Dho 011

LL7 or Meta-LL

Found December 5, 1999
18° 20.7′ N., 54° 11.9′ E. A single 150 g stone was found on the surface of the desert in Oman. It was classified at the Vernadsky Institute (M. Ivanova) as an LL7, S3, W3 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. An identical 414 g stone designated Dhofar 014 was found nearby and is considered to be paired. FissionBreaking apart of a body into smaller fragments. In nuclear physics, fission refers to splitting of a heavy atomic nucleus into two or more lighter nuclei with an associated release of energy. The mass of the nucleus before fission is greater than the combined masses of the resulting fragments; the track thermochronometry indicates that type 7 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 cooled more slowly at greater depths than did those of lower petrologic types (Trieloff et al., 2003). Consequently, type 7 chondrites experienced a longer period of thermal metamorphism within this interior layer, and now exhibit extensively recrystallized textures that are transitional to an achondriteAn achondrite is a type of stony meteorite whose precursor was of chondritic origin and experienced metamorphic and igneous processes. They have a planetary or differentiated asteroidal origin where the chondritic parent body reached a sufficient size that through heating due to radioactive decay of Al (aluminum isotope) and gravitational classification. Type 7 ordinary chondrites were originally defined by Dodd et al. (1975) according to specific petrographic characteristics. They listed three metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. criteria to distinguish between petrologic types 6 and 7:

1. the presence of poorly defined 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 in type 6, but only relict chondrules in type 7
2. low-Ca pyroxenes contain no more than 1.0 wt% CaO (1.0 wt% = ~1.9 mol% Wo) in type 6, but more than 1.0 wt% in type 7; conversely, the CaO content of high-Ca pyroxenes decreases from type 6 to type 7
3. 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. grains gradually coarsen to reach a size of at least 0.1 mm in type 7

In Dhofar 011, chondrules are virtually absent within the coarse-grained, thoroughly recrystallized matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents.. Furthermore, CaO in the low-Ca pyroxenes comprise 1.14 wt%. With reference to Dodd (1981), the classification of Dhofar 011 is consistent with that of type 7.

In the intervening years since Dodd et al. proposed their classification parameters, additional type 7 chondrites have been found and studied. As a result of more recent studies, it was proposed by Wittke and Bunch (pers. comm., 2004) that a type 7 category should not comprise meteorites containing any relict chondrules, but rather, should represent a metamorphic extreme in which no sign of chondrules remains. This definition would lump those meteorites containing ‘poorly defined’ chondrules and ‘relict’ chondrules, such as Dhofar 011/014 and DaG 1022, into the type 6 category.

In further contrast to Dodd et al., Wittke and Bunch (2004) suggest that the relative size of all the silicates, rather than only the feldspar grains, would provide a better gauge of a petrographic type 7 since silicates attain an equigranular texture only under the highest metamorphism. They have also discovered that simple twinning of 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 occurs only in type 7, and suggest that this could be utilized as an additional parameter. Beyond that, it was revealed that modal 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 contents decrease significantly during late metamorphic stages; i.e., low-Ni metal, as well as pyroxenes, are consumed to produce 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, resulting in only small amounts of Ni-rich metal along with lower amounts of 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 Mg substitutes for Fe up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca contents occur and clinopyroxene compared to those amounts present in lower metamorphic grades. In addition, Tomkins (2014) determined other petrogrphic criteria to mark the chondrite–achondrite transition. They found that beyond 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. 6, skeletal plagioclase forms an interconnected network between olivine 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 category. grains, and as a low degree (<5%) of partial meltingAn igneous process whereby rocks melt and the resulting magma is comprised of the remaining partially melted rock (sometimes called restite) and a liquid whose composition differs from the original rock. Partial melting occurs because nearly all rocks are made up of different minerals, each of which has a different melting occurs, plagioclase exhibits wetting textures in contact with olivine and pyroxene grains. This was in fact shown to be the case in the H7 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 Watson 012 studied by Tait et al. (2004). In addition, low-Ca exsolutions were observed in many clinopyroxene grains in meteorites beyond petrologic type 6, and relict chondrules were very limited in number—only 0.25 per cm² in Watson 012.

Among the several type-7 LL chondrites studied by Friedrich et al. (2014), both Uden and EET 92013 exhibit some evidence for very low degrees (incipient) of partial melting and occasional mobilization of both FeNi–FeS and plagioclase, as well as evidence of rare vestigial chondrules; a relict barred 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 was observed in their LL7 Y-82067. In a study of the bulk elemental abundances of several H7 and LL7 chondrites, Yoshioka et al. (2014) observed only slight volatileSubstances which have a tendency to enter the gas phase relatively easily (by evaporation, addition of heat, etc.). 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 depletions (Se), and abundance patterns that exhibit both a negative Eu anomaly and an HREE enrichment through REEOften abbreviated as “REE”, these 16 elements include (preceded by their atomic numbers): 21 scandium (Sc), 39 Yttrium (Y) and the 14 elements that comprise the lanthanides excluding 61 Promethium, an extremely rare and radioactive element. These elements show closely related geochemical behaviors associated with their filled 4f atomic orbital. 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., features likely resulting from plagioclase loss. In their research on the metamorphic transition to type 7, Tait et al. (2014) studied the H7 chondrite Watson 012 employing various techniques including X-ray computed tomography. They observed evidence of incipient 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 partial melting in the form of an interconnected plagioclase network composed of coarse-grained skeletal crystals, often associated with large globular metal–troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. grains encapsulated at triple junction node expansions; in addition, some orthopyroxene and clinopyroxene grains have crystallized from this melt phase. They argue that the partial melt was produced early in Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. history (10–15 m.y. old) through a combination of radiogenic heating and impact-shock heating (>S4), resulting in a combined peak temperature of ~1156°C. This equilibrium-controlled partial melting occurred at a depth of ~15–20 km, and thereafter, a period of slow cooling and annealing ensued. Tait et al. (2014) have proposed a revision to the Van Schmus and Wood (1967) classification 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 to include criteria for petrologic type 7, incorporating updates from Dodd (1981), Sears and Dodd (1988), Brearley and Jones (1998), and Tait et al. (2014), which can be viewed here.

For those chondritic meteorites that experienced metamorphic temperatures high enough for significant metal–sulfide melting to occur, which commonly occurred as a result of impact events (after the decay of most radiogenic 26Al), an igneous texture would be produced (Mittlefehldt and Lindstrom, 2001). In these cases the use of the Van Schmus–Wood classification system is no longer valid, and these meteorites could be characterized as impact melts or even primitive achondrites. Dhofar 011 does not show any evidence of having lost a low-melting fraction of metal–sulfide, and it exhibits an unfractionated chondritic composition consistent with subtype 7. Finally, those meteorites which have undergone more extensive thermal processing and have lost their original geochemical and isotopic features (e.g., members of the HED suite) would be termed achondrites.

Research has been published that identifies specific characteristics that distinguish type 7 chondrites from primitive achondrites. The following characteristics are typically observed in primitive achondrites (Ford et al., 2004):

1. an equigranular (igneous) texture with no extensive segregation
2. experienced temperatures to levels necessary for FeNi-metal, FeS, and silicate partial melting (~1200°C, perhaps by shock meltingComplete melting of target material produced by the shock of a meteoric impact. Different minerals will experience certain shock effects at different pressures and temperatures. For example, dense target rocks like anorthosite will typically experience whole rock melting above 50 to 60 GPa, while chondritic rocks require more than 70 of an already hot parental source)
3. migration of free metal from olivine 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) and chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. as a result of 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 processes (i.e., by reaction with 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), resulting in Mg-rich olivine and chromite and low-Ni metal
4. Cr acting as a chalcophile element during reduction leading to its incorporation into troilite
5. close to chondritic bulk composition

The Dhofar 011 meteorite may be paired with the 414 g LL7 Dhofar 014. The specimen of Dhofar 011 shown above is a 1.29 g partial slice. Pictured below is 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". of Dhofar 011 as found in the rock-covered desert.