohen et al. (2004) suggest that most of the anorthite-rich plagioclase clasts likely derive from ureilitic precursor material. Anorthite-rich clasts (>An90) would be produced from material having Ca/Al ratios 2×CI and which experienced a high degree of fractional batch melting (~18%). Si-bearing metals identified by Ross et al. (2009) in polymict ureilites are considered to be remnants of the disrupted UPB 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 that were subsequently accreted to the regolith of the reconstituted UPB.

Notably, a mm-sized, gabbroic troctoliticTroctolite is an intrusive igneous rock consisting of plagioclase feldspar and olivine. It is a member of gabbroic rocks family. It is compositionally similar to gabbro. The main difference is that it does not contain pyroxene or contains very little while it is a major mineral in gabbro. It can Click on Term to Read More clast (γ-8) with compositions (e.g., trace elements and An of plagioclase; Fe/Mg, Fe/Mn, Mg#, and CaO and Cr2O3 contents of olivine) similar to 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 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 NWA 7325 was found in DaG 319 (Kita et al., 2004; Goodrich et al., 2014). Furthermore, Boyle et al. (2017) identified a Ca-plagioclase clast in the polymict ureilite NWA 10657 with identical An and FeO values to plagioclase in NWA 7325. In addition, a number of other mafic silicates and plagioclase clasts have been identified in polymict ureilites which have textures and certain mineral compositions similar to NWA 7325 (see diagram below). The occurrence of similar clasts in both NWA 7325 and polymict ureilites (the clasts are considered to be exogenous in the ureilites) should not be surprising given the fact that the two parent bodies also share similar O-isotopic compositions and likely formed within the same nebular region. Further comparisons utilizing a possible second sampling of the NWA 7325 parent body, NWA 11119 (photo courtesy of Carl Agee), should be elucidating.
Diagram credit: Boyle et al., 48th LPSC, #1219 (2017) Although the Type I and Type II mafic ureilitic clasts show a range of O-isotopic compositions, their presence within a single polymict ureilite demonstrates that they all were formed on a common parent body. In a similar manner, while O-isotopic deviations among the various ureilite subgroups preclude them from being related by igneous processes, the heterogeneity of the polymict ureilites suggests that there was a common parent body for all ureilite subgroups. Furthermore, the olivine compositions within a single thin sectionThin slice or rock, usually 30 µm thick. Thin sections are used to study rocks with a petrographic microscope. of polymict ureilite EET 87720 was found to span the entire range of olivine compositions recorded for unbrecciated ureilites, and the Mg# distribution is nearly identical to that of unbrecciated ureilites—two more factors which demonstrate a common origin for all ureilites (Downes and Mittlefehldt, 2006). Any measurable differences that do exist among individual olivines can be attributed to the fact that widespread impact gardening occurred subsequent to the collisional disruption and reassembly of the proto-ureilite parent asteroid, thus producing the compositional diversity observed.

The ureilite parent body was just large enough to attain temperatures high enough to produce partial melting (up to ~30%) promoting low degrees of basaltic melt migration, but less than that necessary to produce extensive melting and formation of a magma oceanCompletely molten surfaces of terrestrial planets or moons that formed soon after accretion. Samples returned by the Apollo missions provide evidence of a lunar magma ocean, crystallization of which produced a stratified Moon with a low-density crust formed by accumulation of the mineral plagioclase overlying a higher density mantle of Click on Term to Read More. Due to this low degree of melting, perhaps caused by the sudden onset of cooling following impact disruption and reassembly, coupled with rapid melt extraction due to abundant smelting-produced CO+CO2, ureilites have retained the chemical and isotopic heterogeneity of the original carbonaceous chondrite-like asteroid as represented by the unmelted clast population. From chemical and isotopic compositions, it can be inferred that the composition of the UPB was similar to alkali-rich CV-like chondrite material, and that it was intermediate in size between undifferentiated chondritic asteroids and those asteroids large enough to have experienced melting, 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, and core formation; it probably had a diameter of ~200 km (Goodrich et al., 2007).

In an attempt to identify possible common ejection events among the ureilites, Beard and Swindle (2017) conducted a comparative study of 39 different samples utilizing three parameters: CRE age, Fo content in olivine (Mg#), and Δ17O value. They resolved ten potential clusters, several of which show concordance in their CRE age and Mg# but not in Δ17O value (heterogeneous), and three that are concordant in all three parameters (homogeneous). The oldest cluster they resolved, comprising DaG 084, DaG 319, Goalpara, and Haverö (homogeneous, although the Δ17O values for DaG 084 and DaG 319 have not yet been determined), reflects an ejection event that occurred 20.1 (±1.2) m.y. ago. The CRE age of this cluster is consistent with the average of all CRE age results obtained to date of 19.7 (±2.8) m.y. (Riebe et al., 2017).

An excellent petrographic thin section micrograph of DaG 319 can be seen on John Kashuba’s page. This ureilite has a weathering grade of W2 and shock features consistent with low shock. The DaG 319 specimen shown above weighs 5.7 g.

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