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
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/Metamorphic
Found May 2007
coordinates not recorded Four fragments comprising a single 252 g individual stone 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 was found in Algeria and purchased by G. Hupé in Erfoud, Morocco. This is a friable meteorite in which the fusion crustMelted exterior of a meteorite that forms when it passes through Earth’s atmosphere. Friction with the air will raise a meteorite’s surface temperature upwards of 4800 K (8180 °F) and will melt (ablate) the surface minerals and flow backwards over the surface as shown in the Lafayette meteorite photograph below. Click on Term to Read More has been eroded away by prolonged terrestrial weathering processes, leaving only 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 products on its surface. An analysis was conducted at the University of Washington in Seattle (A. Irving and S. Kuehner), and it was ascertained that NWA 4801 is a plutonic igneous cumulateIgneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber. Click on Term to Read More 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.
This angrite has a metamorphosed texture being that it is a coarse-grained rock (0.1–1.2 mm) with 180° triple junctions (Irving and Kuehner, 2007). It is composed of a variety of multi-colored grains, primarily Al–Ti-rich clinopyroxene, and contains a high abundance of 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
in the form of white crystals and aggregates. Other grains are composed of Cr-pleonaste, Ca-rich 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
, pleonaste, and merrillite. Minor troiliteBrass colored non-magnetic Fe sulfide, FeS, found in a variety of meteorites.
is present in association with FeNi-metal and small oxide grains (Riches et al.
, 2016). While kirschsteinite is present in most angrites, it has not been observed in this one. Notably, NWA 4801 has a greater abundance of merrillite than in most other angrites.
The crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. Click on Term to Read More
age of NWA 4801 based on Pb isotopes is barely resolvable from that of the youngest angrite, Angra dos Reis (Amelin and Irving, 2007). This young age is also very close to that of the plutonic angrites LEW 86010 and NWA 4590—NWA 4801 is ~1.2 m.y. younger than LEW 86010. The crystallization/isotope closure age for NWA 4801 based on Mn–Cr systematics is 4.5643 (±0.0005) b.y. When anchored to the absolute Pb–Pb chronometer, which has now been accurately determined for NWA 4801 to be 4.558 (±0.013) b.y., these ages provide the best agreement between these two chronometers yet obtained for angrites (Shukolyukov et al.
, 2009). The Sm–Nd-based age is concordant with the Pb–Pb-based age, and is identical within error to the angrite NWA 4590 (Sanborn et al.
, 2011). A Lu–Hf isochron for NWA 4801 was determined by Bouvier et al.
(2015) to be 4.563 (±0.05) b.y. The time of the last 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 fractionationConcentration or separation of one mineral, element, or isotope from an initially homogeneous system. Fractionation can occur as a mass-dependent or mass-independent process. Click on Term to Read More
as determined by Mn–Cr and tied to the new NWA 4801 Pb–Pb anchor is consistent with the crystallization age of the oldest known angrites at 4.5646 (±0.0005) b.y.
With the steadily increasing number of unique angrite samples available for study, new models of their formation are now emerging. In an abstract from the Workshop on Chronology of Meteorites 2007, A. Irving and S. Kuehner (UWS) conceive of a rapid progression of events on the 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
following its 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
within ~2 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. Immediately thereafter, the onset of internal heating by 26
Al decay, along with significant impact heating (John T. Wasson, 2016), resulted in 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
of the mantle and formation of a small 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
(core mass fraction of 0.08; Shirai et al.
, 2009). Subsequent to core formation, plutonic and volcanicIgneous rock that forms from cooling magma on the surface of a planet or asteroid.
magmatism, metasomatism, metamorphism, and impact-generated 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
formation occurred within ~4–11 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
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 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
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).
In-depth studies of the diverse angrite samples collected thus far are bringing to light a scenario in which a large planetary body accreted and crystallized over an extended period of time, perhaps as long as 7 m.y., beginning only a couple of m.y. after the formation of the earliest nebular condensates. The refractory bulk composition of this body, along with features such as a high abundance of trapped solar 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
, attest to an origin in close proximity to the SunOur parent star. The structure of Sun's interior is the result of the hydrostatic equilibrium between gravity and the pressure of the gas. The interior consists of three shells: the core, radiative region, and convective region. Image source: http://eclipse99.nasa.gov/pages/SunActiv.html. The core is the hot, dense central region in which the
. The oldest angritic material is recognized in the form of early crustal vesicularVesicles appear in nature when they are produced within lava (extrusive aphanitic igneous rock) whose dissolved gases come out of solution (are released) due to the drop in pressure during an eruption. The resulting lava solidifies around the gas bubbles capturing their shape inside and outside the rock. Vesicles do Click on Term to Read More
rocks represented by such meteorites as Sahara 99555, D’Orbigny, and NWA 1296. Younger angritic material, occurring in the form of impact-mixed extrusiveRefers to igneous rocks erupted on a planetary body's surface. Click on Term to Read More
and intrusiveRefers to igneous rocks that crystallized underground. Click on Term to Read More
magmatic rocks combined with regolith material, is represented by A-881371, LEW 87051, and NWA 1670. The youngest angritic rocks known, represented by the meteorites Angra dos Reis, LEW 86010, NWA 2999, NWA 4590, and NWA 4801, are composed of annealed regolith and late intrusive plutonic lithologies.
It was proposed by Irving and Kuehner (2007) that one or more severe collisional impacts onto the angrite parent body resulted in the stripping of a significant fraction of its 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
and upper mantle, with the dissemination of large sections of this material into 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
that has been maintained for the past 4+ b.y. The source of the delivery of angrite material to Earth might lie within the main 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
, or it could remain associated with the original collisionally-stripped parent body postulated by some to be the planetThe term "planet" originally comes from the Greek word for "wanderer" since these objects were seen to move in the sky independently from the background of fixed stars that moved together through the seasons. The IAU last defined the term planet in 2006, however the new definition has remained controversial. Click on Term to Read More
Mercury (see schematic diagram below). The disparity in FeO content that exists between the angrite group of meteorites (up to 25 wt%) and that which is observed on the surface of Mercury (~5 wt%) may reflect the existence of a redox gradient in which the lower mantle region, now the present surface of Mercury, has a more magnesian composition. click on image for a magnified view
Diagram credit: A. Irving and S. Kuehner, Workshop on Chronology of Meteorites, #4050
(2007) While angrites could possibly be original fragments from ‘Maia’, mother of Hermes (Mercury), they may instead derive from ‘Theia’, mother of Selene (the Moon goddess). In a new study of the Fe/Mn ratio in olivine grains for a number of angrites, Papike et al.
(2017) determined that these meteorites plot along a trend line between the Earth and Moon, which indicates a possible location for the angrite parent body (see diagram below).
Diagram credit: Papike et al.
, 48th LPSC, #2688
(2017) In connection with their in-depth study of NWA 5363/5400
, Burkhardt et al.
(2017) published comparative data for nucleosynthetic anomalies among parent bodies for O, Cr, Ca, Ti, Ni, Mo, Ru and Nd. It is interesting to note that with the exception of ε48Ca
(no angrite data is available for ε100
Ru), NWA 5363/5400 and angrites have values for each of these isotopic anomalies that are nearly the same or overlap within uncertainties. Results of their studies indicate that while both angrites and NWA 5363/5400 have Δ17O
values indistinguishable from Earth, and that other anomaly values for angrites overlap with Earth within uncertainties (ε92Ni
), the ε54Cr
values for angrites are distinct from Earth. Based on their studies, Burkhardt et al.
(2017) concluded that the parent body of NWA 5363/5400, and perhaps by extention that of angrites, originated in a unique nebular isotopic reservoir most similar to that of enstatiteA mineral that is composed of Mg-rich pyroxene, MgSiO3. It is the magnesium endmember of the pyroxene silicate mineral series - enstatite (MgSiO3) to ferrosilite (FeSiO3). Click on Term to Read More
and ordinary 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
The CRE age calculated for NWA 4801 is 31.6 (±1.5) m.y. (Nakashima et al.
, 2008). 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. Click on Term to Read More
analysis conducted by Nakashima et al.
(2018) established a CRE age for NWA 4801 of 26.4 (±6.1) m.y. 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 orbit (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 (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.
, Early View, p. 14 (2018) ‘Noble gases in angrites Northwest Africa 1296, 2999/4931, 4590, and 4801: Evolution history inferred from noble gas signatures’
Although NWA 4801 has been remagnetized by hand magnets, a study by Weiss et al.
(2008) 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 revealed that a magnetic field with a strength of ~10 µT, ~20% of that of present-day Earth, was imparted to the angrite PB during its earliest phase of crystallization (as observed particularly from the angrite D’Orbigny). This magnetic field could be attributed to a number of possible causes such as accretion to an orbit in close proximity to the early T-Tauri phase solar field, or perhaps more likely, to a magnetic field generated by an internal core-dynamo mechanism.
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 a 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). This impactor is considered to have been very large, perhaps at least one km³, and its source object could plausibly reside near the Earth–Moon systemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with
. Interestingly, analyses of other grains obtained from Bahía Blanca impact melt glass have a geochemistryStudy of the chemical composition of Earth and other planets, chemical processes and reactions that govern the composition of rocks and soils, and the cycles of matter and energy that transport Earth's chemical components in time and space. Click on Term to Read More
similar to the Moon (Harris and Schultz, 2017).
Photo credit (left): Schultz et al.
, 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
The number of unique angrites identified today is quite limited, and they have been grouped as basaltic/quenched, sub-volcanic/metamorphic, or plutonic/metamorphic, along with a single dunitic sample in NWA 8535 (photo
courtesy of Habib Naji). In a recent 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 Click on Term to Read More
, Bouvier et al.
(2015) inferred that these two meteorite groups reflect the existence of three distinct crustal reservoirs on their respective parent bodies. These three reservoirs reflect similar chemical differentiation processes on both parent bodies: 1) subchondritic Hf/Sm ratios for the Angra dos Reis angrite and the cumulate 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 plutonic angrites (NWA 4590 and NWA 4801) 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 Click on Term to Read More
Binda. The unique metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. Click on Term to Read More
NWA 2999 pairing group was not included in the Bouvier et al.
(2015) study. The specimen of NWA 4801 shown above is a 0.98 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 NWA 4801, shown courtesy of Peter Marmet. click on image for a magnified view
Photo courtesy of Peter Marmet