MesosideriteOne of two main types of stony-iron meteorite, the other being pallasites. Mesosiderites are a mixture of approximately 50% basaltic, gabbroic and orthopyroxenitic silicates and 50% Ni-Fe metal and sulfides. The name derives from the Greek "mesos" meaning "middle" or "half" and "sideros" for "iron;" hence "half-iron". The silicates are Click on Term to Read More, group 1A Found during or before 1861 25° 45′ S., 70° 30′ W. approx. The first of many large masses of this polymict, 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 thebrecciaWork in Progress ... A rock that is a mechanical mixture of different minerals and/or rock fragments (clasts). A breccia may also be distinguished by the origin of its clasts: (monomict breccia: monogenetic or monolithologic, and polymict breccia: polygenetic or polylithologic). The proportions of these fragments within the unbrecciated material Click on Term to Read More was found by a prospector in Atacama, Chile, ~100 miles SE of Taltal. The discovery was first reported by the Chilean geologist Ignacy Domeyko in 1862, in a landscape described as vegetation-free, sand and gravel covered open plains, crossed by dry river beds. Possibly many tons of Vaca Muerta were found and subsequently broken apart by 19th century miners to retrieve the metal-rich portion for smelting.
In 1985, a mining engineering student named Edmundo Martinez explored the Atacama Desert region from which Vaca Muerta was originally found by miners a century earlier (Pedersen et al., 1992). Field work by Martinez and his associates led to the characterization of many sites in the strewn fieldArea on the surface containing meteorites and fragments from a single fall. Also applied to the area covered by tektites, which are produced by large meteorite impacts. Strewnfields are often oval-shaped with the largest specimens found at one end. Given that the largest specimens go the greatest distance, a meteoroid's and the recovery of both undisturbed and disturbed masses of this mesosiderite, in sizes of 13 g–312 kg and 2 g–850 kg, respectively. In addition, they found a large pile of Vaca Muerta igneous clasts discarded by 19th century miners who were seeking the more metal-rich stones for smelting. Almost 4,000 kg of this 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 has been accounted for, while the pre-atmospheric mass is estimated to have been over 6,000 kg. Calculations indicate that the Vaca Muerta meteoroidSmall rocky or metallic object in orbit around the Sun (or another star). had a pre-atmospheric diameter of ~100–140 cm (Reedy et al., 1994). The extensive strewnfield has dimensions of 11.5 × 2.1 km.
A two-stage irradiation history has been proposed for mesosiderites by Hidaka and Yoneda (2011). In the first stage, occurring <4.4 b.y. ago, the silicates were irradiated near the surface, prior to mesosiderite formation. Subsequent to 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 mesosiderite 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, a low velocity collision of a large (~50–150 km diameter) iron projectile and a large (~200–400 km diameter), still molten iron planetesimal occurred ~4.4 b.y. ago, melting and mixing the cool silicate layer of the planetesimal with the molten Fe-metal of the projectile into complex breccias. The partial or total collisional disruption and gravitational reassembly of the target body is considered a strong likelihood by some (Haack et al., 1996). Others favor a scenario in which the molten metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More that became mixed with cooler silicates was derived from the differentiated 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 of the mesosiderite planetesimal itself through a severe impact, without a catastrophic disruption and reassembly (Scott et al., 2001).
Based on in-depth mineralogical and textural studies of numerous samples of a massive >80 kg mesosiderite, portions of which having been described and classified by several independent labs, Bunch et al. (2014) have proposed an alternate petrogenetic history for the mesosiderites. Because they did not findMeteorite not seen to fall, but recovered at some later date. For example, many finds from Antarctica fell 10,000 to 700,000 years ago. Click on Term to Read More any eucritic inclusions in any of the fragments from this very large mesosiderite, and the O- and Cr-isotopic compositions of the 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 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 in this mesosiderite are virtually identical to those of diogenitesDiogenites belong to the evolved achondrite HED group that also includes howardites and eucrites. They are named after the Greek philosopher Diogenes of Apollonia, of the 5th century BCE, who was the first to suggest that meteorites come from outer space (a realization forgotten for over 2,000 years). They are Click on Term to Read More, they developed a scenario in which a small, previously cooled iron-rich asteroid collided with a still warm (~800°C), previously differentiated diogenitic asteroid at a low angle and relatively low velocity (<6 km/s). The frictional shear forces created in this oblique impact produced small rotating molten metal spherules which mechanically incorporated diogeniteDiogenites belong to the evolved achondrite HED group that also includes howardites and eucrites. They are named after the Greek philosopher Diogenes of Apollonia, of the 5th century BCE, who was the first to suggest that meteorites come from outer space (a realization forgotten for over 2,000 years). They are Click on Term to Read More fragments, resulting in a minimally-shocked, fragmental stony-iron layer of debris.
Each of these scenarios envision a brief period of rapid cooling due to the mixing of warm and cold material, followed by very slow cooling and annealing consistent with deep burial of the mesosiderite precursor material under an extensive debris blanket and/or subsequent 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 flows. Other evolutionary stages considered to have occurred include reductionOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons, while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More processes and episodic impact events, the latter resulting in re-melting, metal–silicate mixing and 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, formation of quench textures, mixing of deep silicates and near-surface silicates, 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 gardening, thermal metamorphism, and degassing which reset the Ar–Ar age chronometer to reflect an age of ~3.6–3.9 b.y. Thereafter, the impact excavation and ejection from depth of the mesosiderite meteoroid (~138 m.y. ago for Vaca Muerta) initiated a second stage of irradiation (Bajo and Nagao, 2011). Other calculated CRE ages of mesosiderites reflect different excavations occurring over the past 10–150 m.y. A terrestrial age of <2000 years was calculated for Vaca Muerta (Jull et al., 2009).
Mesosiderites of subgroup 1 have experienced the least thermal metamorphism and are more commonly richer in eucritic enclaves (Ikeda et al., 1990), some of which have escaped significant recrystallization. Vaca Muerta eucritic enclaves or clasts, also known as ‘eucritic pebbles’, are mostly characterized as monogenic basalts with granular to gabbroic textures, composed primarily of silicate (93–98 wt% as plagioclase and pigeoniteLow-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, found as a major mineral in eucrites and shergottites. In order to be considered pigeonite, the clinopyroxene must contain 5 to 20 mol % of calcium (Wo5 - 20). Chondrites of petrologic types 4 and below contain significant low-Ca clinopyroxene. During metamorphism to higher temperatures, all existing) with minor amounts of tridymiteSilica group mineral in which the tetrahedra occur in sheets. Tetrahedra alternately point up or down to share oxygen with tetrahedra of other sheets, forming six-sided rings perpendicular the sheets. Tridymite has a fairly open structure and accommodates Na+, K+ and Ca2+; charge balance is achieved by Al3+ ↔ Si4+. (derived from both the primary eucritic material and through reduction processes sustained by phosphorus in FeNi-metal), phosphates, oxides, and various opaques. The major and 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 compositions of the Vaca Muerta clasts are most commonly similar to cumulateIgneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber. Click on Term to Read MoreeucritesMost 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 having a more magnesian composition (ilmenite-free; Mg# ≥0.47), whereas other mesosiderites might contain more ferroan clasts (ilmenite-bearing; Mg# ≤0.46) and show similarities to non-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, diogeniteDiogenites belong to the evolved achondrite HED group that also includes howardites and eucrites. They are named after the Greek philosopher Diogenes of Apollonia, of the 5th century BCE, who was the first to suggest that meteorites come from outer space (a realization forgotten for over 2,000 years). They are Click on Term to Read More, and dunite material. The eucritic clasts were formed during the very early stages of magmatism on the mesosiderite parent body ~4.563 (±0.015) b.y. ago, representing both magmatic rocks (ilmenite-bearing) and cumulate and/or residual rocks (ilmenite-free). The significant petrographic differences that exist between these mesosiderite eucritic clasts and the HED meteorites may indicate they originated on separate parent bodies.
Both the Vaca Muerta eucritic clasts (see example 1, example 2, example 3, and right photo above) and the metallic-melt host phase have CRE ages based on Cl–Ar and Sm–Gd of 138 (±11) m.y., which reflects only irradiation received during transitWhen a small celestial body moves in front of a much larger one (as when Mercury or Venus appears in silhouette against the solar disk or when a satellite passes in front of Jupiter or Saturn). The shadow of a satellite may also transit the disk of its primary. to Earth (Albrecht et al., 2000; Hidaka and Yoneda, 2011). The eucritic pebbles have an additional Kr–Kr-based cosmogenic age of >60 m.y., which reflects the additional exposure received by the precursor material during residence at shallow depth on the parent asteroid prior to impact ejection. Studies of cosmogenic 131Xe that was produced while the meteoroid was in transit to Earth indicates that the progenitor of Vaca Muerta had been located deep within the source object (Bajo et al., 2011). In addition, rare examples (0–6 vol%; Prinz et al., 1980) of large, coarse-grained, magnesian olivine (dunite) clasts have been found in Vaca Muerta and some other mesosiderites, but it is still unresolved whether they represent higher-level cumulates or mantleMain silicate-rich zone within a planet between the crust and metallic core. The mantle accounts for 82% of Earth's volume and is composed of silicate minerals rich in Mg. The temperature of the mantle can be as high as 3,700 °C. Heat generated in the core causes convection currents in Click on Term to Read More material (Greenwood et al., 2015).
Mandler and Elkins-Tanton (2013) proposed a formation scenario for such dunites that involves a two-stage 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 process: first, an equilibriumTerm used to describe physical or chemical stasis. Physical equilibrium may be divided into two types: static and dynamic. Static equilibrium occurs when the components of forces and torques acting in one direction are balanced by components of forces and torques acting in the opposite direction. A system in static Click on Term to Read More crystallization process from the late-stage liquid after 60–70% solidification of the global 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; second, a fractional crystallizationA crystallization process in which minerals crystallizing from a magma are isolated from contact with the liquid. It is a key process in the formation of igneous rocks during the process of magmatic differentiation. Also known as crystal fractionation. Click on Term to Read More process within an ascended, high-level (crustal) plutonGeology: Igneous intrusive body that forms when magma is injected into host rocks and solidifies. Plutons occur in the crust of asteroids undergoing differentiation or planets. Named after Pluto, the Roman god of the underworld. Plutonic rocks are the rocks found within a pluton. Astronomy: Category of planet including all Click on Term to Read More composed of the former extracted residual melt, ultimately resulting in the formation of a thin lower-crustal dunite layer along with more shallow 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 diogenite, diogenite, and 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 lithologies. On the other hand, if the dunitic clasts are actually derived from mantle material, a scenario is required to explain how such material was incorporated into the regolith. However, it was argued by Barrat and Yamaguchi (2014) that 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 chamber processes are unable to explain the chemical diversity of the diogenites (e.g., the range of heavy-REE ratios in diogenitic orthopyroxenes), and that neither assimilation of wallrock nor incorporation of a trapped melt component can account for this diversity. They contend that the diversity is more likely the result of variability in the respective initial parental melt compositions.
The diverse suite of silicate clasts (e.g., basalts, gabbros, orthopyroxenites, dunites) present in Vaca Muerta and other mesosiderites have O-isotopes consistent with an origin on the mesosiderite parent body rather than a xenolithic origin (Greenwood et al., 2009, 2013, 2015). Because mesosiderites have isotopic signatures (e.g., 17O, 54Cr, and 50Ti [Rüfenacht et al., 2018]) that are virtually identical to the HED clan meteorites, many investigators consider that mesosiderites originated on the HED parent body, or at least within similar nebular reservoirs (see diagrams below). Further information regarding the origin of the dunitic clasts in our collections can be found on the NWA 2968 page. Diagram credit: Greenwood et al., 2017 For an explanation of the diagram components see the open accessarticle in Chemie der Erde, vol. 77, p. 25 (2017) ‘Melting and differentiation of early-formed asteroids: The perspective from high precision 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 MoreisotopeOne 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 studies’ (http://dx.doi.org/10.1016/j.chemer.2016.09.005)
Diagram credit: Wasson and Göpel, 77th MetSoc, #5446 (2014) However, studies of the Eu/Sm systematics suggest that the HED clan meteorites and the mesosiderites did not originate on a common parent body. The silicates in mesosiderites have a Eu/Sm ratio higher than CI and so exhibit a positive Eu anomaly, while the howardites of the HED clan have a Eu/Sm ratio less than CI and show a negative Eu anomaly. Further evidence supporting separate parent bodies rather than distinct regions on a common parent body includes the fact that the CRE ages are not correlated; most howardites 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 into two clusters of ~21 and ~38 m.y., while the mesosiderites have widely ranging ages from <10 to 340 m.y. Moreover, in contrast to the mesosiderite parent body, the HED parent body shows no evidence of a crustal melting episode, or a metal–silicate mixing event (Rubin and Mittlefehldt, 1993). In accord with this viewpoint, it was shown by D. Mittlefehldt (2014) that mesosiderites contain basaltic clasts that have a wide range of anomalous characteristics, while howardites do not. He cited these anomalies as evidence for the fact that clastA mineral or rock fragment embedded in another rock. Click on Term to Read More formation succeeded metal–silicate mixing on the parent body, and he also noted that petrologic data are indicative of clast formation preceding impact gardening. Therefore, the apparent lack of such anomalies in components of howardites is inconsistent with a common parent body for both HED and mesosiderite meteorites. In another study of the highly 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 (HSE) contents of mesosiderites, Xu et al. (2011) found that the HSE patterns are very similar to those in FeNi-metal from H-group 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, and they suggest a genetic link might exist there.
The ~226-km-diameter M-type asteroid, 16 Psyche, has been considered by some to be the mesosiderite parent body. The visible and near-infrared reflectance spectra of 16 Psyche indicate a surface composed of metal and 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 Click on Term to Read More consistent with mesosiderites. A study of 16 Psyche conducted by Viikinkoski et al. (2018) has determined that its 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 (3.99 [±0.26] g/cm–3) is a good match to values calculated by Britt and Consolmagno (2003) for both mesosiderites (~4.25 g/cm–3) and the stony-iron Steinbach (~4.1 g/cm–3), but is inconsistent with that of iron meteorites (~7.8 g/cm–3).
A recent detailed spectroscopic survey of main-belt and near-Earth asteroids has identified certain differentiated asteroids having high concentrations of high-Ca pyroxene, abundant plagioclase, minor olivine, and a significant metal component. These asteroids, including the S-type asteroids 17 Thetis and members of the Merxia and Agnia families, may be similar to the metal–pyroxene-rich mesosiderites (Sunshine et al., 2004). In a similar way, utilizing near-IR spectrography of asteroids located near the border of the 3:1 orbital resonanceDynamic relationship between bodies in heliocentric orbits in which a small body has an orbital period that is a simple fraction of a nearby larger body such as Asteroid Belt objects and Jupiter. The periodic gravitational tug of the large body induces changes in the orbit of the smaller body Click on Term to Read More located at 2.50 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 (a Kirkwood GapLack of asteroids in regions within the asteroid belt (between Mars and Jupiter) coinciding with orbital periods that are simple fractions of Jupiter’s own orbital period. The absence of asteroids in these gaps is due to resonance with Jupiter’s gravitational influence. Click on Term to Read More associated with Jupiter), Fieber-Beyer et al. (2010, 2011) found that close similarities exist in the absorptionTransfer of energy to a medium as a particle or electromagnetic radiation passes through it. Absorption of electromagnetic radiation is the combined result of Compton scattering, σ, and photoelectric absorption, τ. It may be quantified: where, t = thickness, ρ = density, and μ = mass absorption coefficient, which combines Click on Term to Read More spectra of eleven members (out of twelve members studied) of the MariaBroad low plains surrounded by basin-forming mountains, originally thought to be a sea (pl. maria). This term is applied to the basalt-filled impact basins common on the face of the Moon visible from Earth. Click on Term to Read More asteroid family (total of ~80 members) and the absorption spectra obtained for mesosiderites. The Maria family, which is likely the result of collisional disruption, contains HED-type pyroxenes and exhibits spectral reddening likely caused by FeNi-metal. Fieber-Beyer et al. (2011) ascertained that the near-Earth asteroidAsteroids with orbits that bring them within 1.3 AU (195 million km) of the Sun. NEAs are a dynamically young population whose orbits evolve on 100-million-year time scales because of collisions and gravitational interactions with the Sun and the terrestrial planets. These asteroids are probably ejected from the main belt Click on Term to Read More 1036 Ganymed is mineralogically similar to diogenites, with pyroxenes identical to those in Johnstown, and they also determined that the spectra of the asteroid is consistent with asteroids from the Maria family. Dynamical models of the Maria family predict that it is a probable source of some or all mesosiderites delivered to Earth (see diagram below). Diagram credit: Fieber-Beyer et al., Icarus, vol. 213, #2, p. 534, (2011) ‘The Maria asteroid family: Genetic relationships and a plausible source of mesosiderites near the 3:1 Kirkwood Gap’ (https://doi.org/10.1016/j.icarus.2011.03.009) Based on silicate 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 textures, Vaca Muerta has been placed into group 1A. (see the Bondoc page for further information about the grouping scheme). It is theorized by one investigative team that VestaThird largest and fourth brightest asteroid; it was discovered in 1807 by Heinrich Olbers and named for the ancient Roman goddess of the hearth. 4 Vesta has a basaltic surface composition and an average density not much less than that of Mars. Evidently lava once flowed here indicating that the and the Vestoids, along with the isotopically similar mesosiderites and IIAB irons, were themselves the products of a catastrophic breakup of an even larger parent object which they have named ‘Opis’ (Irving et al., 2009). In Greek mythology, Opis was the wife of Saturn, whose children were Jupiter, Neptune, Pluto, Juno, Ceres, and Vesta. The left photo above shows a 9.1 g end section of a metallic-melt host phase of Vaca Muerta, while the right shows a 51.6 g end section of a naturally faceted, eucritic inclusionFragment of foreign (xeno-) material enclosed within the primary matrix of a rock or meteorite. Click on Term to Read More likely formed from a localized impact melt of an existing cumulate eucrite lithology (photos not to the same scale).
MesosideriteOne of two main types of stony-iron meteorite, the other being pallasites. Mesosiderites are a mixture of approximately 50% basaltic, gabbroic and orthopyroxenitic silicates and 50% Ni-Fe metal and sulfides. The name derives from the Greek "mesos" meaning "middle" or "half" and "sideros" for "iron;" hence "half-iron". The silicates are Click on Term to Read More, group 3A Purchased no coordinates recorded Three fragments of a stony-iron meteoriteMeteorite composed of roughly equal amounts by weight of silicate minerals and Ni-Fe metal. The stony-irons consist of two groups: mesosiderites and pallasites. However, there is gradual shading into metal-rich stony meteorites such as the lodranites (once considered stony-irons) and silicate-rich iron meteorites. Stony-iron meteorites are less abundant than their weighing together 288 g were found in Northwest African and sold to a collector in Rissani, Morocco. 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 was classified at the University of Münster, Germany (A. Bischoff) as a mesosiderite, while a 3A subtype was assigned to this meteorite by the Natural History Museum of Bern, Switzerland (B. Hoffmann). This mesosiderite has been shocked to stage S2 and is relatively unweathered with a grade of W0/1. Further information about the mesosiderite grouping scheme can be found on the Bondoc page.
The specimen of NWA 1961 shown above is a small 1.2 g end section. The top photo below shows the complete, flattened, egg-shaped, 189.8 g mass from which the specimen above was taken, while the bottom photo below shows a complete slice of NWA 1961 exhibiting an intricate mixture of silicates and FeNi-metal.
Photos courtesy of JNMC–Zurich
MesosideriteOne of two main types of stony-iron meteorite, the other being pallasites. Mesosiderites are a mixture of approximately 50% basaltic, gabbroic and orthopyroxenitic silicates and 50% Ni-Fe metal and sulfides. The name derives from the Greek "mesos" meaning "middle" or "half" and "sideros" for "iron;" hence "half-iron". The silicates are Click on Term to Read More, group 2A Found 1985 no coordinates recorded Two fragments with a combined weight of ~7 kg were found near the village of Gillio, Libya by an oil worker. The masses were utilized as bookends until they were purchased from the worker’s daughter by 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 dealer A. Lang in 1998. A sample was submitted to Rutgers University (Roger Hewins) for analysis and classification, and the name Sahara 85001 chosen to be most appropriate given the lack of an exact findMeteorite not seen to fall, but recovered at some later date. For example, many finds from Antarctica fell 10,000 to 700,000 years ago. Click on Term to Read More location. However, results were not forthcoming, and four years later, an additional sample was submitted to Northern Arizona University (Ted Bunch). By this time in 2002, the NWA-series had been established by the Meteorite Nomenclature Committee of the Meteoritical Society and the name NWA 1242 was assigned to this mesosiderite; however, since it is a find location in Libya, this meteorite was included within the NWA-series. In the interim a couple of kilos of this meteorite was sold under the name Sahara 85001, and this name is now a synonym for the official name NWA 1242. Northwest Africa 1242 was classified at Northern Arizona University as a member of the small 2A metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. Click on Term to Read More subgroup. (see the Bondoc page for further information about the Floran (1978) and Hewins (1984) classification schemes).
Northwest Africa 1242 contains scattered, cm-sized, metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More nodules (see photo below), together with lithic clasts consisting of Ca-rich 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 Click on Term to Read More, anorthitic 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, pyrrhotiteIron sulfide group of minerals whose composition ranges widely between its end members pyrrhotite (Fe7S8) whose crystal structure is monoclinic, and troilite (FeS) whose crystal structure is hexagonal. Its general formula is Fe1−xS (where x = 0 to 0.17). The troilite phase is found mainly in meteorites and in the Click on Term to Read More, chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More, and FeNi-metal. This unweathered mesosiderite (W0) is shocked to stage S1. The specimen shown above is an 11.67 g partial slice. The top photo below is a close-up image of a large, etched metal nodule, while the bottom photo shows the main massLargest fragment of a meteorite, typically at the time of recovery. Meteorites are commonly cut, sliced or sometimes broken thus reducing the size of the main mass and the resulting largest specimen is called the "largest known mass". Click on Term to Read More of NWA 1242.
Photos courtesy of Alan Lang—R. A. Langheinrich Meteorites
MesosideriteOne of two main types of stony-iron meteorite, the other being pallasites. Mesosiderites are a mixture of approximately 50% basaltic, gabbroic and orthopyroxenitic silicates and 50% Ni-Fe metal and sulfides. The name derives from the Greek "mesos" meaning "middle" or "half" and "sideros" for "iron;" hence "half-iron". The silicates are Click on Term to Read More, group 3A Fell March 12, 1935 52° 0′ N., 19° 55′ E. At 12:52 A.M. near Lowicz, Poland, a shower of stones was seen and heard to 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 over an area of 9.2 km² along a trajectory from west to east. However, in contrast to most other fall distributions, the smallest fragments were recovered in the farthest part of the strewnfield near Seligow, while the larger pieces were found in the closest part in Krepa. This reversed distribution could be the result of fragmentation low in the atmosphere. At least 83 and perhaps over 100 stones with a total weight of over 60 kg were eventually recovered, but many were subsequently lost during World War II.
Mesosiderites have been divided into different groups (Floran, 1978 and Hewins, 1984) based on their composition and degree of thermal metamorphism, with Lowicz falling into group 3A (see the Bondoc page for further information about the grouping scheme). The formation of mesosiderites has been examined in the context of several different models. See the Estherville page for details concerning the most current models. Lowicz has a CRE age of ~53 m.y. The above specimen is a very thin partial slice approximately 19 mm × 11 mm × 1 mm. It weighs 0.9 g and shows abundant metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More mixed with the silicates.
MesosideriteOne of two main types of stony-iron meteorite, the other being pallasites. Mesosiderites are a mixture of approximately 50% basaltic, gabbroic and orthopyroxenitic silicates and 50% Ni-Fe metal and sulfides. The name derives from the Greek "mesos" meaning "middle" or "half" and "sideros" for "iron;" hence "half-iron". The silicates are Click on Term to Read More, group 3A/4A Fell May 1879 43° 25′ N., 94° 50′ W. At 5:00 P.M. on May 10, in Estherville, Iowa, several large masses and hundreds of small iron nodules fell after a fireballA fireball is another term for a very bright meteor, generally brighter than magnitude -4, which is about the same magnitude of the planet Venus as seen in the morning or evening sky. A bolide is a special type of fireball which explodes in a bright terminal flash at its end, often with visible fragmentation. Click on Term to Read More was seen and sonic booms heard. Over 700 pounds of material was recovered, including one mass of ~437 pounds and one of 151 pounds. The largest mass was divided among the London, Paris, and Vienna Museums while the location of the smaller mass is unknown. Hundreds of the atmospherically ablated iron nodules are preserved at Yale’s Peabody Museum.
This polymict brecciaGeneral term for all breccias that are neither monomict nor dimict. Modified from image source: http://www.saharamet.com/meteorite/gallery/HED/index.html. includes iron inclusions together with large areas of silicates including 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, 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 Click on Term to Read More, and plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite Click on Term to Read More. Mineralogical studies have determined that 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 olivines and olivine clasts are most likely xenoliths from separate parent bodies, which were assimilated together onto the mesosiderite planetesimal during impact late events (Hassanzadeh et al., 1990). Lithic clasts of eucritic and diogenitic material are present.
The formation of mesosiderites on their 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 has been explained through several competing theories. A recent model based on smoothed-particle hydrodynamicsStudy of fluid in motion. Click on Term to Read More calls for the disruption and re-accretion of a 200–400 km differentiated asteroid with a molten coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel. Click on Term to Read More. The impactor is calculated to have been a 50–150 km body with an impact speed of 5 km/s. This event initially caused rapid cooling (~0.1°C/y.) from high temperature equilibration, followed by very slow cooling (~0.5°C/m.y.) as the brecciated material was deeply covered by a massive debris blanket. The relatively young Ar–Ar ages of mesosiderites of 3.7–4.1 b.y. reflect this period of very slow cooling. Weakly shocked olivine was sequestered into the core at the time of the catastrophic impact, as molten metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More was mixed with cold crustal fragments during re-accretion. Recent dating of zircons in Estherville by Haba et al. (2014) places the formation age of this mesosiderite (i.e., metal-silicate mixing and crustal remelting) at 4.520 (±027) b.y. Cooling rate studies conducted by Sugiura and Kimura (2015) on a number of mesosiderite samples indicate that Estherville, Vaca Muerta, NWA 2924, and Dong Ujimqin Qi experienced rapid cooling from peak temperatures down to intermediate temperatures, while others including NWA 1242, NWA 1878, Crab Orchard, ALH 77219, and A-882023 cooled much more slowly over the same temperature range.
A more conventional theory calls for the 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, melting, and 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 of the large parent body ~4.56 b.y. ago. A period of impact-melting and metamorphism ensued until 3.9 b.y. ago, by which time the brecciated nature of the mesosiderite parent body had been established. It was at this time, 3.9 b.y. ago, that a major thermal event occurred, raising temperatures to as high as 500°C. A likely cause for this event is the collisional disruption and gravitational reassembly of the asteroid. The surface breccias were buried under a deep 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 where slow cooling and annealing proceeded. Subsequent impacts excavated this deeply buried material and some of it was ejected into space, establishing a range of cosmic-ray exposure ages for mesosiderites of ~10–340 m.y. Estherville has a Sm–Gd-based CRE age of 70 (±7) m.y. (Albrecht et al., 2000).
A more outdated theory has the basaltic 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 of a molten parent body founder and sink through the 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 to the metallic core where mixing occurred. Subsequent collisions exposed this stony-iron layer and delivered fragments to Earth. It is notable that the O-isotopic values of the mesosiderites are almost identical to those of the HED suite of meteorites, implying that a genetic link exists between these disparate groups (Greenwood et al., 2006). Conversely, multiple line of evidence indicate that separate parent bodies were probably involved.
In the classification scheme of Floran, 1978 and Hewins, 1984, Estherville was assigned as a transitional member to group 3A and 4A (see the Bondoc page for further information about the grouping scheme). Calculations based on cosmogenic radionuclides show that Estherville had a pre-atmospheric diameter of at least 62 cm. Its cosmic-ray exposure ageTime interval that a meteoroid was an independent body in space. In other words, the time between when a meteoroid was broken off its parent body and its arrival on Earth as a meteorite - also known simply as the "exposure age." It can be estimated from the observed effects Click on Term to Read More of ~70 m.y. is similar to that of Crab Orchard and Chinguetti, suggesting a common ejection event for these three mesosiderites. The Estherville specimen shown above is an 18.6 g complete slice, which exhibits a stony matrix containing iron inclusions and numerous olivine crystals.
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