Allende
CV3.2 (>3.6)oxA
Fell February 8, 1969
26 ° 58′ N., 105 ° 19′ W. At 1:05 on a Saturday morning, accompanied by strong detonations, a previously rare carbonaceous chondriteCarbonaceous chondrites represent the most primitive rock samples of our solar system. This rare (less than 5% of all meteorite falls) class of meteorites are a time capsule from the earliest days in the formation of our solar system. They are divided into the following compositional groups that, other than Click on Term to Read More approached rural Mexico from the south–southwest (approximately 215 °) at a low-angle trajectory. Thousands of individual stones fell, creating the largest stone strewnfield recorded, measuring 50 km in length and at least 300 km ² in area. A stone found in the village of Pueblito de Allende was taken to the newspaper office where news of the fallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If Click on Term to Read More was disseminated rapidly. Material was quickly collected by both scientific and commercial interests, and within a few weeks research laboratories around the world were studying this new fall.
The largest recovered specimen was found eight months after the fall by Guadalupe Juarez as he was hunting a rabbit. The 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 created a craterBowl-like depression ("crater" means "cup" in Latin) on the surface of a planet, moon, or asteroid. Craters range in size from a few centimeters to over 1,000 km across, and are mostly caused by impact or by volcanic activity, though some are due to cryovolcanism. Click on Term to Read More 32 inches by 52 inches and 13 inches deep, having a small rim on the north end. This specimen was estimated to have weighed ~110 kg before it was fragmented upon impact with the hard soil. The total recovered weight of the Allende fall was over 3 tons, a record at that time for stone meteorites that was surpassed by the Jilin, China fall in 1976 weighing ~4 tons.
X-ray diffractionAnalytical technique used to determine the structures of crystalline solids. A monochromatic beam of X-rays (usually Cu-Kα) is diffracted off repeating planes of atoms in crystalline samples to produce a diffraction pattern. Through analysis of the diffraction pattern, atomic structures can often be determined. techniques and Mössbauer spectroscopyTechnique of splitting electromagnetic radiation (light) into its constituent wavelengths (a spectrum), in much the same way as a prism splits light into a rainbow of colors. Spectra are not smooth but punctuated by 'lines' of absorption or emission caused by interaction with matter. The energy levels of electrons in have been used by Bland et al. (2004) to determine the modal mineralogy of Allende, and to quantify the compositional range of 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 phases. In addition, the grain densityMass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.). Some substances (like gases) are easily compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible gases and is much Click on Term to Read More can be readily estimated from the mode data, and therefore, in combination with the calculated bulk density, the porosityThe volume percentage of a rock that consists of void space. Vesicular porosity is a type of porosity resulting from the presence of vesicles, or gas bubbles, in igneous rock such as the pumice presented here. Vesicular porosity is very rare in meteorites and is often associated with slag, one Click on Term to Read More can be determined. Notably, from the limited studies performed thus far, the CV3 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 subgroup has a greater porosity than 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 subgroup. The modal mineralogy (vol%) and other physical properties of Allende are as follows:
- Olivine
- Fo100 ———— 23.4
- Fo80 ————– 15.4
- Fo60 ————– 20.6
- Fo50 ————– 20.6
- Fo25 —————- 3.9
- PentlanditeFe-Ni sulfide, (Fe,Ni)9S8, that is often associated with troilite, and found in the matrix and chondrules of CO, CV, CK and CR chondrites. The color is yellow-bronze with light bronze-brown streak and metallic luster. It typically forms during cooling of magmatic sulfide melts during the evolution of parent silicate melt. The Click on Term to Read More —————– 8.0
- Clinoenstatite (En90 —– 6.6
- 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 (An100) —– 1.2
- MagnetiteFe oxide, Fe2+Fe3+2O4, containing oxidized iron (Fe3+) found in the matrix of carbonaceous chondrites and as diagnostic component in CK chondrites. In CK chondrites, magnetite is typically chromian, containing several wt. % Cr2O3. Click on Term to Read More ——————- 0.2
- Fe-metal ——————— 0.1
- TOTAL —————— 100.0
- grain density = 3.67 g per cubic cm
Allende and most other carbonaceous 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 contain refractory inclusionsInclusions found predominantly in carbonaceous chondrites and are rich in refractory elements particularly calcium, aluminum and titanium that in various combinations form minerals such as spinel, melilite, perovskite and hibonite. There are two types of refractory inclusion: • Ca Al-rich inclusions (CAIs) • Amoeboid olivine aggregates (AOAs) Refractory inclusions were Click on Term to Read More rich in calcium and aluminum (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) which, as determined through Hf–W systematics, probably formed ~4.5676 b.y. ago by nebular condensation at temperatures high enough to vaporize the Fe and Mg silicates. One theory places their formation early in nebular history when the heat source was the gravitational energy of the accreting stellar disk. Outward diffusionMovement of particles from higher chemical potential to lower chemical potential (chemical potential can in most cases of diffusion be represented by a change in concentration). Diffusion, the spontaneous spreading of matter (particles), heat, or momentum, is one type of transport phenomena. Because diffusion is thermally activated, coefficients for diffusion Click on Term to Read More mechanisms allowed some CAIs to escape solar 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 and become stabilized in the outer zero-drag envelope of the newly formed Jovian gap. In addition, it was proposed that surface flares of Jupiter might have been one of the diverse mechanisms of chondruleRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine Click on Term to Read More formation. Allende chondrule formation is thought to have occurred ~4.5654 b.y. ago. It has been argued that multiple episodes of recycling ensued, during which time thermal, chemical, mechanical, 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 processing occurred, eventually resulting in both CAIs and 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 being incorporated into a carbonaceous assemblage (Ruzicka et al., 2008). Other theories attribute the formation of the majority of chondrules to shock waves caused by gravitational instabilities. Still other studies support a hypothesis of chondrule formation within dust-rich vapor plumes resulting from hypervelocity impact events.
An absolute age for Allende based on the Pb–Pb chronometer, which represents the time of chondrule 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, was determined to be 4.56545 ( ±0.00045) b.y. (Connelly et al., 2007). This age indicates that chondrules in CV3 chondrites were formed by 1.66 ( ±0.48) 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. This history can be compared to that of other carbonaceous 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 groups in which chondrule formation occurred as early as Allende chondrules to as late as 6 m.y. after CAIs (based on Mn–Cr systematics; Trinquier et al., 2008). In another study of Allende chondrule formation ages conducted by Amelin and Krot (2007) and utilizing Pb–Pb residue–leachate isochrons, it was determined that the average age of Allende chondrules is 4.5666 ( ±0.0010) b.y.–within error margins of the age associated with the formation of CAIs in CV3 chondrites (4.5672 [ ±0.0006] m.y.). An ultra-high-precision Mn–Cr age anchored to D’Orbigny of 4.56791 ( ±0.00076) b.y. was determined by Qing-Zhu Yin et al. (2009). These chondrules are the oldest measured from any meteorite, and the inference can be made that Allende chondrules formed virtually contemporaneously with CAIs over a span of time of at least 1.2 m.y., as resolved by 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 (Amelin and Krot, 2007). This period also overlaps the time interval when basaltic crusts were being segregated and iron cores were being formed on other differentiated asteroids.
By a different approach to chondrule age determination, an absolute age of 4.5683 ( ±0.0007) b.y. was determined for CAI formation based on the Hf–W isochron of several angrites (Burkhardt et al., 2008). Comparing this age with the Al–Mg and U–Pb systematics of chondrules in certain carbonaceous chondrites, it can be inferred that CV chondritesMeteorite class named after the Vigarano meteorite that fell in Italy in 1910. They have abundant large, well-defined rimless (?) chondrules of magnesium-rich olivine (~0.7 mm diameter; 40-65 vol. %), often surrounded by iron sulfide. They also contain 7-20 vol. % CAIs. The often dark-gray matrix is dominated by Fe-rich Click on Term to Read More such as Allende formed later than previously thought, ~2 m.y. after CAIs, while CR chondrites formed ~4 m.y. later than CAIs. This history indicates that formation of carbonaceous chondrites occurred even later than ordinary chondrules and is consistent with the lower degree of radiogenic heating experienced by many carbonaceous chondrites compared to ordinary chondrites. Similarly, the timing for the accretion of the CV3 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 was calculated by Jogo et al. (2017) employing thermal and physical evolution models based on assumptions involving multiple parameters (e.g., water:rock ratio, temperature, pressure) under which fayalitePure* iron end-member (Fe2SiO4) of the olivine solid solution series and an important mineral in meteorites. When iron (Fe) is completely substituted by magnesium, it yields the the pure Mg-olivine end-member, forsterite (Mg2SiO4). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo) Click on Term to Read More formation occurred. They concluded that this body accreted 3.0–3.3 m.y. after CAIs.
Still, in a study of U-isotopic variation among Allende CAIs (Brennecka et al., 2010), it was determined that correlations between 238U/235U values and the original Cm/U in the CAIs proved that these U-isotopic variations were caused by the decay of Curium-247, an 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 created during the r-process in supernovae. The calculated initial 247Cm/235U ratio infers that the interval between the production of the heavy 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 247Cm and its delivery to the molecular cloudAn interstellar gas cloud that is dense enough to allow the formation of molecules and comprised of a cold dense complex mixture of interstellar gas and dust roughly 75% hydrogen and 21-24% helium. Clouds contain trace amounts of other molecules, of which well over 100 different types have now been Click on Term to Read More occurred 110–140 million years before CAI formation. This decay of 247Cm would have resulted in higher 235U levels which affects the equations used in the Pb–Pb dating method; ages for the earliest Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. events will need to be corrected by as much as 5 m.y. less than the ages determined by the previous 238U/235U initial ratio of 137.88.
The matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. Click on Term to Read More of Allende is considered by some to be composed of primary nebulaAn immense interstellar, diffuse cloud of gas and dust from which a central star and surrounding planets and planetesimals condense and accrete. The properties of nebulae vary enormously and depend on their composition as well as the environment in which they are situated. Emission nebula are powered by young, massive Click on Term to Read More condensateIn the solar nebula, product of a chemical condensation reaction where a mineral phase precipitates (condenses) directly from a cooling vapor. Click on Term to Read More material that was formed in an 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 region, while others argue that secondary processing on the parent body is more consistent with the evidence. The matrix FeO-rich olivine grains form a porous aggregate of sub- µm- to µm-sized tabular-shaped grains. It was demonstrated that following accretion, the matrix material experienced a period of low-temperature (<610 °C), fluid-assisted thermal metamorphism, which lasted for ~15 m.y. as revealed by Hohenberg et al. (2004) through I–Xe ages of dark inclusions. This scenario of secondary parent body processing is supported by the observation of a Fa-rich olivine composition among various matrix components, including matrix olivine grains, olivine rims around forsteritePure* magnesium end-member (Mg2SiO4) of the olivine solid solution series and an important mineral in meteorites. When magnesium (Mg) is completely substituted by iron, it yields the the pure Fe-olivine end member, fayalite (Fe2SiO4). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo) Click on Term to Read More chondrule fragments, and olivine cross-cutting chondrule fragments (Cuvillier et al., 2015); moreover, FeNi-metal has been oxidized to magnetite. Chemical zoning profiles associated with these various occurrences of fayalitic olivine were studied by Cuvillier et al. (2015) at the nanoscale level. The zoning profiles indicate the occurrence of a single thermal metamorphism stage that lasted for <2 m.y. and reached a peak temperature of 425–505 °C.
In their study of Mn–Cr in Allende and other carbonaceous chondrites, Scott and Sanders (2008, 2009) reasoned that even though carbonaceous chondrite parent bodies were not formed until a few m.y. after the formation of CAIs, they do share a bulk rock Mn–Cr isochron age coincident with CAI formation of 4.568 ( ±0.001) b.y. They argue that carbonaceous chondrules could not have been formed by direct melting of pristine nebular dust because such a moderately volatileSubstances which have a tendency to enter the gas phase relatively easily (by evaporation, addition of heat, etc.). element 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 would not have persisted within the nebula to be inherited at a much later time by carbonaceous chondrules. They propose instead that chondrules were more likely formed from fine-grained splash or condensate material generated by collisions between very early-accreted planetesimalsHypothetical solid celestial body that accumulated during the last stages of accretion. These bodies, from ~1-100 km in size, formed in the early solar system by accretion of dust (rock) and ice (if present) in the central plane of the solar nebula. Most planetesimals accreted to planets, but many – Click on Term to Read More. In addition, fine-grained dust generated from such collisions may have been subsequently melted and mixed with both a volatile-rich and a refractory-rich nebular dust component to form some carbonaceous chondrules. According to their current scenario, volatility fractionation occurred at this time, separating Mn and Cr grains into separate reservoirs: refractory Cr was rapidly concentrated into a first-generation of forsterite- and FeNi-metal-bearing planetesimals during the early, intensely hot stage of nebular condensation, while moderately volatile Mn persisted within the hot solar gas over a longer period of time, only condensing into planetesimals farther from 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 as temperatures decreased. Both elements remained stored within these ‘precursor’ planetesimals for 1.5–5 m.y.–a time period during which steady bombardment shattered these objects and released fine-grained Cr- and Mn-rich ejectaFractured and/or molten rocky debris thrown out of a crater during a meteorite impact event, or, alternatively, material, including ash, lapilli, and bombs, erupted from a volcano. Click on Term to Read More which eventually became incorporated into chondrules. These chondrules were finally accreted to the various carbonaceous chondrite parent bodies.
An alternate theory of CAI origin places the formation at a later period, when the accretion phase was over and the Sun was in its T Tauri phase. The solar windSupersonic flow of high-speed charged particles continuously blowing off a star (mostly e- and p+). When originating from stars other than the Sun, it is sometimes called a "stellar" wind. The solar wind may be viewed as an extension of the corona into interplanetary space. The solar wind emanates radially swept the volatile-rich gas from the outer layers of the nebular disk leaving behind only refractory-rich dust. Shock-wave heating then evaporated the Fe and Mg silicates, leaving the dust enriched in Al. This dust finally coalesced and was melted to form the CAIs that are found in most carbonaceous meteorites. More recently, studies have shown that some CAIs were accreted rapidly into larger bodies, heated by the decay of 26Al, and thermally metamorphosed. Thereafter, these bodies were disrupted and the CAIs were returned to the nebula to be remelted and recycled into later-forming carbonaceous chondrite bodies. Studies have demonstrated that multiple melting events did occur in some CAIs, and that in some, 16O-poor meliliteGroup of minerals found in the CAIs of meteorites such as CV chondrites. Melilite consists almost exclusively of the binary solid solution gehlenite (Ca2Al2SiO7) – åkermanite (Ca2MgSi2O7). The melilite in CAIs is closer to gehlenite in composition. The first-formed (highest-temperature) melilite crystallizing from a melt is relatively aluminum-rich and becomes progressively Click on Term to Read More has crystallized within the short time interval of 0.4 m.y. after the initial crystallization of 16O-rich melilite (Ito et al., 2006).
According to Krot et al. (2009), the depletion mechanism of 16O is thought to have occurred in the outer region of the protoplanetary diskFlattened and rotating disk of dense gas and dust/solids orbiting a young star from which planets can eventually form. Click on Term to Read More due to isotopic self-shielding during UV photolysis of CO. This process preferentially dissociated isotopically heavy molecules from CO including 17O and 18O, which were subsequently incorporated into water ice. Thereafter, this 16O-depleted water ice/dust mixture migrated to the inner disk region and evaporated, leaving this region depleted in 16O. While CAIs were formed in the inner disk during a low-water ice, high 16O CO interval, most chondrules were formed in an ~4 m.y. period after the 16O-depletion occurred.
Layered rims surrounding some CAIs, referred to as Wark–Lovering rims or accretionary rims, were possibly formed by a flash heating event in a more oxidizing environment of the solar nebulaThe primitive gas and dust cloud around the Sun from which planetary materials formed.. This heating event was measured in fractions of a second and resulted in a loss of volatiles with enrichment of the refractory component, along with the subsequent diffusion of O and Mg. During the flash heating event, temperatures at the rim are inferred to have approached 3000 °C, steeply decreasing to temperatures of ~1700 °C just 1 mm below the rim. Subsequent chemical and isotopic exchange, corresponding to the grain size and porosity of specific minerals, most likely occurred in situ on the parent body.
Through investigations of the systematics of radiogenic Pb–Pb in Allende CAIs, values for Th–U were calculated (Yin et al., 2008). Using these values, a formula was employed to determine the age of the interstellar dustGrains of carbon and silicate ~0.1-1.0 mm in size. Dust grains are a major component of the interstellar medium. Dust blocks visible light causing interstellar extinction and scatters incident starlight, particularly blue light (which has a wavelength comparable to the dust grain's size), causing reddening. Cooling of interstellar gas and Click on Term to Read More clouds from which the condensation of the CAIs occurred:
Tgalaxy = 21.8*[log(U/Th)0–log(1/κ)]; where Tgalaxy is expressed in billion years, and (U/Th)0 is the production ratio, and κ is the time integrated model 232Th/238U ratios (Cowan et al., 1999)
According to Cowan et al. (1999), actinide chronometers have been used to determine Galactic ages by (1) predicting 232Th/238U and 235U/238U ratios in r-process calculations; (2) applying them in Galactic evolution models, which include assumptions about the histories of starSelf-luminous object held together by its own self-gravity. Often refers to those objects which generate energy from nuclear reactions occurring at their cores, but may also be applied to stellar remnants such as neutron stars. formation rates and r-process production; and finally (3) comparing these ratios with meteoritic data which provide the Th/U and U/U ratios at the time of formation of the solar system. Age ranges of 10.0–10.9 b.y. and 13.6–14.2 b.y. were determined utilizing two separate Allende CAIs. These ranges are consistent with the age of the galaxyConcentration of 106 to 1012 stars, dust and gas, that are gravitationally bound. Our galaxy contains ~2 × 1011 stars. There are four main types of galaxies: • Elliptical
• Lenticular
• Spiral
• Irregular
Click on Term to Read More as determined through astronomical observations.
The CV group has been subdivided into three subgroups (McSween, 1977; Weisberg et al., 1997):
- Reduced subgroup: e.g., Arch, Efremovka, Leoville, Vigarano, and QUE 93429
- Oxidized-Allende subgroup: e.g., Allende, Axtell, Tibooburra, and ALH 84028
- Oxidized-Bali subgroup: e.g., Bali, Grosnaja, Kaba, and Mokoia
The three subgroups reflect varying degrees of aqueous/oxidative alteration, which has been found to be correlated with the amount of ice-bearing matrix that was initially accreted (Ebel et al., 2009). Strong foliation (planar alignment) as well as lineation (much longer in one dimension than in the other two) was observed in Allende by Tait et al. (2016), the latter of which they suggest could have been produced by accreted ices or through impact compaction. They speculate that these structural features could have enabled a preferential flow of oxidizing fluids on the CV parent body, thereby establishing the difference in redox trends among the CV subgroups.
It was shown that the oxidized-Allende subgroup contains twice the amount of matrix and half the amount of chondrules as the reduced subgroup. As a consequence, the oxidized-Allende subgroup has experienced significant metasomatic alteration of primary minerals producing nepheline, sodalite, andradite, sulfides, magnetite, and other secondary phases (Amelin and Krot, 2007). The oxidized-Bali subgroup exhibits a still higher degree of aqueous alteration than the oxidized-Allende subgroup, which can be observed in the replacement of the primary mesostasisLast material to crystallize/solidify from a melt. Mesostasis can be found in both chondrules, in the matrix around chondrules, and in achondrites as interstitial fine-grained material such as plagioclase, and/or as glass between crystalline minerals. Click on Term to Read More and FeNi-metal by phyllosilicatesClass of hydroxyl-bearing silicate minerals with a sheet-like structure. They result from aqueous alteration are dominantly serpentine and smectite in meteorites; found in the matrixes of carbonaceous chondrites. Phyllosilicates consist of repeating sequences of sheets of linked tetrahedra (T) and sheets of linked octahedra (O). The T sheet consists of Click on Term to Read More, magnetite, sulfides, and other secondary phases. The reduced subgroup exhibits a much lower degree of alteration than the oxidized subgroups (for more mineralogical relationships, see Appendix I, Carbonaceous Chondrites).
A recent study was undertaken by Bonal et al. (2004, 2006) to refine the subtypes of several CV3 chondrites. They utilized several methods to obtain their data, including Raman spectrometry of organicPertaining to C-containing compounds. Organic compounds can be formed by both biological and non-biological (abiotic) processes. Click on Term to Read More material, a petrologic study of Fe zoning in olivine phenocrysts, presolar grain abundance, and a 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 study. These methods are in contrast to that of TL sensitivity data of feldsparAn alumino-silicate mineral containing a solid solution of calcium, sodium and potassium. Over half the Earth’s crust is composed of feldspars and due to their abundance, feldspars are used in the classification of igneous rocks. A more complete explanation can be found on the feldspar group page. Click on Term to Read More which is typically used to determine subtypes of ordinary chondrites and which was previously applied to the CV3 chondrites. They suggest that TL sensitivity data are not applicable to aqueously altered carbonaceous chondrites because of loss of feldspars through dissolution, leading to an underestimate of the petrologic subtypes. They have redefined the petrologic subtypes of the common CV3 members as follows:
| Raman | TL | |
|---|---|---|
| Allende | >3.6 | 3.2 |
| Axtell | >3.6 | 3.0 |
| Grosnaja | ~3.6 | 3.3 |
| Mokoia | ~3.6 | 3.2 |
| Bali | >3.6 | 3.0 |
| Efremovka | 3.1-3.4 | 3.2 |
| Vigarano | 3.1-3.4 | 3.3 |
| Leoville | 3.1-3.4 | 3.0 |
| Kaba | 3.1 | 3.0 |
These differences in petrologic subtype are explained by Greenwood et al. (2009) in their study of CV and CK chondriteClass of carbonaceous chondrite named for the Karoonda meteorite that fell in Australia in 1930. They are more oxidized than all other carbonaceous chondrites and genetically distinct from CV chondrites. CK chondrites appear dark-gray or black due to a high percentage of Cr-rich magnetite dispersed in a matrix of dark Click on Term to Read More relationships. They assert that there is a decoupling between the 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 and organic components with respect to measurements involving thermal metamorphism.
Allende contains other inclusions such as diamonds and silicon carbidePresolar interstellar dust grain found in CM and E chondrites; its formula is SiC. Click on Term to Read More grains that have unusual isotopic and rare-gas element compositions. Recent investigations have discovered other forms of ‘poorly-graphitized carbon’ having three-dimensional, closed structures, including carbonElement commonly found in meteorites, it occurs in several structural forms (polymorphs). All polymorphs are shown to the left with * indicating that it been found in meteorites and impact structures: a. diamond*; b. graphite*; c. lonsdalite*; d. buckminsterfullerene* (C60); e. C540; f. C70; g. amorphous carbon; h. carbon nanotube*. Click on Term to Read More spheres (fullerenes), graphene sheets, and various other pyrocarbon phases (Vis et al., 2002). It is considered by some investigators (e.g., Verchovsky et al., 2002; Matsuda et al., 2010) that an amorphousMaterial without the regular, ordered structure of crystalline solids. Amorphous substances, like glass, lack a definite repeating pattern in their atomic structures (crystallinity). There may be small regions of order, but, overall there is disorder. Click on Term to Read More phase of carbon experienced implantation through ionAtom with a net electrical charge because it has lost, or gained, one or more electrons relative to the number possessed by a neutral atom of the same element. A positively charged ion (cation) has fewer electrons than a neutral atom; a negatively charged ion (anion) has more. Click on Term to Read More irradiation of planetary 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 (the ‘plasmaFourth state of matter: a gas in which many or most of the atoms are ionized. In the plasma state the atoms have split into positive ions and negative electrons, which can flow freely, so the gas becomes electrically conducting and a current can flow. Click on Term to Read More model’), and that this phase now serves as the carrier of the Q-gases. These Q-gases, specifically, He, Ne, Ar, Kr, and Xe, are then released through 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 processes resulting in a rearrangement of the carbon structure. An in-depth investigation into the carbonaceous carrier of the Q-phase was conducted by Fisenko et al. (2018) utilizing the L4 chondrite Saratov. They contend that the carrier of the Q-gases is a nongraphitizing carbon phase present as curved, few-layer, graphene-like sheets which were likely formed in the protoplanetary nebula.
Coarse-grained lithic clasts composed of forsterite grains with annealed, granoblastic textures, were identified within type-I chondrules of some CV chondrites by Libourel and Krot (2006). Due to the lack of O-isotopic equilibration between the olivine clasts and the host 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, it was conjectured that the clasts may represent relict material of early-accreted, differentiated and metamorphosed planetesimals that formed prior to chondrule formation (Krot et al., 2009).
There is evidence in the form of unvaporized interstellar material as well as in short-lived radionuclides, especially in the presence of 60Fe that is produced only through stellar nucleosynthesis, that one or more nearby type-Ia and/or type-II supernovae and/or massive asymptotic giant branch (AGB) starsStars on the Asymptotic Giant Branch, which represents a late stage of stellar evolution that all stars with initial masses < 8 Msun go through. At this late stage of stellar evolution, gas and dust are lifted off the stellar surface by massive winds that transfer material to the interstellar Click on Term to Read More seeded our stellar nursery during its earliest stages, but after the first solids had already formed (Huss et al., 2007). Another investigation of short-lived radionuclideRadioactive isotope - Atomic nuclide that decays radioactively . Click on Term to Read More origins conducted by Huss et al. (2009) suggests that intermediate-mass AGB starsStars on the Asymptotic Giant Branch, which represents a late stage of stellar evolution that all stars with initial masses < 8 Msun go through. At this late stage of stellar evolution, gas and dust are lifted off the stellar surface by massive winds that transfer material to the interstellar Click on Term to Read More, supernovae of type-II having >20 M⊙, and supernovae of type-Ib or -Ic having >~33 M⊙ could each have been possible progenitors which injected short-lived radionuclides into the pre-collapse stage of the molecular cloud. They also found that r-process elements likely came from type-II supernovae having ≤11 M⊙ and 12–25 M⊙. Based on an alternative model, Trigo-RodrÃguez et al. (2009) assert that a nearby massive AGB starStars on the Asymptotic Giant Branch, which represents a late stage of stellar evolution that all stars with initial masses < 8 Msun go through. At this late stage of stellar evolution, gas and dust are lifted off the stellar surface by massive winds that transfer material to the interstellar Click on Term to Read More of ~6.5 M⊙, which had experienced the third dredge-up and hot bottom burning, injected a volume of 0.01 M⊙ of short-lived radionuclides from its envelope into the protosolar cloud. This injection, the volume of which was subsequently diluted by a factor of 300, occurred 0.53 m.y. prior to the formation of the first nebular solids.
From a study of O-isotopic anomalies of the Sun, Lee et al. (2008) recognized that the Sun must have formed within a stellar cluster coincident with a massive star. According to Ouellette et al. (2007, 2009), the mechanism of collapse for the protosolar molecular cloud is consistent with that of a shock waveAbrupt perturbation in the temperature, pressure and density of a solid, liquid or gas, that propagates faster than the speed of sound. from a supernova(e) (~15–25 M⊙) having a speed of 5–30 km/sec. They envision a prior Wolf-Rayet phase in which many short-lived radionuclides are ejected by the star and homogenized into the protoplanetary disk, including 26Al and 41Ca. Only later, during the actual supernova(e) explosion did the initial injection of Fe-enriched dust grains occur through a magnetic hydrodynamic mechanism, followed thereafter by intermediate-mass elements, and finally, unburned C+O; this scenario is consistent with the lack of 60Fe in some very early-accreted (~0.5–0.7 m.y. after CAIs) planetesimals. Carbonaceous chondrites would have accreted later than most other bodies, after the late-ejection of C+O by the supernova(e), within a region of low-temperature conditions (Ustinov, 2006).
While the late injection model may be correct, a related scenario was investigated by Nittler (2007) in an effort to understand the origin of a specific group of presolar grainsMineral grains that formed before our solar system. These tiny crystalline grains are typically found in the fine-grained matrix of chondritic (primitive) meteorites. Most grains probably formed in supernovae or the stellar outflows of red giant (AGB) stars before being incorporated in the molecular cloud from which the solar system Click on Term to Read More having unusual isotopic ratios. He argued that during the explosion of a Type II 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, a jet of 16O-rich material from the inner-zone was mixed with material from the outer-zone to form these unusual presolar grains. This supernova may have also injected the short-lived radionuclides into an already collapsed solar disk. He submitted that the late injection of 26Al into the solar disk during a possible Wolf-Rayet phase, as proposed by Ouellette et al. (2007) (see above), would result in the loss of the star’s entire envelope before the burning and mixing of the various zones required to produce the studied grains could be accomplished. However, Boss (1995) determined that a slowed shock front (<50 km/sec; Foster and Boss, 1996) from a supernova located at a distance of several parsecs from the protoplanetary disk could both trigger the collapse and inject radionuclides from different shells to account for the short-lived radionuclide abundances observed in Allende. The supernova could have been much closer, <1 parsec, had the solar disk already been concentrated.
Another model was presented by Sahijpal and Gupta (2007) in which low-mass star formation occurs first as a result of local density fluctuations, and thereafter, a massive star (>40 M⊙) is formed within ~25 parsecs, perhaps through stellar mergers. This massive star underwent 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 collapse to become a supernova within a short interval of ~3–5 m.y., injecting short-lived nuclides into the assemblage of protoplanetary disks. More detailed solar formation scenarios by Sahijpal and Gupta (2009) and others can be found on the Ningqiang page.
Meteorite studies in recent years have revealed the existence of a group of meteorites that are texturally-evolved chondrites, some of which have O-isotopic compositions that plot in the CV chondrite field; the term metachondriteTerm used to describe a metamorphosed chondrite. Also referred to as a type 7 chondrite. Metachondrites are texturally evolved rocks derived from chondritic precursors and some have been classified as primitive achondrites. Click on Term to Read More has been used to more accurately describe these meteorites (Irving et al., 2005). In addition, O-isotopic compositions determined for members of other meteorite groups also plot in the CV chondrite field, supporting the inference of a large differentiated planetary body that underwent a catastophic disruption early in solar system history. At one time, this object could have consisted of multiple lithological zones including a metallic core (iron), a core–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 boundary (pallasiteOne of two main classes of stony-iron meteorite, the other being mesosiderites. Pallasites are igneous in nature and characterized by crystals of olivine, sometimes peridot (green gem quality clear olivine crystals), embedded in a matrix of Fe-Ni metal. The type specimen, weighing 680 kg, was found in the mountains near Click on Term to Read More), an upper mantle impact-melted zone composed of metal+silicate assemblages (silicated iron), a high-temperature zone (dunite), an intensely thermally metamorphosed strataOriginally horizontal layers of rock. (metachondrite), and a primitive, insulating chondritic 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 which has experienced impact-gardening and metasomatism. See ‘The Breakup of Antaeus’ for additional details.
In support of such a differentiated asteroid model, the detection of a strong natural remanent magnetization has been confirmed in Allende, estimated to be 3–60 microTeslas ( µT) (Weiss et al., 2009, 2010; Emmerton, 2011). This paleomagnetic field intensity is consistent with an internally-generated dynamo, and the acquisition timing (~8–10 m.y. after solar system formation; Carporzen et al., 2010, 2011 [‘Magnetic Evidence for a Partially Differentiated Carbonaceous Chondrite Parent Body’, PNAS, vol. 108, #16 Supporting Information]) and long term directional stability excludes an origin from either shock-induced magnetization or that produced by any external source such as the early T Tauri phase of the Sun (see diagram below). Further studies by Elkins-Tanton and Weiss (2009) led them to conclude that following the early, rapid accretion (by ~1.7–3 m.y.) of the CV parent body, a metallic iron core and an internally-generated convecting 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 were formed, covered by an insulating, unmelted chondritic 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; the planetesimal was probably at least 400 km in diameter. Metasomatism occurring over a period of several million years lead to the acquisition of a remanent magnetization by minerals in the chondritic crust (Elkins-Tanton et al., 2011). In a contrary interpretation of the magnetic remanence data obtained for Allende matrix, Muxworthy et al. (2015, 2017) suggest that impact shock heating of a highly porous matrix with non-porous chondrules is a plausible source for the magnetism. Paleomagnetic studies of the Kaba CV3 meteorite were conducted by Gattacceca et al. (2016) utilizing the abundant pseudo-single domain magnetite. They measured a natural remanent magnetization of 3 µT that was attained through an internally-generated core dynamo ~10 to several tens of m.y. after CAIs, but that no significant magnetic field existed during the earlier stage of aqueous alteration ~4–6 m.y. after CAIs. Employing multiple investigation techniques, Shah et al. (2017) investigated the paleointensity of 19 Vigarano chondrules and found values of 1.1–150 µT. The observed magnetic remanence is considered to have been acquired during brecciationThe formation of a breccia through a process by which rock fragments of of various types are recemented or fused together. Click on Term to Read More events that occurred ~5 m.y. after initial parent body accretion, with impact shock pressures reaching 10–20 GPa. Therefore, they reason that the original paleofield would have been ~40 µT, which is too high to be attributable to the solar wind field, but is in the range of that expected for a planetary core dynamo. The observed magnetic remanence is considered to have been acquired during brecciation events that occurred ~5 m.y. after initial parent body accretion, with impact shock pressures reaching 10–20 GPa. Therefore, they reason that the original paleofield would have been ~40 µT, which is in the range of that expected for a planetary core dynamo.
Paleointensities Obtained for Allende
click on photo for a magnified view
Diagram credit: Carporzen et al., PNAS, vol. 108, #16 (2011, open access link)
‘Magnetic Evidence for a Partially Differentiated Carbonaceous Chondrite Parent Body’
(https://doi.org/10.1073/pnas.1017165108)
Some investigators (e.g., Greenwood et al., 2003 and Wasson et al., 2013) have proposed that the CK chondrites could represent an extension of the CV group. This subgroup is considered to reflect varying degrees of metamorphism including impact-generated crushing, thermal alteration, and recrystallization processes (Wasson et al., 2013). However, subsequent studies (e.g., Dunn et al., 2016; Yin et al., 2017) present petrographic, geochemical, mineralogical, and isotopic evidence which is more consistent with separate CV and CK parent bodies; details of these studies can be found on the Dhofar 015 page.
Allende has a 21Ne-based CRE age of 5.2 m.y. It was shocked to stage S1 (1.7 GPa), perhaps as part of an impact-breccia lens. The specimen of Allende shown above is a 10.2 g partially fusion-crusted individual stone, along with a 7.2 g partial slice exhibiting abundant chondrules and CAIs within a dark matrix. The Allende stone was the first meteorite acquired for the Weir Meteorite Collection in 1983, purchased from Robert ‘Meteorite Man’ Haag pictured below holding his meteorwrong Rachel. 
