Al-Mg and U-Pb chronological records of Erg Chech 002 ungrouped achondrite meteorite
Philip M. RegerYvonne RoebbertW. NeumannAbdelmouhcine GannounMarcel RegelousW. SchwarzThomas LudwigM. TrieloffStefan WeyerAudrey Bouvier
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Achondrite
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Radiogenic nuclide
Pyroxene
Abstract We studied 149 pyroxenes from 69 pyroxene‐bearing micrometeorites collected from deep‐sea sediments of the Indian Ocean and South Pole Water Well at Antarctica, Amundsen‐Scott South Pole station. The minor elements in pyroxenes from micrometeorites are present in the ranges as follows: MnO ~0.0–0.4 wt%, Al 2 O 3 ~0.0–1.5 wt%, CaO ~0.0–1.0 wt%, Cr 2 O 3 ~0.3–0.9 wt%, and FeO ~0.5–4 wt%. Their chemical compositions suggest that pyroxene‐bearing micrometeorites are mostly related to precursors from carbonaceous chondrites rather than ordinary chondrites. The Fe/(Fe+Mg) ratio of the pyroxenes and olivines in micrometeorites shows similarities to carbonaceous chondrites with values lying between 0 and 0.2, and those with values beyond this range are dominated by ordinary chondrites. Atmospheric entry of the pyroxene‐bearing micrometeorites is expected to have a relatively low entry velocity of <16 km s −1 and high zenith angle (70–90°) to preserve their chemical compositions. In addition, similarities in the pyroxene and olivine mineralogical compositions between carbonaceous chondrites and cometary particles suggest that dust in the solar system is populated by materials from different sources that are chemically similar to each other. Our results on pyroxene chemical compositions reveal significant differences with those from ordinary chondrites. The narrow range in olivine and pyroxene chemical compositions are similar to those from carbonaceous chondrites, and a small proportion to ordinary chondrites indicates that dust is largely sourced from carbonaceous chondrite‐type bodies.
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Abstract Meteoritical Bulletin 104 contains 2279 meteorites including 12 falls (Annama, Cartersville, Creston, Diepenveen, Famenin, Izarzar, Nkayi, Porangaba, San Juan de Ocotán, Trâpeăng Rônoăs, Xinglongquan, Žd’ár nad Sázavou), with 1847 ordinary chondrites, 138 carbonaceous chondrites, 128 HED achondrites, 38 lunar meteorites, 24 ureilites, 22 Martian meteorites, 19 iron meteorites, 17 primitive achondrites, 14 enstatite chondrites, 10 mesosiderites, 9 Rumuruti chondrites, 5 pallasites, 4 ungrouped achondrites, 2 enstatite achondrites, 1 ungrouped chondrite, and 1 Kakangari chondrite, and with 996 from Antarctica, 790 from Africa, 337 from Asia, 111 from South America, 30 from North America, 11 from Oceania, and 4 from Europe. Note: 1 meteorite from Russia was counted as European.
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Aspects of the crystallization history of the primitive crust of the HED (howardites, eucrites, diogenites) achondrite parent body have been deciphered from the microtextures and chemical compositions of pyroxene and plagioclase in some Antarctic HED achondrites intermediate to diogenite and eucrite. Five new specimens related to Yamato 75032 and three eucritic clasts in Y791186 and Y791208 have been investigated by electron microprobe, analytical transmission electron microscope, and X ray diffraction techniques. Their pyroxene compositions cover the entire range from Fe‐rich diogenite to eucrite. The pyroxene textures of the Y75032‐type achondrites indicate that many of them crystallized as low‐Ca pigeonite, and some as orthopyroxene. Later shock events modified the original inversion and exsolution textures. The chemical compositions and the interstitial occurrence of plagioclase crystals in pyroxene indicate crystallization from intercumulus trapped liquid. The pyroxene crystallization and the relation between the anorthite content (An) of plagioclase and the Mg/(Mg + Fe) number of the coexisting pyroxene in these specimens reveal two crystallization trends: One trend is intermediate to that of the Mg‐rich suites of the lunar highland crust and that of ferroan anorthosite, and the other is a trend with a steep decrease in the An content, while the Mg/(Mg + Fe) of the pyroxene remains effectively constant. A model involving crystal fractionation in a shallow magma ocean or intrusion in the parent body has been proposed.
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The cooling histories of individual meteorites can be empirically reconstructed by using ages from different radioisotopic chronometers with distinct closure temperatures. For a group of meteorites derived from a single parent body such data permit the reconstruction of the cooling history and properties of that body. Particularly suited are H chondrites because precise radiometric ages over a wide range of closure temperatures are available. A thermal evolution model for the H chondrite parent body is constructed by using all H chondrites for which at least three different radiometric ages are available. Several key parameters determining the thermal evolution of the H chondrite parent body and the unknown burial depths of the H chondrites are varied until an optimal fit is obtained. The fit is performed by an 'evolution algorithm'. Empirical data for eight samples are used for which radiometric ages are available for at least three different closure temperatures. A set of parameters for the H chondrite parent body is found that yields excellent agreement (within error bounds) between the thermal evolution model and empirical data of six of the examined eight chondrites. The new thermal model constrains the radius and formation time of the H chondrite parent body (possibly (6) Hebe), the initial burial depths of the individual H chondrites, the average surface temperature of the body, the average initial porosity of the material the body accreted from, and the initial 60Fe content of the H chondrite parent body.
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