Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.447t&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 447
Mathematics
Logic
4
Acapulco, Achondrites, Augite, Caddo County, Iab Iron Meteorites, Lodran, Neutron Activation Analysis, Partial Melting, Plagioclase, Rare-Earth Elements, Residual Liquid, Silicates
Scientific paper
Caddo County is an IAB-iron meteorite with silicate inclusions. Its silicate inclusion with olivine and clinopyroxene as major minerals was studied by INAA by Palme et al. [1]. We report mineralogy and INAA data of an inclusion with abundant augite and plagioclase, which will provide us with information on the missing materials removed from sourse regions of the primitive achondrites. The PTS of Caddo County, 7.5 x 5.5 mm in size has been studied by mineralogical techniques including electron probe microanalysis (EPMA) and scanning electron microscopy (SEM) with digital imaging utilities. A thick slice of the specimen next to the PTS was crushed and separated into metal- rich and a silicate-rich fractions for determination of lithophile and sidelophile element concentrations by INAA. The PTS shows aligned textures of silicate inclusions in metallic matrix, suggesting that some parts are intruded into this place. Augite (Aug) and plagioclase fragments are rounded and anhedral, and numerous metal veins fill interstices of the silicate grains. The silicate parts are mainly composed of augite and plagioclase with additional small amounts of minor orthopyroxene (Opx). Olivine is present only as small grains. Crystals of augite reach up to 0.8 x 0.4 mm in size and plagioclase crystals up to 0.6 x 0.3 mm in size. Many plagioclase grains in the metal matrix within one region shear a common crystallographic orientation as indicated by polysynthetic twin lamellae. The Opx distributes in a limited region and Opx and Aug are in contact with curved boundaries even within a grain. The pyroxene compositions are Aug (Ca44.5Mg52.5Fe3) and Opx (Ca2Mg91.5Fe6.5). The plagioclase compositions (Or3Ab80An17) are Na rich and show small ranges from grain to grain. The compositions of olivine (Fa97.5) are similar to that of winonaites [2]. The modal abundances are: Aug 20 vol%, plagioclase 28%, Opx 3%, metallic irons etc. 49% (excluding minor olivine and Opx). Na, Al and REE are enriched in the silicate phases in Caddo County: Na (5.79X CI), Al (6.17X CI) in contrast with Bild and Wasson's [3] results on Lodran, which showed depletion of Ca relative to Mg or Si. Mg-normalized abundance ratios of Ca in the silicates of Caddo County (11.8X CI) are one order of magnitude larger than Acapulco (0.9X CI). Ni content of metal phase of Caddo County is about 7%, and Ir is 2.5 ppm. The Ru content in the FeNi metal is 2 to 5 ppm. Abundances of Ni, Ir, and Ru are in the range of IAB iron meteorite in agreement with those by Palme et at. [1]. According to the formation model of lodranites [4], Ca-Al-rich melt and Fe-Ni- S eutectic melt are removed from source materials. Kracher and Wasson [5] suggested that the type IAB iron meteorites have been formed out of the Fe-Ni- S melt. It is interesting to note that Ca-Al-rich melts removed during formation of lodranitelike materials crystallize plagioclase and Aug. The Ca- Al-rich silicates like plagioclase and Aug are major minerals in the silicate inclusion of Caddo County. From bulk chemistry by INAA, Ca, Al, REE, typically absent in lodranites, were enriched in Caddo County. These facts indicate that this type of meteorite was formed from the Fe-Ni-S-rich and Ca-Al-rich melts. However, we have to admit that silicate inclusions in the IAB group have different oxygen isotopic abundances from those of lodranites. Lodranitelike meteorites in mineralogy and bulk chemistry are missing in the winonaite-IAB group and those of Caddo County have not been found in the Acapulcoite- lodranite group. References: [1] Palme H. et al. (1991) LPSC XXII, 1015-1016. [2] Kimura M. et al. (1992) Proc. NIPR Symp. Antarct. Meteorites, 5, 165-190. [3] Bild R. W. and Wasson J. T. (1976) Mineral. Mag., 40, 721-735. [4] Takeda H. et al. (1992) Proc. Japan Academy, 68, B, 115-120. [5] Kracher A. and Wasson J. T. (1982) GCA, 46, 2419.
Baba Takayasu
Ebihara Mitsuru
Otsuki Michio
Saiki Koichiro
Takeda Hidenori
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