NWA 1670

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
Basaltic/Quenched, Picritic

Purchased January 1, 2001
no coordinates recorded A single 30.6 g stone that was found in 2001 in Morocco, possibly in Attamina, was subsequently sold in Erfoud, Morocco to Bruno Fectay and Carine Bidaut. Northwest Africa 1670 was classified at two French institutions, the Université Pierre & Marie Curie (A. Jambon and O. Boudouma) and the Université d’Angers (J-A. Barrat), and has been described as a highly shocked 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 representative of an impact melt.

Northwest Africa 1670 has been described (Mikouchi et al., 2003; Jambon et al., 2008) as having a porphyritic texture, primarily consisting of a very fine-grained groundmass (82 vol%) composed of lath-shaped grains up to 1 mm in size consisting primarily of fassaite 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 Click on Term to Read More. Embedded within the groundmass are highly-magnesian (Fo96–88) 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 xenocrysts (~18–20 vol%) measuring 0.5–3 mm in size, but it is considered likely they were all in the larger size range before sectioning (Mikouchi, 2014). Along the rims of the olivine xenocrysts in NWA 1670 are <1 mm-sized euhedral olivine phenocrysts that crystallized from the groundmass melt. As reported in D’Orbigny and A-881371, xenocrysts in NWA 1670 contain ~5–10 µm-sized inclusions consisting of FeNi-metal and sulfides, while some contain traces of fluid inclusions (Mikouchi et al., 2011; Mikouchi, 2014). These magnesian olivine xenocrysts were formed 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 Click on Term to Read More conditions before they were incorporated into the 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 Click on Term to Read More parental melt of the groundmass (Mikouchi et al., 2015). The olivine xenocrysts are proposed to be zoned 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 which was incorporated into an ascending 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 and was subsequently quenched upon eruption onto the surface of a relatively large angrite protoplanet. However, an alternative formation scenario through a severe impact melting event is still under consideration (Mikouchi, 2014). Either way, angrites represent some of the earliest known differentiated material from a Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. object, and with a U-corrected Pb–Pb age of 4.56437 (±0.00019) b.y., NWA 1670 is the oldest known angrite (Bizzarro et al., 2013).

As in other angrites, the plagioclase is nearly chemically pure 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 Click on Term to Read More (An99–100), but is more Fe-enriched. Lesser amounts of calcic olivine are incorporated as patches within the fassaite. Accessory phases include spinelMg-Al oxide, MgAl2O4, found in CAIs. (both xenocrystic and groundmass types), FeS, kirschsteinite, Ti-magnetite, and Ca-silicophoshate. Ca-carbonate droplets (up to 5 µm) are trapped in 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. Alkalies such as Na and K are lacking, possibly as a result of loss during impact events. 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 Click on Term to Read More and REEOften abbreviated as “REE”, these 16 elements include (preceded by their atomic numbers): 21 scandium (Sc), 39 Yttrium (Y) and the 14 elements that comprise the lanthanides excluding 61 Promethium, an extremely rare and radioactive element. These elements show closely related geochemical behaviors associated with their filled 4f atomic orbital. Click on Term to Read More data for NWA 1670 are similar to that for the other quenched angrites, and along with the similar mineralogies, indicates a common magmatic origin (Sanborn and Wadhwa, 2010; Mikouchi et al., 2011).

Northwest Africa 1670 is typical in many respects to other angrites, being derived from a primary angritic source melt—the apparent differences among them can be attributed in large part to the accumulation of xenocrystic, highly magnesian olivine and to pyroxene accumulation. The groundmass texture and olivine zoning profiles in NWA 1670 are consistent with that of a more rapidly quenched melt located at very shallow depths. The low Si content and the overabundance of Ca in many mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More phases of NWA 1670 attests to melting in the presence of carbonateMineral or compound containing carbon and oxygen (i.e. calcium carbonate, CaCO3, calcite). Click on Term to Read More (Jambon et al., 2005). This is a process unique to angrites, which might illustrate one of the earliest stages of 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 evolution.

It was proposed by Mikouchi et al (2001) that a rapidly cooling magma (~10–50°C/hour) entrained locally variable amounts of magnesian olivine xenocrysts derived from the mantle into the groundmass melt. Cooling rate data acquired with respect to chemical zoning of olivine xenocrysts gave consistent rates of 7–13°C/hour (Mikouchi et al., 2008). The lower Mn/Cr ratios obtained by Sugiura et al. (2003) are also consistent with rapid cooling within a thin lavaHot molten or semifluid rock derived from a volcano or surface fissure from a differentiated and magmatically active parent body. Click on Term to Read More flow at a depth of ~0.5–2 m. In further contrast to other angrites (with the exception of strongly shocked NWA 7203; Hayashi et al., 2018), NWA 1670 exhibits signs of a severe shock event, as evidenced by mosaicism and undulose extinctionIn astronomy, the dimming of starlight as it passes through the interstellar medium. Dust scatters some of the light, causing the total intensity of the light to diminish. It is important to take this effect into account when measuring the apparent brightness of stars. The dark bands running across portions Click on Term to Read More in olivine xenocrysts, and by the presence of cracks and impact-melt veins. In view of the shock deformation features present in NWA 1670 (and other quenched angrites) olivine xenocrysts, late metamorphism associated with an impact-shock event is considered a possibility (Jambon et al., 2008; Mikouchi et al., 2015, 2017).

The Mn–Cr ages of NWA 1670, Asuka 881371/12209, D’Orbigny, and Sah 99555 are identical and represent the oldest angrite 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 ages. Despite the fact that D’Orbigny and Sah 99555 lack olivine xenocrysts, NWA 1670 likely originated from a common magma source, as did the two other olivine xenocryst-bearing (picritic) quenched angrites LEW 87051 and Asuka 881371/12209. By inferring the amount of dissolved olivine xenocrysts each of these angrites should contain, it was ascertained that they, along with NWA 1296, have similar bulk elemental compositions supporting a common magma source controlled 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 with or without addition and resorption of Mg-rich olivine xenocrysts (Mikouchi and Bizzarro, 2012). Furthermore, the chemical composition of the NWA 2999 pairing group shows that it also derives from a picritic source magma, which thereafter experienced further fractional melting, metamorphism, and annealing, along with incorporation of an exogenous 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 component (Baghdadi et al., 2015). NWA 1670 contains the most magnesian (Fo96) olivine xenocrysts of any angrite (or 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) known and also contains FeNi-metal grains, which suggests that it originated on a large, 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 angrite 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 having a significant 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. Click on Term to Read More; the xenocrysts subsequently experienced a period of 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 prior to incorporation into the parental melt (Mikouchi et al., 2017).

In order to better constrain the properties of the differentiated angrite parent body core, van Westrenen et al. (2016) conducted a study modeling 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 depletions along with their 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 partitioningThe tendency of elements to prefer one mineral phase relative to another or to preferentially enter the solid or remain in the liquid during crystallization. Click on Term to Read More behavior for the hypothesized angrite parental melt composition. A CV 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 mantle composition was used for their calculations, along with a temperature and pressure (0.1 GPa) appropriate for a solidifying melt on a small planetesimal. Their results indicate that the observed siderophile element depletions of angrites are consistent with a core mass fraction of 0.12–0.29 composed of Fe and Ni in a ratio of ~80:20 (with a low S content), and that it was formed under redoxOxidation 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 conditions (oxygen fugacityUsed to express the idealized partial pressure of a gas, in this case oxygen, in a nonideal mixture. Oxygen fugacity (ƒO2) is a measure of the partial pressure of gaseous oxygen that is available to react in a particular environment (e.g. protoplanetary disk, Earth's magma, an asteroid's regolith, etc.) and Click on Term to Read More) of ΔIW–1.5 (±0.45).

A CRE age of ~15–18 m.y. was calculated for the both NWA 1670 and LEW 86010 angrites, possibly representing a single ejection event (Eugster et al., 1991; Herzog and Caffee, 2014). This event might also include the angrite NWA 7812 with a CRE age of 20–21 m.y., since this age should be considered an upper limit based on the possibility that it contains a solar cosmic ray Ne component (Wieler et al., 2016). In addition, a similar CRE age of 20.3 (±2.2) m.y. was calculated by Takenouchi et al. (2019) for the quenched angrite NWA 7203. They recognized that NWA 1670 and NWA 7203 are the only angrites that exhibit shock features, which are manifest in the form of melt veins. An Ar–Ar age of 3.80 (±0.44) b.y. was ascertained, which Takenouchi et al. (2019) believe best represents the timing of this shock event.

Multiple episodes of impact, disruption, and dissemination of the crustOutermost layer of a differentiated planet, asteroid or moon, usually consisting of silicate rock and extending no more than 10s of km from the surface. The term is also applied to icy bodies, in which case it is composed of ices, frozen gases, and accumulated meteoritic material. On Earth, the Click on Term to Read More 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 Click on Term to Read More (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 Click on Term to Read More) 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 (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 Click on Term to Read More) 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, Early View, p. 14 (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. Click on Term to Read More in angrites Northwest Africa 1296, 2999/4931, 4590, and 4801: Evolution history inferred from noble gasElement occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. Click on Term to Read More signatures’
(http://dx.doi.org/10.1111/maps.13039)
In a study of remanent magnetismAlso called residual magnetism, refers to the permanent magnetization preserved within the ferromagnetic minerals inside rocks, like meteorites. The presence of a magnetic field assumes and requires a differentiated parent body that contained a liquid core sometime in its past. When a rock cools below its Curie temperature, it acquires Click on Term to Read More in angrites, Weiss et al. (2008) discovered that a magnetic field with a strength of ~10 µT (microteslas), ~20% of that of present-day Earth, was imparted to the angrite PB during its earliest phase of crystallization. This magnetic field may be attributable to a number of possible causes; e.g., 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 to an orbit in close proximity to the early T-Tauri phase solar field, or perhaps more plausible, a magnetic field generated through an internal core-dynamo mechanism. Subsequent paleomagnetic intensity studies conducted for D’Orbigny, Sahara 99555, and Angra dos Reis by Wang et al. (2015) have established a natural remanent magnetization value for Angra dos Reis of ~15 µT (microteslas), demonstrating that this lithology formed under the influence of a significant core dynamo which existed ~11 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. By comparison, no natural remanent magnetization (paleointensity) > ~1 µT was detected for the earlier formed angrites D’Orbigny and Sahara 99555, which constrains the onset of the APB core dynamo to later than ~4 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. It was also recognized that the strong solar nebula-generated magnetic field which had existed ~1.2–3 m.y. after CAIs (~50 µT, measured in Semarkona 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) had virtually disappeared by the time the earliest angrites were formed, indicating that the solar nebulaThe primitive gas and dust cloud around the Sun from which planetary materials formed. had already been largely dissipated.

Diagram credit: Wang et al., 46th LPSC, #2516 (2015)
A limited number of unique angrites are represented in our collections today, and they have been grouped as basaltic/quenched, sub-volcanic/metamorphic, or plutonic/metamorphic, along with a single dunitic sample NWA 8535 (photo courtesy of Habib Naji). Another quenched angrite, NWA 7203 (photo courtesy of Labenne Meteorites), exhibits a striking variolitic texture. Interestingly, small fine-grained 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 clasts exhibiting textures and mineralogy generally consistent with a quenched angrite-like impactor are preserved in impact melt glass fragments recovered from the significant impact event that occurred ~5.28 m.y. ago near Bahía Blanca, Argentina (Schultz et al., 2006; Harris and Schultz, 2009, 2017; see photo below). The specimen of NWA 1670 pictured above is a 0.25 g partial slice.

Photo credit (left): Schultz et al., MAPS, vol. 41, #5, p. 755 (2006) (http://dx.doi.org/10.1111/j.1945-5100.2006.tb00990.x)
Diagram credit (right): Harris and Schultz, 40th LPSC, #2453 (2009)