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
Found May 1999
y° 13′ 53′ N., x° 32′ 01′ W. A single fusion-crusted 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 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 weighing 2.71 kg was found by the Labenne Family during their 1999 meteorite expedition in the Sahara Desert. The 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 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 composition and texture of Sah 99555 is very similar to that of D’Orbigny and the groundmass component of Asuka 881371. However, where Asuka 881371 contains only small vugs, Sah 99555 contains large mm-sized vugs within a greenish-gray, coarse-grained 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. Sah 99555 also lacks the 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 that the other two contain.
Prior studies based on a somewhat limited sampling of 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
have shown them to be igneous rocks composed of mostly clinopyroxene in the rare form of Al,Ti–diopside-hedenbergite, formerly known as fassaite. Sah 99555 has a higher content of 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
(33 vol%) than clinopyroxene (24 vol%), together with significant amounts of Mg-rich olivine (23 vol%), Ca,Fe-rich olivine (19 vol%), and low-Ca kirschsteinite (8.5 vol%). In addition, minor high-Ca kirschsteinite, titano-magnetite, troiliteBrass colored non-magnetic Fe sulfide, FeS, found in a variety of meteorites.
, and a late-stage Ca silico-phosphate (determined to be silico-apatite by Mikouchi et al.
, 2015) are present. As with other angrites, Sah 99555 is highly depleted in volatiles such as Na and K and highly enriched in oxidizedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons, while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More
elements such as FeO, TiO and CaO—characteristics which separate this class from all others, and suggest a precursor that was extremely CAI-rich, probably similar to the CV-type 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
. In fact, in a study of the least metamorphosed members D’Orbigny and Sah 99555, it was demonstrated by Jurewicz et al.
(2004) that these angrites were compositionally similar to, though not identical to, devolatilized Allende 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
melts formed under low pressures at elevated 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
Angrites are extremely ancient meteorites, with absolute ages ranging from ~4.557 b.y. to ~4.564 b.y., only slightly younger than 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 Allende (~4.5685 b.y.; Burkhardt et al.
, 2007). Angrite 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
formation occurred 1.7–2.8 m.y. after these first nebular condensates (Markowski et al.
, 2006). Various radioactive isotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: 1H, 2H (deuterium), and 3H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. Click on Term to Read More
chronometers have been employed to establish the date for the formation of angrites. These extensive isotopic studies establish angrites as an early planetary differentiate undisturbed since their formation. Based on the Pb–Pb chronometer, an age of 4.5662 (±0.0001) b.y. was derived for Sah 99555 and NWA 1296 by Baker et al.
(2005), while a slightly younger Pb–Pb age of 4.56441 (±0.00065) b.y. was determined for Sah 99555 by Amelin (2007). A highly precise progressive dissolution technique, which successfully accounts for three Pb components, was recently conducted by Connelly et al.
(2008) and Amelin (2008). A revised Pb–Pb age of 4.56458 (±0.00014) b.y. was determined to be the best estimate for 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 Sah 99555. This revised Pb–Pb age is now consistent with that of D’Orbigny. On the other hand, a number of other radionuclideRadioactive isotope - Atomic nuclide that decays radioactively . Click on Term to Read More
chronometers reveal an age ~2 m.y. younger than the Pb–Pb age, and this discrepancy has not been resolved thus far.
High precision measurements conducted on the Al–Mg 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
have established a crystallization age for Sah 99555 of ~4.11 m.y. after CAIs (given that CAIs formed 4.5683 [±0.0007] b.y. ago). More specifically, 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
was formed 3.0–3.5 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, followed by ~1.5–2.0 m.y. of 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
ocean evolution prior to eruption and crystallization (Schiller et al.
, 2010). This corresponds to an absolute age of 4.5635 (±0.0005) b.y. Relative to Efremovka CAIs, an Al–Mg age of 4.5624 (±0.0002) b.y. was determined by Spivak-Birndorf et al
(2009). Almost within error margins, a Mn–Cr age for Sah 99555 was determined to be 4.5637 (±0.0004) b.y., while a Hf–W age was determined to be 4.5628 (±0.0008) b.y., which is concordant with other extinct radionuclide chronometers; however, all ages are slightly younger than the Pb–Pb age. This very early period of Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids.
history corresponds to a time when the short-lived isotopes 26
Al and 60
Fe were still extant and could have initiated parent body melting. In their studies of the 176
Hf excess in Sah 99555, Thrane et al.
(2007) demonstrated that it was derived from the rapid decay of 176
Lu, the nuclei of which were excited by cosmic raysHigh-energy subatomic particles mainly originating outside the Solar System that continuously bombard the Earth from all directions. They represent one of the few direct samples of matter from outside our solar system and travel through space at nearly the speed of light. These charged particles – positively charged protons or Click on Term to Read More
generated from a supernovaStellar explosion that expels much or all of the stellar material with great force, driving a blast wave into the surrounding space, and leaving a supernova remnant. Supernovae are classified based on the presence or absence of features in their optical spectra taken near maximum light. They were first categorized Click on Term to Read More
explosion that occurred after the crystallization of the angrite PB.
According to Sanders and Scott (2007), any body that accreted to a diameter >60 km (i.e.
, large enough to minimize heat loss from the surface through conductionTransfer of heat as a result of collisions between molecules; when one end of an object is heated or excited, the molecules vibrate faster and their energy is transferred sequentially to their neighbors. Click on Term to Read More
) within ~2 m.y. of CAI formation (the oldest objects dating to 4.567 b.y. ago) as the angrites did, must contain enough 26
Al to produce global melting and 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
. In contrast, Senshu and Matsui (2007) determined that 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 a diameter of only ~14 km occurring within 2 m.y. of CAI formation was all that was required for global differentiation to occur, while a diameter of 40–160 km occurring within 1.5 m.y. was cited by Hevey and Sanders (2006) and Sanders and Taylor (2005) as the minimums. Only at large heliocentricCentered around a sun. Our own Solar System is centered around the Sun so that all planets such as Earth orbit around the Sun. Note that 25% of Americans incorrectly believe the Sun revolves around the Earth. Click on Term to Read More
distances (>~2.8 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 Click on Term to Read More
) would accretion proceed too slowly for sufficient 26
Al to accumulate and initiate global melting prior to a body growing too large to melt, considered to be ~200 km diameter (Nyquist and Bogard, 2003).
Be that as it may, John T. Wasson (2016) presented evidence that the slow heating generated entirely by the decay of 26
Al is insufficient to melt asteroids, and that an additional heat source would have been required; e.g.
, the rapid heating incurred from major impact events. He determined that the canonical 26
Al ratio of 0.000052 is much too low to cause any significant melting, and that a minimum ratio of 0.00001 would be required to produce a 20% melt fraction on a well-insulated body having a significant concentration of 26
Al. The initial ratio of 0.0000004–0.0000005 calculated for the angrites Sah 99555 and D’Orbigny based on their 26
Mg isochrons is too low to have generated any significant melting without an additional heat source.
Kurat et al.
(2004) have conducted an extensive study of D’Orbigny and other angrites, in which they utilized mutiple sources of data (i.e.
, structural, textural, chemical, and 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
evidence). They concluded that the angrites are most consistent with a non-igneous origin from refractory solar nebulaThe primitive gas and dust cloud around the Sun from which planetary materials formed.
condensates—basically an asteroid-sized version of a CAI—which record unusual circumstances in the early foundation of the solar system. Some further details on their proposed angrite petrogenesis can be found on the D’Orbigny
Sahara 99555 has a K–Ar age of 3.54 (±0.15) b.y., reflecting a late isotopic disturbance. Interestingly, the D’Orbigny 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
Sm–Nd data show a disturbance at 3.08 (±0.05) b.y. Trace and major element compositions, textures, and crystallization ages of Sah 99555 and D’Orbigny are almost identical (Nyquist et al.
, 2003; Floss et al.
, 2003), suggesting a possible genetic relationship. They are considered to represent the earlier formed crustal lithology on the angrite parent body. In addition, Asuka 881371 and LEW 87051 have trace element trends similar to D’Orbigny and Sah 99555, suggesting that they may all share a common origin, or at least have experienced similar petrographic histories. Trace element trends for LEW 86010 and AdoR are significantly different from each other and from the other angrites, which suggests that they represent distinct lithological sources and that they, along with NWA 4590 and NWA 4801, crystallized a few m.y. later than the oldest angrites. It has been suggested that these latter angrites represent 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
igneous intrusions into the 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
(Irving and Kuehner, 2007).
The results of CRE age studies based on cosmogenic nuclideA nuclear species characterized by Z protons and N neutrons. Click on Term to Read More
data infer a CRE age for Sah 99555 of 6.6 (±0.8) m.y. (Bischoff et al.
, 2000). This age is similar to that calculated for Asuka 881371 of 5.3 m.y., and these two angrites might share a common ejection event. Portions of the angrite asteroid must be 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
) from which spallationThe formation of new nuclides by interactions of high-energy cosmic ray protons with target nuclei that commonly produce several smaller product nuclides.
has continued to occur over the past ~56 m.y. as indicated by the broad range
in angrite CRE ages. <!– Crystallization of the angrites proceeded as a two-stage process beginning with 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
from a CV-like chondritic source composed of olivine, 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
, and clinopyroxene at low pressure, followed by slow cooling to ~650 °C when it was rapidly quenched (1–50 °C/hr) during a severe impact event. Severe outgassing of volatiles occurred at this time, possibly hastened by 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
strength of the gravitational field of the fragmented asteroid. Based on studies relating kirschsteinite lamellae profiles to cooling rates, as well as results of crystallization experiments, the burial depth of the angrites as they quickly crystallized in 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 is inferred to have been within 1 m of the surface.
–> The number of unique angrites represented in our collections today is very limited, and they have been grouped by some as basaltic/quenched, sub-volcanic/metamorphic, or plutonic/metamorphic, along with a single dunitic sample in NWA 8535 (photo
courtesy of Habib Naji). The specimen of Sah 99555 pictured above is a 1.97 g partial slice measuring 20 × 10 × 3 mm. A tiny vug reflecting the incident light can be seen just left of center. Much larger vugs are present in this angrite, which are apparent in the following photo shown courtesy of Labenne Meteorites:
The top photo below shows Marc Labenne as he removed the angrite meteorite from its shallow depression. The bottom photo below shows a 470 g end section in the collection of the University of New Mexico.
Photos courtesy of Labenne Meteorites