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  • 1
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    A compositional classification scheme for meteoritic chondrules (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: A compositionally based classification scheme for chondrules is proposed that will help in systematizing the wealth of data available and disentangling the effects of nebular and subsequent processes. The classification is not by texture or the composition of a single phase, or a mixture of these two, but rather is a comprehensive, systematic approach which uses the composition of the two main chondrule components. This scheme is applicable to over 95 percent of the chondrules and is easily applied using an electron microprobe. It stresses the original diversity of the chondrules and the complex yet facile way in which they respond to parent-body metamorphism. Results using this classification scheme suggest that arguments against an important role of chondrules in determining the compositional trends of the chondrites have been premature.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Nature (ISSN 0028-0836); 357; 6375,
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  • 2
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    Comment on 'Update on terrestrial ages of Antarctic meteorites' by K. Nishiizumi, D. Elmore and P. W. Kubik (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Earth and Planetary Science Letters (ISSN 0012-821X); 99; 4, Se; 380-382;
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  • 3
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    Metamorphism of eucrite meteorites studied quantitatively using induced thermoluminescence (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Induced thermoluminescence studies provide a new and quantitative means of determining relative metamorphic intensities for eucrite meteorites, the simplest and most ancient products of basaltic volcanism. Using this technique, it is shown that the eucrites constitute a continuous metamorphic series and not, as commonly assumed, two groups of metamorphosed and nonmetamorphosed meteorites. It is suggested that the method may have applications to other basalts.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Nature (ISSN 0028-0836); 349; 516-518
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  • 4
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    Cathodoluminescence observations of in situ aqueous destruction of chondrules in the Murchison CM chondrite (1993)
    In:  Other Sources
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    Publication Date: 2019-01-25
    Description: A low-power cathodoluminescence (CL) mosaic of a 1x2 cm section of the Murchison CM chondrite contains chondrules of two types: one contains olivines with red CL and is thought to be the equivalent of group A chondrules of Sears et.; and one without observable CL, which is probably equivalent to their group B chondrules. Both types of chondrule contain dust mantles with a characteristic uniform red CL due to very fine-grained forsterites, but the mantles of group A chondrules have a complex internal structure and are much thicker than the relatively thin rims on the group B chondrules. The variety of internal textures and the progressive integration of the objects with the surrounding matrix suggests to us that these features are large group A chondrules in the process of being destroyed by aqueous alteration. Prior to aqueous alteration, group A chondrules in CM chondrites were larger than group B chondrules, which is contrary to the situation in ordinary chondrites and were apparently destroyed more rapidly than group B chondrules by aqueous alteration.
    Keywords: ASTROPHYSICS
    Type: Lunar and Planetary Inst., Twenty-Fourth Lunar and Planetary Science Conference. Part 3: N-Z; p 1269-1270
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  • 5
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    Thermal metamorphism (1988)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Most chondrites have experienced thermal metamorphism, resulting in changes in texture, mineralogy and possibly chemical composition. The physical conditions for metamorphism range from approximately 400 to 1000 C at low lithostatic pressure. Metamorphism may have resulted from decay of short-lived radionuclides, electromagnetic induction or accretion of hot materials. Several thermal models for chondrite parent bodies have been proposed. The least metamorphosed type-3 chondrites probably carry the most information about the early solar system, but even these have been affected to some degree by thermal processing.
    Keywords: LUNAR AND PLANETARY EXPLORATION
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  • 6
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    Overview and classification of meteorites (1988)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Before interpreting properties of meteorites in terms of putative processes and conditions in the early solar system, it is necessary to understand just what sort of objects meteorites are. Such understanding begins with classification. In this chapter, the current taxonomy of meteorites is summarized, and it is shown how certain stone meteorites, the chondrites, possess chemical and petrographic features that make them potentially attractive as probes of the early solar system. The prevalence of secondary alteration effects, often capable of perturbing the primitive record even in chondrites, is also emphasized.
    Keywords: LUNAR AND PLANETARY EXPLORATION
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  • 7
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    The classification and complex thermal history of the enstatite chondrites (1995)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: We have carried out instrumental neutron activation analysis of 11 enstatite chondrites and electron microprobe analyses of 17 enstatite chondrites, most of which were previously little described. We report here the third known EH5 chondrite (LEW 88180) and an unusual EL6 chondrite (LEW 87119), new data on four EL3 chondrites (ALH 85119, EET 90299, PCA 91020, and MAC 88136, which is paired with MAC 88180 and MAC 88184), the second EL5 chondrite (TIL 91714), and an unusual metal-rich and sulfide-poor EL3 chondrite (LEW 87223). The often discussed differences in mineral composition displayed by the EH and EL chondrites are not as marked after the inclusion of the new samples in the database, and the two classes apparently experienced a similar range of equilibrium temperatures. However, texturally the EL chondrites appear to have experienced much higher levels of metamorphic alteration than EH chondrites of similar equilibration temperatures. Most of the petrologic type criteria are not applicable to enstatite chondrites and, unlike the ordinary chondrites, texture and mineralogy reflect different aspects of the meteorite history. We therefore propose that the existing petrologic type scheme not be used for enstatite chondrites. We suggest that while 'textural type' reflects peak metamorphic temperatures, the 'mineralogical type' reflects equilibration during postmetamorphic (probably regolith) processes. Unlike the ordinary chondrites and EH chondrites, EL chondrites experienced an extensive low-temperature metamorphic episode. There are now a large number of enstatite meteorite breccias and impact melts, and apparently surface processes were important in determining the present nature of the enstatite chondrites.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 100; E5; p. 9417-9438
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  • 8
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    Two chondrule groups each with distinctive rims in Murchison recognized by cathodoluminescence (1993)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Two groups of chondrules in the Murchison CM chondrite, which have previously been identified on the basis of FeO in the chondrule grains, are readily identified from cathodoluminescence (CL) and belong to those of the ordinary chondrite group A and B chondrules of Sears et al. (1992a). All chondrules are surrounded by fine-grained rims containing forsterite with bright red CL, but on group A chondrules an outer thin rim grades into a much thicker rim, with a lower density of forsterite grains, which in turn grades into the central chondrule. Group B chondrules have only the thin outer rim with a high density of small forsterite grains. This is the first time an unequivocal correlation has been observed between chondrule rim thickness and the composition of the object on which the rim is located. We suggest that while all objects in the meteorite (group B chondrules, refractory inclusions, mineral and chondrule fragments, clasts) acquired a very thin rim during processing in a wet regolith, the thick rims on group A chondrules were formed by aqueous alteration of precursor metal- and sulfide-rich rims which are a characteristic of group A chondrules in ordinary chondrites.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Meteoritics (ISSN 0026-1114); 28; 5; p. 669-675
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  • 9
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    Phreatic Explosions During Basaltic Fissure Eruptions: Kings Bowl Lava Field, Snake River Plain, USA (2018)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Physical and compositional measurements are made at the approx. 7 km-long (approx. 2200 years B.P.) Kings Bowl basaltic fissure system and surrounding lava field in order to further understand the interaction of fissure-fed lavas with phreatic explosive events. These assessments are intended to elucidate the cause and potential for hazards associated with phreatic phases that occur during basaltic fissure eruptions. In the present paper we focus on a general understanding of the geological history of the site. We utilize geospatial analysis of lava surfaces, lithologic and geochemical signatures of lava flows and explosively ejected blocks, and surveys via ground observation and remote sensing.
    Keywords: Geophysics
    Type: GSFC-E-DAA-TN53229 , Journal of Volcanology and Geothermal Research (ISSN 0377-0273); 351; 89-104
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  • 10
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    The natural thermoluminescence of meteorites. III - Lunar and basaltic meteorites (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Natural thermoluminescence (TL) data were obtained to investigate recent thermal and radiation histories of the lunar meteorite MacAlpine Hills 88104/5 and 65 eucrites, howardites, diogenites, and mesosiderites. All these meteorites have low levels of natural TL compared to chondrites, which is primarily because they display anomalous fading. Some meteorites have especially low natural TL which must reflect heating within the last 100,000-1,000,000 y. The parameters for TL decay were determined assuming plausible values for cosmic ray dose rate and that the natural TL of MAC88104/5 was totally drained by ejection from the moon. The obtained parameters for TL decay suggest that the moon-earth transit times for MAC88104 and MAC88105 were 2,000 and 1,800 y, respectively, compared with 19,000 and 2,500 y for Y791197 and ALHA81005, respectively.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geochimica et Cosmochimica Acta (ISSN 0016-7037); 55; 3167-318
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