SAH 97096
EH3.1–3.4
(EHa3 in Weyrauch et al., 2018)
Found 1997
y° 01′ 43′ N., x° 32′ 21′ W.
| ENSTATITE CHONDRITEType of meteorite high in the mineral enstatite and also referred to as E-chondrites. Although they contain substantial amounts of Fe, it is in the form of Ni-Fe metal or sulfide rather than as oxides in silicates. Their highly reduced nature indicates that they formed in an area of the Click on Term to Read More SUBGROUPS Weyrauch et al., 2018 |
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|---|---|---|---|---|
| EHa | EHb | ELa | ELb | |
| Troilite | Cr <2 wt% | Cr >2 wt% | Cr <2 wt% | Cr >2 wt% |
| (Mn,Mg,Fe)S | Fe <20 wt% | Fe >20 wt% | Fe <20 wt% | Fe >20 wt% |
| Daubréelite | Abundant | Missing | Abundant | Missing |
| Kamacite | Ni <6.5 wt% | Ni >6.5 wt% | Ni <6.5 wt% | Ni >6.5 wt% |
A few other E chondrites with intermediate mineralogy have also been identified, including LAP 031220 (EH4), QUE 94204 (EH7), Y-793225 (E-an), LEW 87223 (E-an), and PCA 91020 (possibly related to LEW 87223). Studies have determined that these meteorites were not derived from the EH or EL source through any metamorphic processes, and some or all of them could represent separate E chondrite asteroids. The revised E chondrite classification scheme of Weyrauch et al. (2018) including selected examples from their 80-sample study can be found here. It was determined that the Sahara 97096 pairing group is part of the EHa subgroup.
Spherical metal–sulfide nodules are abundant (37 vol%) in 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 and contain a wide variety of sulfides (e.g., Ca-bearing oldhamiteMn-Ca sulfide, (Mn,Ca)S, is a pale to dark brown accessory mineral found in minor amounts in highly reduced meteorites such as many enstatite chondrites, and some aubrites and enstatite achondrites. Oldhamite in enstatite chondrites likely formed by solar nebular gas condensation. CaS Oldhamite was also found in the most fresh Click on Term to Read More, a mineral which might have formed as the highest-temperature 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 during cooling of a solar gas under reducingOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons, while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More conditions), phosphides (e.g., schreibersiteNi-Fe phosphide mineral, (Fe,Ni)3P, yellowish in color and predominantly found in iron and stony-iron meteorites. Schreibersite can also be found in a variety of other meteorites including some acapulcoites, aubrites, enstatite chondrites and achondrites, lunars, ureilites, winonaites and a smattering of other meteorite types like CM, CO and CB. Schreibersite Click on Term to Read More and perryite), and 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 mineral phases (Weisberg et al., 2006; Lehner et al., 2014). Moreover, it was revealed that a continuum exists for these spherical nodules, in which one end member consists of metal–sulfide nodules that contain no silicates, and the other end member consists of pyroxene-rich chondrules that contain no sulfides (Lehner et al., 2014). They suggest that when taken together, the compositions of each of these components (transitional objects and 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) manifest a complementary relationship. 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 studies of Sahara 97096 by Jacquet et al. (2015) led them to conclude that formation of these opaque nodules was the result of an extreme sulfidizing event in which oldhamite (CaS) was expelled from chondrules, thus negating the need for enstatite chondrite silicates to have initially condensed under unique reducing conditions associated with high C/O ratios. A characteristic which is unique to this meteorite compared to other E chondrites is the presence in some portions of abundant 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. It has been shown by Tagle and Berlin (2007) that the E chondrites are typically depleted in those refractory elementsUsing research by Wood (2019), any of the elements with a relatively high condensation temperature of 1291 K < TC,50 < 1806 K in the solar nebula. They are the first elements to condense out of a cooling gas. Refractory elements are the main building blocks of rocky planets, dwarf Click on Term to Read More that condense before 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, but that the CV/CK chondrites are enriched in these same elements to a complementary degree. Below the temperature at which refractory inclusions give way to the formation of chondrules, this complementarity is maintained, although it is reversed. They expressed support for a scenario whereby the refractory component was transferred from the E-chondriteType of meteorite high in the mineral enstatite and also referred to as E-chondrites. Although they contain substantial amounts of Fe, it is in the form of Ni-Fe metal or sulfide rather than as oxides in silicates. Their highly reduced nature indicates that they formed in an area of the Click on Term to Read More formation region to the CV/CK region. Sahara 97096 also contains an unusual abundance of FeO-rich silicates, mostly low-Ca 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. However, as shown by Kimura et al. (2003), the O-isotopic compositions of both FeO-rich and FeO-poor silicates are identical, indicating that they both formed from a common oxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only Click on Term to Read More reservoir (the same oxygen reservoir as that associated with the Earth and Moon). Notably, FeO-rich silicates with identical O-isotopic values are present in Kakangari (Berlin et al., 2007). These silicates show evidence for 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 in both meteorite groups, but to a greater degree in E chondrites than in Kakangari. It was suggested that the FeO-rich silicates in both of these groups may have had an early common precursor, despite the differences that now exist in their degree of reduction. Rubin et al. (2009) identified a clastic matrix component incorporated among the chondrules, 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 fragments, and opaque assemblages in EH chondrites. The matrix is typically present in relatively low abundance in EH chondrites, and constitutes ~21.5 vol% in Sah 97096 (Lehner et al., 2014). It consists of coarse angular particles of silicate (20–30 vol%) and opaque (25–30 vol%) minerals having a similar mineralogy to the minerals common in the bulk EH chondrite; therefore, it has been considered likely that the matrix component represents a disaggregated component of these same mineral phases. Contrariwise, Lehner et al. (2011) determined that the matrix and chondrule compositions of Sah 97096 are different from each other and are not complementary. They found that the matrix is not composed of a mixture of components from the bulk meteorite, and that the silicate component of the matrix is more depleted in refractory elements than are the chondrules. Instead, Lehner et al. (2014) determined that the composition of the matrix consists primarily of pulverized pyroxene-rich chondrules that have undergone sulfidation in a hot nebula environment, along with a minor component of 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 clasts. Utilizing transmission electron microscopy (TEM), Weisberg et al. (2014) found that the matrix material in Sah 97096 and other studied EH chondrites is composed of a unique, fine-grained, reduced component that was not derived from chondrules, but rather from primary dust and debris inherent to the EC formation region. Employing TEM and other advanced techniques, Lehner et al. (2014) ascertained that the matrix consists of both 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 and crystalline grains of enstatite (~45 vol%) and cristobaliteHigh temperature polymorph of silicon dioxide (SiO2). Has the same chemical composition as coesite, stishovite, seifertite and tridymite but possesses a different crystal structure. This silica group mineral occurs in terrestrial volcanic rocks, martian and lunar meteorites, chondrites and impact glasses like Libyan Desert Glass. Cristobalite has a very open Click on Term to Read More (up to 30 vol%), along with typical opaque phases (15–20 vol%, as kamacite and troilite), as well as minor oldhamite, niningerite, and C-rich spherules. The glassy silicaSilicon dioxide, SiO2. grains were shown by Zolensky et al. (2014) to contain inclusions of sulfide, 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, schreibersite, and enstatite. Present in the matrix is a presolar nebular dust component (45–50 vol%) which occurs as fine-grained (nm- to sub-µm-sized), amorphous or finely crystalline, Al- or Mg-rich silicate particles, and also includes SiC grains and C-anomalous grains. The matrix constitutes only 2–5 vol% of EH chondrites and exhibits an enrichment in alkalis such as Na and K, possibly due to their recondensation onto nebular dust during chondrule heating. These sub-µm-sized nebular fines comprise the minerals kamacite, troilite, niningerite, oldhamite, Cu-rich sulfide, schreibersite, enstatite, and silica; Fe–FeS spherules are abundant near shock-melt veins. It was demonstrated that all of the matrix phases originated from the same O-isotopic reservoir as the other EH chondrite components, but are different in significant ways from the matrix material in EL chondrites; e.g., the abundance of silica is higher and the abundance of albitic plagioclase is lower in EH compared to EL chondrites. Schreibersite particles in EH chondrites are observed to occur separatly from FeNi-metal, and they are thought to represent an early condensate (Lehner and Buseck, 2010). An anomalous 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 grain was identified in Sah 97096 that has a unique texture and composition compared to other chondrule components (Weisberg et al., 2011). Its O-isotopic ratios are more similar to those of R chondrites, and it is considered that this olivine possibly represents a relict grainCoarse-grained precursor material typically found in chondrules, that survived the melting event that formed the host chondrule. In other words, the relict grain did not crystallize in situ from the host melt. In fact, relict grains can be survivors from previous chondrule-forming events that have been recycled into the most Click on Term to Read More acquired from a different generation and distinct reservoir of chondrules that was preserved due to incomplete melting. Computer modeling of the genesis of enstatite chondrite chondrules was conducted by Blander et al. (2009). They demonstrated that high temperature and high pressure conditions initially present in the nebular condensation region created a barrier to the nucleation of Fe, but which was conducive to the formation of FeO. They contend that a cloud of supercooled liquid droplets in 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 with the nebular gas of solar compositionµat a pressure of ~0.1–1.0 barUnit of pressure equal to 100 kPa. (near 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 at a distance consistent with the orbitThe elliptical path of one body around another, typically the path of a small body around a much larger body. However, depending on the mass distribution of the objects, they may rotate around an empty spot in space • The Moon orbits around the Earth. • The Earth orbits around Click on Term to Read More of Mercury) and a high temperature of ~1625°C resulted in the initial condensation of the more refractory elements such as Ca, Al, Mg, and Si. As the temperature decreased, these Ca,Al,Mg,Si-oxide droplets (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) were gravitationally removed from the condensation region as is reflected by the composition of the later formed enstatite chondrites. Weisberg et al. (2011) concluded that unlike the locally-formed chondrules, all CAIs presently associated with the various chondrule groups were formed in a distinct nebular location and were subsequently redistributed to diverse 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 regions. As temperatures continued to decrease below ~1325°C, Fe was precipitated and most of the previously formed FeO underwent reduction. As the temperature reached ~1125°C, the supercooled oxide droplets rapidly solidified to form chondrules of various textures and compositions, consisting primarily of near-pure enstatite (58 wt%) with lesser amounts of olivine (26 wt%), along with a silica-rich liquid phase (16 wt%) that eventually became the chondrule 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. As the temperature decreased below ~400°C, the formation of niningerite [(Mg,Fe,Mn)S], troilite (FeS), and oldhamite (CaS) occurred through the sulfidation of ferromagnesian silicates as Mg became volatilized within an H-poor, C- and S-rich gaseous reservoir (Lehner et al., 2013). The Fe-rich chondrules resulting from this entire process are consistent with those constituting enstatite chondrites, and are similar in composition to the planetThe term "planet" originally comes from the Greek word for "wanderer" since these objects were seen to move in the sky independently from the background of fixed stars that moved together through the seasons. The IAU last defined the term planet in 2006, however the new definition has remained controversial. Click on Term to Read More Mercury; it can be inferred that Mercury is composed of these same constituents. Chondrule-sized, shock-melted, spheroidal lumps have been described in studies of Sah 97096 (Lehner and Buseck, 2009). They were formed in an impact event, probably on the EH parent asteroid, prior to consolidation and lithification of the Sah 97096 host rock. This scenario is evidenced by the sintering of the 5–40 µm-sized metallic and silicate fragments by an Fe metal–sulfide melt phase, and by the presence of melt veins and metal spherules both within the lump and throughout the bulk meteorite. Moreover, the composition of the lumps is similar to bulk Sah 97096. The discovery of these lumps led the investigating team to conclude that Sah 97096 is a primitive brecciaWork 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. However, the chemical composition of Sah 97096 as exhibited in its high Fs content in pyroxene, high Ti content in troilite, and low Cr content in olivine may be more consistent with a low degree of metamorphism (Komatsu et al., 2011). In a study of 16 different E chondrites conducted by Macke et al. (2009), Sah 97096 was shown to have a higher 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 of 12.6% than all of the others, which typically ranged from 0.3% to 6.4%. Other compositional details suggest there was a wide variation in oxygen fugacityUsed to express the idealized partial pressure of a gas, in this case oxygen, in a nonideal mixture. Oxygen fugacity (Æ’O2) is a measure of the partial pressure of gaseous oxygen that is available to react in a particular environment (e.g. protoplanetary disk, Earth's magma, an asteroid's regolith, etc.) and Click on Term to Read More (related to the partial pressure of available oxygen) during accretion of Sah 97096. However, 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 identified in Sah 97096, such as graphiteOpaque form of carbon (C) found in some iron and ordinary chondrites and in ureilite meteorites. Each C atom is bonded to three others in a plane composed of fused hexagonal rings, just like those in aromatic hydrocarbons. The two known forms of graphite, α (hexagonal) and β (rhombohedral), have Click on Term to Read More, pyroxene, and grains of C surrounded by troilite and metal, reflect their stability under the redoxOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons, while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More conditions that existed during formation of the E chondrite 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, and are indicative of a highly reduced environment (Ebata and Yurimoto, 2009). Recently, a presolar oxide grain (corundumCrystalline form of aluminium oxide, Al2O3, found in Ca-Al-rich inclusions (CAIs). Corundum-bearing CAI are a rare class of high-temperature condensates from the inner regions of the protoplanetary disk. Click on Term to Read More) was identified in one of the paired fragments of this meteorite, the first ever found in an E chondrite. This oxide grain is isotopically consistent with an origin in a red giantGiant and highly luminous red star in the later stages of stellar evolution after it has left the main sequence. These red stars have a relatively cool surface whose core has burned most of its hydrogen. Red giants lose parts of their atmospheres and thus provide new elements into interstellar Click on Term to Read More or 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. Some earlier studies suggested that the EL and EH chondrites originate from different layers on the same parent body. Employing multiple lines of evidence including chemical, petrographic, metamorphic, and 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 data, a sequence from the 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 to the surface of EH6, EH5, EH4, EH3, EL3, EL4, EL5, and EL6 was derived. The theory provides for the inner EH layers to be metamorphosed by internal heating, probably during accretion, while the outer EL layers were metamorphosed by external heating, probably by the Sun’s early activities. Studies have determined that the E chondrites formed at a location closer to the Sun—at a distance of at least 1 AU outward to 1.4 AU—than the current location in the asteroid beltBelt located between 2.12 and 3.3 AU from the Sun and located between the orbits of Mars and Jupiter containing the vast majority of asteroids. The asteroid belt is also termed the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such Click on Term to Read More which they now occupy. More recently, very precise measurements were made of a statistically larger sampling of E chondrites and aubritesAubrites are named for the Aubres meteorite that fell in 1836 near Nyons, France. They are an evolved achondrite that is Ca-poor and composed mainly of enstatite (En100) and diopside (En50Wo50) with minor amounts of olivine (Fa0) and traces of plagioclase (An2-8). They contain large white crystals of enstatite as Click on Term to Read More. Although their O-isotopic data were identical, a three-isotope plot did resolve the EH group from both the EL and aubriteAubrites are named for the Aubres meteorite that fell in 1836 near Nyons, France. They are an evolved achondrite that is Ca-poor and composed mainly of enstatite (En100) and diopside (En50Wo50) with minor amounts of olivine (Fa0) and traces of plagioclase (An2-8). They contain large white crystals of enstatite as Click on Term to Read More groups by its slightly steeper slope. The EL and aubrite groups still plotted on the terrestrial 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 line. By using 53Mn/53Cr isotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: H, H (deuterium), and H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. Click on Term to Read More systematics as a chronometer for absolute ages, Shukolyukov and Lugmair (2004) found that the EL6 Khairpur is ~4–5 m.y. younger than the EH4 chondrites Abee and Indarch, possibly representing an extended cooling history for Khairpur on a common parent body, or perhaps an origin on a distinct parent body. In a similar manner, age data based on I–Xe for EL and EH chondrites was attained by Hopp et al. (2013, 2015). They found significant age variations exist for meteorites both among members of the same group and between the two groups, so this precise chronometer does not resolve the two groups; e.g., the age of EH3 Sah 97096 at ~4.5544 b.y. vs. EH4 Abee at ~4.5618 b.y. vs. EH5 St. Marks at ~4.5612 b.y. vs. EL6 Neuschwanstein at ~4.5584 b.y. vs. EL6 LON 94100 at ~4.5579 b.y. Employing a broader range of EL and EH petrologic types exemplifying differences in formation temperatures, Hopp et al. (2014) were able to resolve these two groups using the K–Ar 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. They determined a lower corrected age range for metamorphic cooling of EL5 and EL6 meteorites of 4.48–4.51 b.y., and also found evidence of a more complex thermal history for the EH parent body indicative of multiple impact resetting events and homogenization during the period 2–4.5 b.y. ago. Their studies of Sah 97096 and other EH chondrites revealed a late partial metamorphic resetting event ~2.2 b.y. ago, while the oldest Ar–Ar age of ~4.53 b.y. was measured for the EH parent body in the LAP 02225 impact melt. A third possible grouplet with intermediate mineralogy has been identified, represented by the meteorite Y-793225. Studies have determined that it was not derived from either the EH or EL groups through any metamorphic processes, and thus may represent a unique enstatite parent body. Still, since Y-793225 contains the SiN mineral sinoite, which has only been found to occur in the EL group, this anomalous E chondrite may be related to that group. Further analyses of many EL3 and EH3 chondrites has identified both 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 breccias containing trapped solar rare gases and those which are solar gas-free. No regolith breccia or solar rare gases have been found in other E-chondrite petrologic types, supporting the theory that EL3 and EH3 members represent the surface material from separate parent bodies. In another study, both Fe- and Zn-isotope compositions are fractionated to different degrees between EL and EH chondrites—EL chondrites are heavier than EH chondrites, indicating that they experienced higher volatilization during formation closer to the Sun (Mullane et al., 2005). A radically different conclusion about the origins of E chondrites has recently been drawn from studies of trapped 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 (A. Patzer and L. Schultz, 2002). The trapped primordial noble gases found in these meteorites are present as a mixture of specific components, with each component containing a different ratio of 36Ar, 132Xe, and 84Kr. One component has ‘solar’ ratios of these noble gases, which is typically found in regolith breccias (~30% of E3 chondrites). Another component which is present in both of the enstatite chondrite groups (EH and EL) as well as in ordinary and carbonaceous chondrites is called the ‘Q’ component (formerly known as the planetary or common 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 component). In addition, an unusually Ar-rich component with an elemental composition intermediate between solar and Q ratios has been identified and labeled ‘subsolar’. It has been argued that subsolar gas originates from fractionated solar gas that was implanted in chondrule precursors (Okazaki et al., 2010). Finally, a component with ratios lower than those in Q was identified and given the name ‘sub-Q’. In contrast to ordinary and 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 groups, these various noble gas components in E chondrites appear to be segregated based on petrologic typeMeasure of the degree of aqueous alteration (Types 1 and 2) and thermal metamorphism (Types 3-6) experienced by a chondritic meteorite. Type 3 chondrites are further subdivided into 3.0 through 3.9 subtypes. instead of genetic (parent body) relationships. For example, all E chondrites of petrologic type 4–6 have both a Q and a subsolar gas component, while all those of type 3 have a Q and a sub-Q component. However, these noble gas compositions do not correspond to variations in thermal metamorphism because subsolar gas abundances throughout the range E4–6 are similar. Moreover, since the subsolar component in E4–6 chondrites is less fractionated than the Q component present in E3 chondrites, the subsolar gas cannot be derived through thermal metamorphism of type 3 chondrites. Therefore, these differences in noble gas compositions that exist among E chondrites must have been established at the time of nebular condensation and accretion. The inferred scenario calls for solar-type and Q-type noble gases to be incorporated into separate parent bodies, with a subsequent metamorphic event fractionating these components into subsolar and sub-Q compositions. It was concluded by Okazaki et al. (2010) that the sub-Q component was derived from fractionation incurred during terrestrial weathering. In addition to those parent body distinctions which can be made through studies of trapped noble gas compositions, other characteristics also suggest an independent nebular origin for the E3 and the E4–6 chondrites; e.g., O-isotopes, Ni content in kamacite, Si content in metal, and the lack of an anticorrelation between 36Ar and petrologic type (Hopp et al., 2014 and references therein). These characteristics, along with the trapped noble gas data, are consistent with a separate formation of E3 and E4–6 chondrites on separate parent bodies. This scenario would be a radical departure from the commonly cited onion-skin model which serves as the basis for petrologic type divisions in other chondrite groups. Piani et al. (2009) employed multiple techniques to study the insoluble organicPertaining to C-containing compounds. Organic compounds can be formed by both biological and non-biological (abiotic) processes. Click on Term to Read More matter (IOM) component of Sah 97096, located primarily within matrix material surrounding chondrules and in opaque nodules. They identified only a small number of aromatic compounds, including benzene and naphthalene, compared to the wide diversity of both aromatic and aliphatic compounds present in CI and CM carbonaceous chondrites. Many differences exist between IOM in primitive carbonaceous chondrites compared to Sah 97096, including a low abundance of IOM compounds in Sah 97096, and these can be attributed to the higher temperature conditions experienced by Sah 97096 prior to accretion or during parent body metamorphism. Utilizing Raman micro-spectroscopy, Robin et al. (2008) analyzed the maturity (degree of structural order) of the IOM in a fine-grained matrix component and in inclusions within metal nodules of several E chondrites in order to better resolve the metamorphic grades and to assign petrologic types. Other indicators which measure the degree of thermal metamorphism were also employed, including textural and opaque mineral petrography. Based on their results, and through comparisons with similar studies conducted previously on carbonaceous and ordinary chondrites, an accurate petrologic type for Sah 97096 was determined to be 3.1–3.4; this is among the lowest found thus far in members of the EH group. The thermal history of both EH and EL enstatite chondrite asteroids, including the relationship between petrologic type and the closure temperature of opaque phases, is consistent with a simple onion shell model (Quirico et al., 2011). Notably, a new type of meteorite classified as a forsterite chondriteForsterite Chondrites or F-Chondrites are presently only known as inclusions in other meteorites and described by certain lithologies of the Cumberland Falls aubrite. They are thought to have derived from a small and primitive asteroid of F chondritic composition that collided with the aubrite parent body shortly after their formation Click on Term to Read More/achondrite with EH3 affinities has been discovered in a pairing of Sah 97096 (Boyet et al., 2011; abstract). The photo above shows a 19.4 g partial slice of Sah 97096 that was cut from the 2,516 g mass.
