Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.203b&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 203
Mathematics
Logic
2
Scientific paper
The mineralogy and chemistry of the unique chondrites ALH 85085 and Acfer 182 (Bischoff et al., 1989, 1992a), Renazzo and the CR-type chondrite Acfer 059/El Djouf 001 (Bischoff et al., 1992b) were studied in detail. Several significant mineralogical and chemical similarities were noted, although some distinct differences are present. Differences: One major difference is the average size of chondrules. The chondrule size is increasing from ALH 85085 (16 micrometers; Scott, 1988) to Acfer 182 (60 micrometers; Bischoff et al., 1992a) and to the CR-type chondrite Acfer/El Djouf (1.0 mm; Bischoff et al., 1992b). In the same sequence the metal content is decreasing (ALH 85085: 20 vol% (Scott, 1988); Acfer 182: 15 vol% (estimated, pre- terrestrial (Bischoff et al., 1992a)); Acfer/El Djouf: ~10 vol% (Bischoff et al., 1992b)). Similarities: There are mineralogical and chemical (compare Palme and Spettel, 1992) similarities among ALH 85085, Acfer 182, the Acfer/El Djouf CR-type chondrite, and Renazzo that distinguish these meteorites from other types of C- chondrites: all these samples have on average low FeO contents of olivine and pyroxene and high Cr2O3- contents in olivines (about 0.5 wt%; Bischoff et al., 1992a,b). These meteorites contain abundant fine-grained, dark inclusions. Spectacular Ca,Al-rich inclusions were found within these samples. Except for Renazzo Ca- dialuminate (CaAl4O7) is an abundant phase which is in other chondrite groups very rare. In Acfer 182 Ca- dialuminate is a dominating phase in Ca,Al-rich inclusions (Weber and Bischoff, 1992). Similarities among these chondrites in oxygen isotopic composition and the presence of a unique component of subsolar gases were reported (Bischoff et al., 1992a,b). All these meteorites have low contents of refractory and moderately volatile elements (Bischoff et al., 1992a,b; Palme and Spettel, 1992). Grossman et al. (1988) and Weisberg et al. (1988, 1990) noted that the Bencubbin chondrite breccia has some important chemical and isotopic similarities to CR-type chondrites and ALH 85085. An investigation of the silicate portion of Bencubbin revealed that the finely barred olivine portions are texturally similar to the up to 1-cm-sized chondrule-like objects in the Acfer/El Djouf meteorite and to several large fragments in Acfer 182. Conclusions: Based on chemical and mineralogical similarities it is suggested that the components of these meteorites were formed under similar conditions. However, a single common parent body for these meteorites is unlikely in view of the size distribution of individual components. It is possible that small different parent bodies were formed in a closely related region of the nebula after the components have passed through a process of grain-size- sorting. References: Bischoff A., Metzler K., Stoffler D., Palme H., and Spettel B. (1989) Lunar Planet. Sci. (abstract) 20, 80- 81. Bischoff A., Palme H., Ash R.D., Clayton R.N., Schultz L., Herpers U., Stoffler D., Grady M.M., Pillinger C.T., Spettel B., Weber H., Grund T., Endress M., and Weber D. (1992a) Geochim. Cosmochim. Acta (submitted). Bischoff A., Palme H., Schultz L., Weber D., Weber H.W., and Spettel B. (1992b) Geochim. Cosmochim. Acta (submitted). Grossman J.N., Rubin A.E., and MacPherson G.J. (1988) Earth Planet. Sci. Lett. 91, 33-54. Palme H. and Spettel B. (1992) Meteoritics, this volume. Scott E.R.D. (1988) Earth Planet. Sci. Lett. 91, 1-18. Weber D. and Bischoff A. (1992) Meteoritics, this volume. Weisberg M.K., Prinz M., and Nehru C.E. (1988) Earth Planet. Sci. Lett. 91, 19-32. Weisberg M.K., Prinz M., and Nehru C.E. (1990) Meteoritics 25, 269-279.
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