Classification of meteorites is largely based on their mineralogical and petrographic characteristics and their whole-rock chemical and O-isotopic compositions According to the currently used classification scheme, meteorites are divided into chondrites, primitive achondrites, and achondrites There are 15 chondrite groups, including 8 carbonaceous (CI, CM, CO, CV, CK, CR, CH, CB), 3 ordinary (H, L, LL), 2 enstatite (EH, EL), and R and K chondrites Several chondrites cannot be assigned to the existing groups and may represent the first members of new groups Some groups are subdivided into subgroups, which may have resulted from asteroidal processing of a single group of meteorites Each chondrite group is considered to have sampled a separate parent body Some chondrite groups and ungrouped chondrites show chemical and mineralogical similarities and are grouped together into clans The significance of this higher order of classification remains poorly understood The primitive achondrites include ureilites, acapulcoites, lodranites, winonaites, and silicate inclusions in IAB and IIICD irons and probably represent recrystallization or residues from a low-degree partial melting of chondritic materials The genetic relationship between primitive achondrites and the existing groups of chondritic meteorites remains controversial Achondrites resulted from a high degree of melting of chondrites and include asteroidal (angrites, aubrites, howardites-diogenites-eucrites, mesosiderites, 3 groups of pallasites, 15 groups of irons plus many ungrouped irons) and planetary (martian, lunar) meteorites

/pdf/systematics-and-evaluation-of-meteorite-classification-4k05qo1ksk.pdf

Systematics and Evaluation of Meteorite Classification

https://www.ipgp.fr/~kaminski/web_doudoud/JavoyEA_EPSL_2010.pdf

The chemical composition of the Earth: Enstatite chondrite models

Chondrites are extraordinary mixtures of materials with diverse origins that formed around other stars, in the solar nebula, and in their parent asteroids. Most chondrites were so severely altered by aqueous fluids, thermal metamorphism, and impacts that the original characteristics of their components have been largely erased. But a few pristine chondrites have preserved an exquisite mineralogical, chemical, isotopic, and chronological record of the first few million years of solar system history. The properties of diverse types of carbonaceous, ordinary, and enstatite chondrites focusing on the most pristine samples are reviewed to establish the chemical, isotopic, and mineralogical properties and origins of their components and to elucidate the asteroidal processes that modified them. Refractory inclusions – amoeboid olivine aggregates and Ca–Al-rich inclusions – were the first solids to form in the solar nebula near to the protosun. Chondrules and associated metallic Fe–Ni grains were still forming several million years later when the earliest planetesimals, which melted due to heat from 26 Al decay, were colliding. In the least-altered chondrites, matrix material, which coats chondrules and other components, is largely composed of micrometer-sized silicates and amorphous materials, which formed at high temperatures, plus small amounts (up to 200 ppm) of presolar oxides and silicates.

Chondrites and Their Components

/pdf/nature-and-origins-of-meteoritic-breccias-50dvs6azxx.pdf

Nature and Origins of Meteoritic Breccias

Contributors to chromium isotope variation of meteorites

We examined 15 bulk samples of the acapulcoite-lodranite clan for their major, minor, and trace element concentrations using INAA techniques. Among the analyzed meteorites are 2 new acapulcoites (Dhofar [Dho] 290, Thiel Mountains [TIL] 99002) as well as an additional acapulcoite that has been described previously only in very brief form (Graves Nunataks [GRA] 98028). The petrographic attributes of these 3 samples are addressed thoroughly. We also include petrographic information on 2 acapulcoites from Africa: Northwest Africa (NWA) 725 and NWA 1058. In general, our study strongly supports the widely accepted idea that acapulcoites and lodranites evolved through partial melting and melt migration of metal/sulfide phases and plagioclase. Furthermore, we concur with previous researchers that the original bimodal classification scheme for acapulcoites and lodranites proves to be too simple. Based on our data set, we introduce an alternative, extended scheme. With respect to their elemental distribution patterns, we distinguish 5 subtypes comprising primitive, typical, transitional, and enriched acapulcoites on one hand and lodranites on the other. The chemical distinction between the primitive, typical, and transitional acapulcoites is rather subtle and gradual. It stands in contrast to the clear modifications observed for the signatures of the enriched acapulcoites and the lodranites. The definition of subcategories basically reflects the concentrations of 2 key elements: K and Se. We note, however, that the assignment of subgroups may not be exclusively inferred from elemental abundances but should also consider additional petrographic information.

/pdf/evolution-and-classification-of-acapulcoites-and-lodranites-46whx2u77o.pdf

Evolution and classification of acapulcoites and lodranites from a chemical point of view

— Itqiy is a unique coarse-grained, metal-rich enstatite meteorite that was found in the Western Sahara and consists of two rocks together weighing 4.72 kg, which are both completely coated with fusion crust. We report results from our electron microprobe and instrumental neutron activation analysis techniques.



Itqiy consists of subhedral, equigranular, millimeter-sized enstatite, −25 vol% of millimeter-sized kamacite and a few tiny intergrowths of sulfides and kamacite. Relic chondrules are absent. Pyroxene (Fs0.2) is chemically similar to enstatite in EL chondrites, but the metal is closer in composition to that in EH chondrites. Sulfides resemble those in E chondrites but their compositions are distinct from those in both EL and EH chondrites. Itqiy clearly formed under very reducing conditions, but it does not appear to have formed from EH or EL chondrites.



Two thermal events can be distinguished. Silicate compositions including rare earth element abundances indicate loss of partial melt and slow cooling. Heterogeneous sulfides indicate a subsequent reheating and quenching event, which may have been due to shock as many enstatite grains show shock stage S3 features.

/pdf/itqiy-a-metal-rich-enstatite-meteorite-with-achondritic-kbpb1hupym.pdf

Itqiy: A metal‐rich enstatite meteorite with achondritic texture

Dar al Gani 872 (DaG 872) is a new meteorite from Libya that we classified by means of Instrumental Neutron Activation Analysis (INAA), electron microprobe, and optical microscopy. According to our results, DaG 872 is a Mg-rich main group eucrite, i.e., a monomict noncumulate basaltic eucrite displaying a predominant coarse-grained relict subophitic and a fine-grained granulitic texture. The meteorite also shows pockets of late-stage mesostasis and is penetrated by several calcite veins due to terrestrial weathering. Finally, it exhibits shock phenomena of stage 12 including heavily fractured mineral components, undulose extinction of plagioclase, kinked lamellae, and mosaicism in pyroxenes corresponding to peak pressures of ~20 GPa. In view of petrographic criteria as well as compositional and exsolution characteristics of its pyroxenes, the sample represents a metamorphic type 5 eucrite. Assuming the metamorphic type to be a function of burial depth on the parent body and taking into account the relatively high shock stage, the excavation of DaG 872 was likely induced by a major impact event. Prior to this point, DaG 872 apparently underwent a 4-stage geological evolution that is reflected by intricate textural and mineralogical features.

/pdf/new-eucrite-dar-al-gani-872-petrography-chemical-composition-3j3588c9xh.pdf

New eucrite Dar al Gani 872: Petrography, chemical composition, and evolution

Mineralogical and chemical studies of Dar al Gani 983 show that this meteorite is a eucrite. Its texture is that of an impact breccia. It contains cumulate pyroxene and feldspar megacrysts, a variety of recrystallized melt clasts, clasts of subophitic basalt, and mesostasis. These components are embedded in a matrix of fragmental pyroxene and plagioclase. In addition, the entire rock is penetrated by glassy melt veins and patches, and displays features of strong shock. The mineralogical and chemical evidence obtained for DaG 983 indicates that this meteorite experienced a complex evolutionary history. The presence of cumulate silicate crystals implies substantial, large scale cratering events on the HED asteroid. As a result of these impacts, rocks from different intrusive bodies to extrusive surface layers were laterally and vertically transported to form a thoroughly mixed megaregolith. DaG 983 represents a sample of this megabreccia.

/pdf/the-new-polymict-eucrite-dar-al-gani-983-petrography-4zbzvj0sws.pdf

The new polymict eucrite Dar al Gani 983: Petrography, chemical composition, noble gas record, and evolution

Introduction: In two recent studies, the noble gas records of 57 enstatite chondrites (E-chondrites) are presented [1,2]. Here, we report the rare gas content of one new (SAH00225) and three so far undetermined Echondrites recovered in Antarctica (ALHA81260, MAC88180, MAC88184) (tab. 1). Both meteorites from the Mac Alpine Hills are EL3s and have been considered to be paired with MAC88136 [3,4]. In accordance with those findings, all the three of them show the same mineralogical type [5] and also reveal clear Cr excesses [6]. However, another work concluded on the basis of textural differences that they are not paired [7]. An additional well-established parameter to prove pairing is the noble gas record of meteorites. The noble gas concentrations and isotopic ratios of MAC88136 have been reported in [1,2]. Here, we present the rare gas content of MAC88180 and 88184. The other two E-chondrites investigated in this study are ALHA81260 and SAH00225, both EL6s. Additionally, we determined the noble gas record of Zaklodzie, an unusual enstatite meteorite that was found in Poland recently [8]. We will compare it with our rare gas results for Itqiy to be presented at this conference as well [9]. Itqiy is also classified as an unusual enstatite meteorite and was recovered from the Sahara in 2000 [10]. Results and discussion: MAC88180+88184. MAC88180 clearly exhibits solar noble gases and all isotopic ratios closely resemble those of MAC88136. Its Ne cosmic-ray exposure age (T21) is 2.8 Ma, compared to 3.7 Ma of MAC88136. MAC88184 reveals a similarly low T21 of 2.7 Ma. The noble gases of this meteorite, however, are not dominated by the solar component. He and Ar are basically radiogenic or trapped whereas the isotopic composition of Ne plots on a mixing line of the cosmogenic and solar signature. ALHA81260 and SAH00225. These two meteorites contain no solar rare gases but show a subsolar component. T21 are 30.6 Ma and 34.6 Ma, respectively. According to [2], the noble gas content of Echondrites leads to two different patterns of trapped heavy noble gases depending on the petrologic type (1). All E-chondrites of type 4 to 6 contain a mixture of trapped planetary rare gases (Q) and a subsolar component (average elemental ratios for EL6s are Ar/Xe = 990 ± 58 and Ar/Kr = 368 ± 16). In contrast to that, E3-chondrites, if solar gas free, show a mixture of Q and a so-called sub-Q signature with lower elemental ratios (Ar/Xe = 23.1 ± 4.1 and Ar/Kr = 34 ± 17). Trapped solar noble gases only occur in type 3 E-chondrites. These data are supported by the new results with a subsolar component in ALHA81260 and SAH00225, a solar signature in MAC88180 and a sub-Q pattern in MAC88184, which is mainly solar gas free. Zaklodzie. Unlike Itqiy, Zaklodzie exhibits radiogenic He and Ar as well as essentially Q-type trapped noble gases. Its T21 of about 77 Ma as well as its Xe concentration are considerably higher than the corresponding values of Itqiy. The only similarity between Zaklodzie and Itqiy is the cosmogenic Ne signature.

New Noble Gas Data of Four Enstatite Chondrites and Zaklodzie

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