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Allan Hills Meteorite

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B. Spettel – One of the best experts on this subject based on the ideXlab platform.

  • Acfer 182 and paired samples, an iron-rich carbonaceous chondrite: Similarities with ALH85085 and relationship to CR chondrites
    Geochimica et Cosmochimica Acta, 1993
    Co-Authors: Addi Bischoff, H. Palme, Ludolf Schultz, D. Weber, H. W. Weber, B. Spettel

    Abstract:

    Three samples of a new, Fe-rich chondrite were found in the Sahara in 1990 and 1991 (Acfer 182, Acfer 207, Acfer 214). The samples are paired and the Meteorite will be designated as Acfer 182. The chondrite is chemically, texturally, and mineralogically similar to the Allan Hills Meteorite ALH85085. One important difference between the two Meteorites is the smaller average chondrule size in ALH85085.

    The major components of Acfer 182 (in decreasing abundance) are
    1.
    (1) highly altered (by terrestrial weathering) matrix

    2.
    (2) mineral and polymineralic silicate fragments and aggregates

    3.
    (3) chondrule fragments

    4.
    (4) chondrules

    5.
    (5) metal

    6.
    (6) fine-grained, dark inclusions. The abundance of chondrules is lower and the average chondrule size ($90 μm) smaller than in most other chondrites. Chondrule fragments are often so large that they cannot be derived from the present chondrule population. Apparently, size sorting has prevented accumulation of the larger parent chondrules. Several spectacular Ca,Al-rich inclusions were found, rich in Ca-dialuminate, hibonite, or Zr-, Y-, Sc-bearing phases.

    The chemical composition of Acfer 182 and of ALH85085 are almost indistinguishable. Major chemical signatures are
    1.
    (1) uniform enrichment of Fe and other nonvolatile metals relative to CI-chondrites by about 70%

    2.
    (2) absence of enrichment in refractory lithophiles, characteristic of most type 2 and 3 carbonaceous chondrites

    3.
    (3) strong depletion of volatile and moderately volatile elements. Based on the oxygen isotopic composition, the chemical composition, and the abundances of chondrules and matrix, Acfer 182 should be classified as a carbonaceous chondrite. Considering their affinity to carbonaceous chondrites and their high bulk iron content the two Meteorites, Acfer 182 and ALH85085, are designated as CH-chondrites.

    There are mineralogical and chemical similarities among Acfer 182, ALH85085, and CR chondrites which distinguish these Meteorites from other types of carbonaceous chondrites:
    1.
    (1) low FeO contents of olivine and pyroxene and correspondingly high metal contents

    2.
    (2) high Cr-content in olivine

    3.
    (3) abundant fine-grained dark inclusions

    4.
    (4) abundant Ca-dialuminate (CaAl4O7) in CAIs

    5.
    (5) similarities in oxygen isotopic composition

    6.
    (6) low contents of moderately volatile elements

    7.
    (7) low refractory element contents

    8.
    (8) presence of a unique component of subsolar rare gases.

    These observations suggest similar conditions of formation for the components of these Meteorites. A single common parent body is unlikely in view of the differences in chemical composition and in the size distribution of individual components.

P. Sterzai – One of the best experts on this subject based on the ideXlab platform.

  • Ice dynamics of the Allan Hills Meteorite concentration sites revealed by satellite aperture radar interferometry
    Meteoritics & Planetary Science, 2003
    Co-Authors: F. Coren, G. Delisle, P. Sterzai

    Abstract:

    The ice flow conditions of a 100 x 100 km area of Victoria Land, Antarctica were analyzed with the synthetic aperture radar (SAR) technique. The area includes a number of Meteorite concentration sites, in particular the Allan Hills ice fields. Regional ice flow velocities around the Mid- western and Near-western ice fields and the Allan Hills main ice field are shown to be ≤2.5 m yr-1. These sites are located on a horseshoe-shaped area that bounds an area characterized by higher ice flow velocities of up to 5 m yr-1. Meteorite find locations on the Elephant Moraine are located in this “high ice flow” area. The SAR derived digital elevation model (DEM) shows atypical low surface slopes for Antarctic conditions, which are the cause for the slow ice movements. Numerous ice rises in the area are interpreted to cap sub-ice obstacles, which were formed by tectonic processes in the past. The ice rises are considered to represent temporary features, which develop only during warm stages when the regional ice stand is lowered. Ice depressions, which develop in warm stages on the lee side of ice rises, may act as the sites of temporary build-up of Meteorite concentrations, which turn inoperative during cold stages when the regional ice level rises and the ice rises disappear. Based on a simplified ice flow model, we argue that the regional ice flow in cold stages is reduced by a factor of at least 3.

Addi Bischoff – One of the best experts on this subject based on the ideXlab platform.

  • Acfer 182 and paired samples, an iron-rich carbonaceous chondrite: Similarities with ALH85085 and relationship to CR chondrites
    Geochimica et Cosmochimica Acta, 1993
    Co-Authors: Addi Bischoff, H. Palme, Ludolf Schultz, D. Weber, H. W. Weber, B. Spettel

    Abstract:

    Three samples of a new, Fe-rich chondrite were found in the Sahara in 1990 and 1991 (Acfer 182, Acfer 207, Acfer 214). The samples are paired and the Meteorite will be designated as Acfer 182. The chondrite is chemically, texturally, and mineralogically similar to the Allan Hills Meteorite ALH85085. One important difference between the two Meteorites is the smaller average chondrule size in ALH85085.

    The major components of Acfer 182 (in decreasing abundance) are
    1.
    (1) highly altered (by terrestrial weathering) matrix

    2.
    (2) mineral and polymineralic silicate fragments and aggregates

    3.
    (3) chondrule fragments

    4.
    (4) chondrules

    5.
    (5) metal

    6.
    (6) fine-grained, dark inclusions. The abundance of chondrules is lower and the average chondrule size ($90 μm) smaller than in most other chondrites. Chondrule fragments are often so large that they cannot be derived from the present chondrule population. Apparently, size sorting has prevented accumulation of the larger parent chondrules. Several spectacular Ca,Al-rich inclusions were found, rich in Ca-dialuminate, hibonite, or Zr-, Y-, Sc-bearing phases.

    The chemical composition of Acfer 182 and of ALH85085 are almost indistinguishable. Major chemical signatures are
    1.
    (1) uniform enrichment of Fe and other nonvolatile metals relative to CI-chondrites by about 70%

    2.
    (2) absence of enrichment in refractory lithophiles, characteristic of most type 2 and 3 carbonaceous chondrites

    3.
    (3) strong depletion of volatile and moderately volatile elements. Based on the oxygen isotopic composition, the chemical composition, and the abundances of chondrules and matrix, Acfer 182 should be classified as a carbonaceous chondrite. Considering their affinity to carbonaceous chondrites and their high bulk iron content the two Meteorites, Acfer 182 and ALH85085, are designated as CH-chondrites.

    There are mineralogical and chemical similarities among Acfer 182, ALH85085, and CR chondrites which distinguish these Meteorites from other types of carbonaceous chondrites:
    1.
    (1) low FeO contents of olivine and pyroxene and correspondingly high metal contents

    2.
    (2) high Cr-content in olivine

    3.
    (3) abundant fine-grained dark inclusions

    4.
    (4) abundant Ca-dialuminate (CaAl4O7) in CAIs

    5.
    (5) similarities in oxygen isotopic composition

    6.
    (6) low contents of moderately volatile elements

    7.
    (7) low refractory element contents

    8.
    (8) presence of a unique component of subsolar rare gases.

    These observations suggest similar conditions of formation for the components of these Meteorites. A single common parent body is unlikely in view of the differences in chemical composition and in the size distribution of individual components.