Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.460w&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 460-460
Computer Science
22
Chondrites, Ordinary, Weathering, Meteorites, Antarctic
Scientific paper
Weathering categories A, B, and C are used by the Meteorite Working Group at the NASA Johnson Space Center in Houston for Antarctic meteorite finds, denoting minor, moderate, and severe rustiness of hand specimens. A different scale can be set up from the weathering effects seen in polished sections with the microscope. These weathering effects finally lead to the disintegration of the meteorite; they are important in connection with its terrestrial age and an estimate of the true fall rate of meteorites. In order to avoid confusion with the hand specimen classification A, B, C, the weathering grades determined on polished sections were named W1 to W6. Weathering affects first the metal grains, later troilite, and finally the silicates. The following progressive stages can be distinguished: W0: No visible oxidation of metal or sulfide. A limonitic staining may already be noticeable in transmitted light. Fresh falls are usually of this grade, although some are already W1. W1: Minor oxide rims around metal and troilite, minor oxide veins. W2: Moderate oxidation of metal, about 20-60% being affected. W3: Heavy oxidation of metal and troilite, 60-95% being replaced. W4: Complete (>95%) oxidation of metal and troilite, but no alteration of silicates. W5: Beginning alteration of mafic silicates, mainly along cracks. W6: Massive replacement of silicates by clay minerals and oxides. More or less massive veining with iron oxides can already be found in stage W2. These veins develop independently from the weathering grade, apparently in cracks that form through mechanical forces. Broad cracks are often filled with carbonates. Grades W5 and W6 are rare. The silicate alteration affects first the olivines; it starts inside the grains, not from the rim. In stage W6 intact chondrules were found, where olivines were completely replaced by a mixture of clay minerals and iron oxides, the feldspathic mesostasis being unaffected. A correlation between these weathering grades and the terrestrial ages was shown for meteorite finds from Roosevelt County, New Mexico [1]. In these climatic conditions the weathering grades W2 to W6 develop in the following times: W2, 5000 to 15,000 yr; W3, 15,000 to 30,000 yr; W4, 20,000 to 35,000 yr; W5 and W6, 30,000 to >45,000 yr. Similar terrestrial ages were found for chondrites of these weathering grades from the Lybian and Algerian Sahara [2,3]. Antarctic meteorite finds weather much more slowly. A check of 53 Antarctic ordinary chondrites (of hand specimen weathering categories A to C) showed only 9 of grade W2, the rest being W1. Among the W1s is ALHA77278 (category A) with a terrestrial age of 320,000 yr [4]. References: [1] Jull A. J. T. et al. (1991) LPSC XXII, 665. [2] Jull A. J. T. et al. (1990) GCA, 54, 2895. [3] Jull A. J. T. et al. (1993), this volume. [4] Nishiizumi K. et al. (1989) EPSL, 93, 299.
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