Physics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.254m&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 254
Physics
1
Scientific paper
Desert regions can be productive meteorite recovery locations because of low humidity, minimal ground cover and, in favorable situations, high prevailing winds to deflate surfaces. For these reasons we have made reconnaissance searches of a particularly arid Atacama Desert tract between Antofagasta and Mejillones, Chile (approx. 23 degrees 15'S, 70 degrees 30'W). One of us (EMR) had previously collected new ordinary chondrites from this same area, which has been named Pampa (a), (b), (c), and (e). The Pampa de Agua Blanca (PAB) chondrite is also apparently from this same area. In our brief 1991 reconnaissance we found additional specimens of (a) and (c). All of these meteorites have been found as multiple stones, necessitating pairing studies. Consequently, we examined our finds (a, b, c, and e), borrowed specimens from the Field Museum (a, b, PAB) and Robert Haag (c), and have performed the first detailed studies of the Pampa meteorites. Here we summarize results of these studies. Shock levels refer to the classification developed by Stoffler et al. (1991). PAMPA (a): This weathered meteorite is represented by several partially fusion-crusted fragments with a combined mass of approximately 380 g. No distinct chondrules are evident in thin section, although some chondrule fragments are present; considerable recrystallization is evident and plagioclase (Ab(sub)80Or5-Ab(sub)83Or(sub)5) is present as clear grains. Diopside and hydroxylapatite are also present. Olivine (Fo(sub)75.0 mean, 0.4% mean deviation-PMD) exhibits no shock effects. We classify Pampa (a) as L6, shock level 1. PAMPA (b): This is a weathered meteorite represented by numerous individual stones with a combined weight of approximately 10 kg. We examined thin sections from four individual stones of (b). A few distinct barred chondrules are evident in thin section, along with numerous chondrule and aggregate fragments. Olivine (Fo(sub)74.8 mean, 0.9 PMD) exhibits undulatory extinction, planar fractures, minor mosaicism and very rare (<25%) planar deformation features. We classify Pampa (b) as L4/5, shock level 4. PAMPA (c): This is a highly weathered, desert-varnished meteorite represented by numerous stones with a combined weight of approximately 25 kg. We examined sections from three stones of (c); the most notable feature of the meteorite is considerable darkening. Metal is almost entirely weathered. A few distinct barred and excentroradial chondrules are evident, along with numerous chondrule and aggregate fragments. Olivine (Fo(sub)75.3 mean, 0.8 PMD) exhibits undulatory extinction, planar fractures, mosaicism, planar deformation features, and melt veins and pockets. Some turbid glass is present. We classify Pampa (c) as L4, shock level 6. The classification of Pampa (c) given in the Meteoritical Bulletin (Graham, 1989) (L6) is incorrect. PAMPA (e): This is a weathered meteorite represented by numerous individual stones with an unknown total mass. No distinct chondrules are present in the single thin section available to us. Considerable recrystallization is evident, and clear plagioclase grains are present. Olivine (Fo(sub)75.0 mean, 0.07 PMD) exhibits no shock effects. We classify Pampa (e) as L6, shock level 1. PAMPA de AGUA BLANCA: This is a weathered meteorite represented by specimens totaling 10 g. No distinct chondrules are evident in thin section. Olivine (Fo(sub)75.2 mean, 0.2 PMD) exhibits undulatory extinction, planar fractures, minor mosaicism and very rare (<25%) planar deformation features. Clear plagioclase (Ab(sub)76Or(sub)8-Ab(sub)84Or(sub)5) grains are present. We classify Pampa de Agua Blanca as L6, shock level 4. We conclude by noting that the Atacama Desert in general, and the Antofagasta area in particular, appears to be a fruitful region for future meteorite searches. We note with curiosity that all chondrites found to date in the Antofogasta-Mejillones area have been of the L persuasion. METEORITE OLIVINE (Fo) PYROXENE (En) GLASS? SHOCK CLASS. MEAN PMD CaO MEAN PMD Pampa (a) L6 75.0 0.4 <=0.05 77.2 0.7 NO 1 Pampa (b) L4/5 74.8 0.9 <=0.07 77.3 1.2 NO 4 Pampa (c) L4 75.3 0.8 0.05-0.15 78.2 2.0 YES 6 Pampa (e) L6 75.0 0.07 0.01-0.03 77.0 0 NO 1 Pampa Agua Blanca L6 75.2 0.2 <=0.05 77.5 0.6 NO 4 References: [1] Stoffler et al., (1991) GCA 55, 3845; [2] Graham (1989) Meteoritics 24, 59. We thank Robbie Score for assistance with excentroradial chondrules, and Ignacio Casanova and Robert Haag for samples.
Martinez Rafael
Martinez de Los Rios E.
Zolensky Michael
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