NWA 2999

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
PlutonicGeology: Igneous intrusive body that forms when magma is injected into host rocks and solidifies. Plutons occur in the crust of asteroids undergoing differentiation or planets. Named after Pluto, the Roman god of the underworld. Plutonic rocks are the rocks found within a pluton. Astronomy: Category of planet including all/Metamorphic

Purchased August 2004
no coordinates recorded Twelve individual fragments constituting a single meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and, with a combined total weight of 392 g, were visually distinguished by Greg Hupé from an assortment of meteorites he had purchased in Morocco. Samples from different stones were sent for analysis to Northern Arizona University (T. Bunch and J. Wittke) and the University of Washington in Seattle (A. Irving and S. Kuehner). A preliminary analysis found similarities to known angrites, and a sample was sent to the Carnegie Institute, Washington D.C. (D. Rumble III) for O-isotopic analysis. By this method it was verified that these meteorites were in fact a new sampling of 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 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.. Because of the importance and uniqueness of this findMeteorite not seen to fall, but recovered at some later date. For example, many finds from Antarctica fell 10,000 to 700,000 years ago., a sample from each of the twelve fragments was submitted for analysis. Numerous other pairings have been independently analyzed and given separate NWA series numbers, with the total combined weight of this pairing group being ~7.8 kg.

Only a small number of unique angrites are currently represented in our collections, which some investigators have resolved into four subgroups: basaltic/quenched, sub-volcanic/metamorphic, plutonic/metamorphic, and dunitic. In a study based on a comparison of Hf/Sm ratios for a diverse sampling of both angrites and 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, Bouvier et al. (2015) inferred that these meteorite subgroups reflect the existence of three distinct crustal reservoirs on their respective parent bodies. These three reservoirs reflect similar chemical 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 processes on both parent bodies: 1) subchondritic Hf/Sm ratios for the Angra dos Reis angrite and the cumulateIgneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber. eucrites (such as Moama); 2) chondritic Hf/Sm ratios for the quenched angrites (such as D’Orbigny and Sahara 99555) and the basaltic eucrites; 3) superchondritic Hf/Sm ratios for the sub-volcanic and plutonic angrites (NWA 4590 and NWA 4801, respectively) and the unusual cumulate eucriteMost 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 Binda. The metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. NWA 2999 pairing group was not included in the Bouvier et al. (2015) study.

In contrast to other angrites, NWA 2999 exhibits a polygonal-granular texture consistent with a relatively slowly-cooled and annealed lithology, more similar to the sub-volcanic and plutonic angrites than to the quenched angrites. Evidence in support of a plutonic origin for NWA 2999 can be found in the homogenous 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. compositions compared to the wider compositional range that exists in some other angrites (Kuehner et al., 2006). However, evidence also exists for an extended residence within a regolith—large (up to 6 mm) anorthiteRare compositional variety of plagioclase and the calcium end-member of the plagioclase feldspar mineral series with the formula CaAl2Si2O8. Anorthite is found in mafic igneous rocks such as anorthosite. Anorthite is abundant on the Moon and in lunar meteorites. However, anorthite is very rare on Earth since it weathers rapidly, spinelMg-Al oxide, MgAl2O4, found in CAIs., and 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 rock fragments are present within the fine-grained groundmass. Moreover, while other angrites contain only minor FeNi-metal (<2 vol%), the NWA 2999 pairing group contains up to 9 vol% (NWA 3164 pairing) coarse FeS and FeNi-metal having chondritic abundance patterns (Baghdadi et al., 2015). The FeNi-metal in NWA 2999 has elemental ratios that are inconsistent with what would be expected from incomplete 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. separation (Jambon et al., 2012), and neither could this high abundance of 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 have been derived through partial 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 of iron. Instead, it is considered more plausible that the FeNi-metal was incorporated from an exogenous source during an impact event on the angrite parent body. The impactor was most likely a metallic object unrelated to any known chondritic or iron chemical group (Humayun et al., 2007; Jambon et al., 2012).

Consistent with this finding, an increased level of other siderophile elements such as Co, Ir, and AuThe astronomical unit for length is described as the "mean" distance (average of aphelion and perihelion distances) between the Earth and the Sun. Though most references state the value for 1 AU to be approximately 150 million kilometers, the currently accepted precise value for the AU is 149,597,870.66 km. The support the presence of a significant meteoritic component. However, it is unknown if this exogenous FeNi-metal source can also explain the increased Mg content and 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 concentration of refractory elementsUsing research by Wood (2019), any of the elements with a relatively high condensation temperature of 1291 K < TC,50 < 1806 K in the solar nebula1. They are the first elements to condense out of a cooling gas. Refractory elements are the main building blocks of rocky planets, dwarf (e.g., Ca, Al, and Ti) observed in this angrite. Since a chondritic impactor would also have necessarily carried an O-isotopic composition close to that of the TFL, an alternate scenario was proposed by Gellissen et al. (2007) and then by Irving and Kuehner (2007) to account for the observed anomalous elemental abundances. They suggest that a large impact onto the angrite parent body occurred, perhaps by an evolved iron object, which created a mixture of diverse lithologies from within the angrite target body. These diverse lithologies which constitute NWA 2999 were then deeply buried (~120 cm based on depth profiles of 22Ne/21Ne ratios; Nakashima et al., 2018) where they underwent thermal metamorphism and annealing to produce the observed granulitic texture. The chemical composition of the NWA 2999 pairing group shows that it derives from a picritic source magmaMolten silicate (rock) beneath the surface of a planetary body or moon. When it reaches the surface, magma is called lava., which thereafter experienced further fractional melting, metamorphism, and annealing, along with incorporation of an exogenous metal component (Baghdadi et al., 2015).

Northwest Africa 2999 preserves some unique metamorphic features (previously observed in some terrestrial metamorphic rocks) which initially were thought to reflect a decompression stage followed by rapid cooling. Investigators presumed that these events were initiated during an extensive multi-km-deep thrust faulting event on a large parent body, postulated by some to be Mercury (Irving et al., 2005). These metamorphic features include the presence of clinopyroxene–spinel symplectites between 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 and olivine clasts (reflecting a decompression phase), and plagioclase coronas surrounding portions of spinel grains (reflecting a rapid cooling phase).

An alternate explanation for these unique metamorphic features has been proposed by Ruzicka and Hutson (2006), who argue that under low-pressure oxidizingOxidation 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 conditions at various degrees of melt formation, both plagioclase coronas and clinopyroxene–spinel symplectites can be produced as cooling proceeds. Improved models of these symplectite and coronaExtended outer atmosphere of the Sun. The glow of the corona is a million times less bright than that of the photosphere; it can only be seen when the disk of the Sun is blocked during a total solar eclipse, or by using a coronagraph, which artificially blocks the disk textures by Irving and Kuehner (2007) led them to conclude these features are more likely the result of the percolation of a S-bearing fluid during a metasomatic phase. These unique corona microstructures have been further interpreted by Baghdadi et al. (2012, 2013), who reason that a granulitic peridotitic lithology, which originated either as a slowly-cooled plutonGeology: Igneous intrusive body that forms when magma is injected into host rocks and solidifies. Plutons occur in the crust of asteroids undergoing differentiation or planets. Named after Pluto, the Roman god of the underworld. Plutonic rocks are the rocks found within a pluton. Astronomy: Category of planet including all or possibly as an annealed brecciated rock at depth (P <0.9 GPa), was intruded by a hot magma that increased the temperature to 1000–1200°C. This thermal event resulted in the formation of solid state metamorphic coronas at 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 contacts (‘contact metamorphism’) over an extended time interval through the following reaction: clinopyroxene + spinel ⇒ olivine + anorthite (with the reverse reaction occurring upon re-cooling). Eventual ejection of this angrite from its likely planetary-sized parent body produced the fracturing observed within the coronas and throughout this meteorite.

Other features consistent with a very rapid melting and cooling event on the angrite PB have been identified in the angrite NWA 4590. Glass present along mineral grain boundaries attests to a late mobilization of primary phases consistent with a decompression event (Kuehner and Irving, 2007). It has been postulated that the angrite meteorites might represent the impact-related dissemination of a more FeO-rich outer layer during the early history of Mercury, thereby explaining the chemical and mineralogical differences observed on Mercury compared to the angrites; e.g., the higher FeO-abundance of angrites compared to that on the present surface of Mercury, and the reversed Fe/Mn values for both olivine and pyroxene as compared to those of other planetary bodies. Nevertheless, even accepting the occurrence of collisional-stripping of a hypothetical FeO-rich basaltic (angritic) 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 on Mercury, Hutson et al. (2007) find it implausible that Mercury initially differentiated under oxidizing conditions to form the angritic crust, and then subsequently differentiated under reducingOxidation 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 conditions to form the surface that we observe today. They have also argued that other mineralogical features identified in angrites (e.g., reaction coronas), which on one hand may be attributed to rapid decompresion on a planetary-sized body such as Mercury, may just as well be consistent with the typical cooling processes that occurred during crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. of a melt.

In contrast to some other angrites, neither kirschsteinite nor 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 has been found in NWA 2999, and vesicles are absent. Based on Hf–W systematics, NWA 2999 formed ~5 m.y. later than Sahara 99555 and D’Orbigny (Markowski et al., 2007). However, Jambon et al. (2012, #1758) contend that due to the exogenous FeNi-metal present in this meteorite, the Hf–W chronometer is not reliable. It was concluded by Kleine et al. (2009) that both NWA 2999 and AdoR were derived from a parental source magma that had higher Hf–W than other angrites, likely the result of extended differentiation after core formation. A precise crystalization age based on the Mn–Cr 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 indicates an age for NWA 2999 of 4.5579 (±0.0011) b.y., indistinguishable from that of AdoR and LEW 86010. As deduced by Shukolyukov and Lugmair (2008), two age clusters encompass all of the angrites studied thus far, and this attests to a very early period of magmatic activity.

A CRE age of 73.4 (±6.6) m.y. was calculated for NWA 2999 by Nakashima et al. (2008), while an age of 69.6 (±11.2) m.y. was calculated for the paired NWA 4931. A more precise 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. analysis conducted by Nakashima et al. (2018) established a CRE age for NWA 2999 and NWA 4931 of 47.2 (±6.1) m.y. and 51.7 (±6.4) m.y., respectively. Multiple episodes of impact, disruption, and dissemination of the crust can be inferred by the wide range of CRE ages determined for the angrites—<0.2–56 m.y. for thirteen angrites measured to date, possibly representing as many ejection events (Nakashima et al., 2008; Wieler et al., 2016; Nakashima et al., 2018). This range is consistent with a single large parent body enduring multiple impacts over a very long period of time, which would suggest that the parent object resides in a stable 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 (planetary or 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) permitting continuous sampling over at least the past 56 m.y. Alternatively, Nakashima et al. (2018) consider it plausible that there is currently at least two angrite (daughter) objects occupying distinct orbits: one representing the fine-grained (quenched) angrites with the shorter CRE age range of <0.2–22 m.y., and another representing the coarse-grained (plutonic) angrites with the longer CRE age range of 18–56 m.y. (see diagram below). Cosmic-ray Exposure Ages of Angrites

Diagram credit: Nakashima et al., MAPS, vol. 53, #5, p. 965 (2018)
‘Noble gasesElement 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. in angrites Northwest Africa 1296, 2999/4931, 4590, and 4801: Evolution history inferred from noble gas signatures’
(http://dx.doi.org/10.1111/maps.13039) Taken together, all of the anomalous characteristics observed for the NWA 2999 pairing group could be attributed to contamination through exotic impact event(s), or as speculated by some, it could be that the NWA 2999 pairing group might even represent a unique parent body with similar O-isotopic values to those measured for the angrite PB. The specimen of NWA 2999 shown above is a 0.228 g partial slice. The photo below is an excellent petrographic thin sectionThin slice or rock, usually 30 µm thick. Thin sections are used to study rocks with a petrographic microscope. micrograph of the pairing NWA 6291, shown courtesy of Peter Marmet.
click on image for a magnified view
Photo courtesy of Peter Marmet